3168 lines
117 KiB
ReStructuredText
3168 lines
117 KiB
ReStructuredText
.. _cgroup-v2:
|
|
|
|
================
|
|
Control Group v2
|
|
================
|
|
|
|
:Date: October, 2015
|
|
:Author: Tejun Heo <tj@kernel.org>
|
|
|
|
This is the authoritative documentation on the design, interface and
|
|
conventions of cgroup v2. It describes all userland-visible aspects
|
|
of cgroup including core and specific controller behaviors. All
|
|
future changes must be reflected in this document. Documentation for
|
|
v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgroup-v1>`.
|
|
|
|
.. CONTENTS
|
|
|
|
1. Introduction
|
|
1-1. Terminology
|
|
1-2. What is cgroup?
|
|
2. Basic Operations
|
|
2-1. Mounting
|
|
2-2. Organizing Processes and Threads
|
|
2-2-1. Processes
|
|
2-2-2. Threads
|
|
2-3. [Un]populated Notification
|
|
2-4. Controlling Controllers
|
|
2-4-1. Enabling and Disabling
|
|
2-4-2. Top-down Constraint
|
|
2-4-3. No Internal Process Constraint
|
|
2-5. Delegation
|
|
2-5-1. Model of Delegation
|
|
2-5-2. Delegation Containment
|
|
2-6. Guidelines
|
|
2-6-1. Organize Once and Control
|
|
2-6-2. Avoid Name Collisions
|
|
3. Resource Distribution Models
|
|
3-1. Weights
|
|
3-2. Limits
|
|
3-3. Protections
|
|
3-4. Allocations
|
|
4. Interface Files
|
|
4-1. Format
|
|
4-2. Conventions
|
|
4-3. Core Interface Files
|
|
5. Controllers
|
|
5-1. CPU
|
|
5-1-1. CPU Interface Files
|
|
5-2. Memory
|
|
5-2-1. Memory Interface Files
|
|
5-2-2. Usage Guidelines
|
|
5-2-3. Memory Ownership
|
|
5-3. IO
|
|
5-3-1. IO Interface Files
|
|
5-3-2. Writeback
|
|
5-3-3. IO Latency
|
|
5-3-3-1. How IO Latency Throttling Works
|
|
5-3-3-2. IO Latency Interface Files
|
|
5-3-4. IO Priority
|
|
5-4. PID
|
|
5-4-1. PID Interface Files
|
|
5-5. Cpuset
|
|
5.5-1. Cpuset Interface Files
|
|
5-6. Device
|
|
5-7. RDMA
|
|
5-7-1. RDMA Interface Files
|
|
5-8. HugeTLB
|
|
5.8-1. HugeTLB Interface Files
|
|
5-9. Misc
|
|
5.9-1 Miscellaneous cgroup Interface Files
|
|
5.9-2 Migration and Ownership
|
|
5-10. Others
|
|
5-10-1. perf_event
|
|
5-N. Non-normative information
|
|
5-N-1. CPU controller root cgroup process behaviour
|
|
5-N-2. IO controller root cgroup process behaviour
|
|
6. Namespace
|
|
6-1. Basics
|
|
6-2. The Root and Views
|
|
6-3. Migration and setns(2)
|
|
6-4. Interaction with Other Namespaces
|
|
P. Information on Kernel Programming
|
|
P-1. Filesystem Support for Writeback
|
|
D. Deprecated v1 Core Features
|
|
R. Issues with v1 and Rationales for v2
|
|
R-1. Multiple Hierarchies
|
|
R-2. Thread Granularity
|
|
R-3. Competition Between Inner Nodes and Threads
|
|
R-4. Other Interface Issues
|
|
R-5. Controller Issues and Remedies
|
|
R-5-1. Memory
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
Terminology
|
|
-----------
|
|
|
|
"cgroup" stands for "control group" and is never capitalized. The
|
|
singular form is used to designate the whole feature and also as a
|
|
qualifier as in "cgroup controllers". When explicitly referring to
|
|
multiple individual control groups, the plural form "cgroups" is used.
|
|
|
|
|
|
What is cgroup?
|
|
---------------
|
|
|
|
cgroup is a mechanism to organize processes hierarchically and
|
|
distribute system resources along the hierarchy in a controlled and
|
|
configurable manner.
|
|
|
|
cgroup is largely composed of two parts - the core and controllers.
|
|
cgroup core is primarily responsible for hierarchically organizing
|
|
processes. A cgroup controller is usually responsible for
|
|
distributing a specific type of system resource along the hierarchy
|
|
although there are utility controllers which serve purposes other than
|
|
resource distribution.
|
|
|
|
cgroups form a tree structure and every process in the system belongs
|
|
to one and only one cgroup. All threads of a process belong to the
|
|
same cgroup. On creation, all processes are put in the cgroup that
|
|
the parent process belongs to at the time. A process can be migrated
|
|
to another cgroup. Migration of a process doesn't affect already
|
|
existing descendant processes.
|
|
|
|
Following certain structural constraints, controllers may be enabled or
|
|
disabled selectively on a cgroup. All controller behaviors are
|
|
hierarchical - if a controller is enabled on a cgroup, it affects all
|
|
processes which belong to the cgroups consisting the inclusive
|
|
sub-hierarchy of the cgroup. When a controller is enabled on a nested
|
|
cgroup, it always restricts the resource distribution further. The
|
|
restrictions set closer to the root in the hierarchy can not be
|
|
overridden from further away.
|
|
|
|
|
|
Basic Operations
|
|
================
|
|
|
|
Mounting
|
|
--------
|
|
|
|
Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
|
|
hierarchy can be mounted with the following mount command::
|
|
|
|
# mount -t cgroup2 none $MOUNT_POINT
|
|
|
|
cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All
|
|
controllers which support v2 and are not bound to a v1 hierarchy are
|
|
automatically bound to the v2 hierarchy and show up at the root.
|
|
Controllers which are not in active use in the v2 hierarchy can be
|
|
bound to other hierarchies. This allows mixing v2 hierarchy with the
|
|
legacy v1 multiple hierarchies in a fully backward compatible way.
|
|
|
|
A controller can be moved across hierarchies only after the controller
|
|
is no longer referenced in its current hierarchy. Because per-cgroup
|
|
controller states are destroyed asynchronously and controllers may
|
|
have lingering references, a controller may not show up immediately on
|
|
the v2 hierarchy after the final umount of the previous hierarchy.
|
|
Similarly, a controller should be fully disabled to be moved out of
|
|
the unified hierarchy and it may take some time for the disabled
|
|
controller to become available for other hierarchies; furthermore, due
|
|
to inter-controller dependencies, other controllers may need to be
|
|
disabled too.
|
|
|
|
While useful for development and manual configurations, moving
|
|
controllers dynamically between the v2 and other hierarchies is
|
|
strongly discouraged for production use. It is recommended to decide
|
|
the hierarchies and controller associations before starting using the
|
|
controllers after system boot.
|
|
|
|
During transition to v2, system management software might still
|
|
automount the v1 cgroup filesystem and so hijack all controllers
|
|
during boot, before manual intervention is possible. To make testing
|
|
and experimenting easier, the kernel parameter cgroup_no_v1= allows
|
|
disabling controllers in v1 and make them always available in v2.
|
|
|
|
cgroup v2 currently supports the following mount options.
|
|
|
|
nsdelegate
|
|
Consider cgroup namespaces as delegation boundaries. This
|
|
option is system wide and can only be set on mount or modified
|
|
through remount from the init namespace. The mount option is
|
|
ignored on non-init namespace mounts. Please refer to the
|
|
Delegation section for details.
|
|
|
|
favordynmods
|
|
Reduce the latencies of dynamic cgroup modifications such as
|
|
task migrations and controller on/offs at the cost of making
|
|
hot path operations such as forks and exits more expensive.
|
|
The static usage pattern of creating a cgroup, enabling
|
|
controllers, and then seeding it with CLONE_INTO_CGROUP is
|
|
not affected by this option.
|
|
|
|
memory_localevents
|
|
Only populate memory.events with data for the current cgroup,
|
|
and not any subtrees. This is legacy behaviour, the default
|
|
behaviour without this option is to include subtree counts.
|
|
This option is system wide and can only be set on mount or
|
|
modified through remount from the init namespace. The mount
|
|
option is ignored on non-init namespace mounts.
|
|
|
|
memory_recursiveprot
|
|
Recursively apply memory.min and memory.low protection to
|
|
entire subtrees, without requiring explicit downward
|
|
propagation into leaf cgroups. This allows protecting entire
|
|
subtrees from one another, while retaining free competition
|
|
within those subtrees. This should have been the default
|
|
behavior but is a mount-option to avoid regressing setups
|
|
relying on the original semantics (e.g. specifying bogusly
|
|
high 'bypass' protection values at higher tree levels).
|
|
|
|
memory_hugetlb_accounting
|
|
Count HugeTLB memory usage towards the cgroup's overall
|
|
memory usage for the memory controller (for the purpose of
|
|
statistics reporting and memory protetion). This is a new
|
|
behavior that could regress existing setups, so it must be
|
|
explicitly opted in with this mount option.
|
|
|
|
A few caveats to keep in mind:
|
|
|
|
* There is no HugeTLB pool management involved in the memory
|
|
controller. The pre-allocated pool does not belong to anyone.
|
|
Specifically, when a new HugeTLB folio is allocated to
|
|
the pool, it is not accounted for from the perspective of the
|
|
memory controller. It is only charged to a cgroup when it is
|
|
actually used (for e.g at page fault time). Host memory
|
|
overcommit management has to consider this when configuring
|
|
hard limits. In general, HugeTLB pool management should be
|
|
done via other mechanisms (such as the HugeTLB controller).
|
|
* Failure to charge a HugeTLB folio to the memory controller
|
|
results in SIGBUS. This could happen even if the HugeTLB pool
|
|
still has pages available (but the cgroup limit is hit and
|
|
reclaim attempt fails).
|
|
* Charging HugeTLB memory towards the memory controller affects
|
|
memory protection and reclaim dynamics. Any userspace tuning
|
|
(of low, min limits for e.g) needs to take this into account.
|
|
* HugeTLB pages utilized while this option is not selected
|
|
will not be tracked by the memory controller (even if cgroup
|
|
v2 is remounted later on).
|
|
|
|
pids_localevents
|
|
The option restores v1-like behavior of pids.events:max, that is only
|
|
local (inside cgroup proper) fork failures are counted. Without this
|
|
option pids.events.max represents any pids.max enforcemnt across
|
|
cgroup's subtree.
|
|
|
|
|
|
|
|
Organizing Processes and Threads
|
|
--------------------------------
|
|
|
|
Processes
|
|
~~~~~~~~~
|
|
|
|
Initially, only the root cgroup exists to which all processes belong.
|
|
A child cgroup can be created by creating a sub-directory::
|
|
|
|
# mkdir $CGROUP_NAME
|
|
|
|
A given cgroup may have multiple child cgroups forming a tree
|
|
structure. Each cgroup has a read-writable interface file
|
|
"cgroup.procs". When read, it lists the PIDs of all processes which
|
|
belong to the cgroup one-per-line. The PIDs are not ordered and the
|
|
same PID may show up more than once if the process got moved to
|
|
another cgroup and then back or the PID got recycled while reading.
|
|
|
|
A process can be migrated into a cgroup by writing its PID to the
|
|
target cgroup's "cgroup.procs" file. Only one process can be migrated
|
|
on a single write(2) call. If a process is composed of multiple
|
|
threads, writing the PID of any thread migrates all threads of the
|
|
process.
|
|
|
|
When a process forks a child process, the new process is born into the
|
|
cgroup that the forking process belongs to at the time of the
|
|
operation. After exit, a process stays associated with the cgroup
|
|
that it belonged to at the time of exit until it's reaped; however, a
|
|
zombie process does not appear in "cgroup.procs" and thus can't be
|
|
moved to another cgroup.
|
|
|
|
A cgroup which doesn't have any children or live processes can be
|
|
destroyed by removing the directory. Note that a cgroup which doesn't
|
|
have any children and is associated only with zombie processes is
|
|
considered empty and can be removed::
|
|
|
|
# rmdir $CGROUP_NAME
|
|
|
|
"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy
|
|
cgroup is in use in the system, this file may contain multiple lines,
|
|
one for each hierarchy. The entry for cgroup v2 is always in the
|
|
format "0::$PATH"::
|
|
|
|
# cat /proc/842/cgroup
|
|
...
|
|
0::/test-cgroup/test-cgroup-nested
|
|
|
|
If the process becomes a zombie and the cgroup it was associated with
|
|
is removed subsequently, " (deleted)" is appended to the path::
|
|
|
|
# cat /proc/842/cgroup
|
|
...
|
|
0::/test-cgroup/test-cgroup-nested (deleted)
|
|
|
|
|
|
Threads
|
|
~~~~~~~
|
|
|
|
cgroup v2 supports thread granularity for a subset of controllers to
|
|
support use cases requiring hierarchical resource distribution across
|
|
the threads of a group of processes. By default, all threads of a
|
|
process belong to the same cgroup, which also serves as the resource
|
|
domain to host resource consumptions which are not specific to a
|
|
process or thread. The thread mode allows threads to be spread across
|
|
a subtree while still maintaining the common resource domain for them.
|
|
|
|
Controllers which support thread mode are called threaded controllers.
|
|
The ones which don't are called domain controllers.
|
|
|
|
Marking a cgroup threaded makes it join the resource domain of its
|
|
parent as a threaded cgroup. The parent may be another threaded
|
|
cgroup whose resource domain is further up in the hierarchy. The root
|
|
of a threaded subtree, that is, the nearest ancestor which is not
|
|
threaded, is called threaded domain or thread root interchangeably and
|
|
serves as the resource domain for the entire subtree.
|
|
|
|
Inside a threaded subtree, threads of a process can be put in
|
|
different cgroups and are not subject to the no internal process
|
|
constraint - threaded controllers can be enabled on non-leaf cgroups
|
|
whether they have threads in them or not.
|
|
|
|
As the threaded domain cgroup hosts all the domain resource
|
|
consumptions of the subtree, it is considered to have internal
|
|
resource consumptions whether there are processes in it or not and
|
|
can't have populated child cgroups which aren't threaded. Because the
|
|
root cgroup is not subject to no internal process constraint, it can
|
|
serve both as a threaded domain and a parent to domain cgroups.
|
|
|
|
The current operation mode or type of the cgroup is shown in the
|
|
"cgroup.type" file which indicates whether the cgroup is a normal
|
|
domain, a domain which is serving as the domain of a threaded subtree,
|
|
or a threaded cgroup.
|
|
|
|
On creation, a cgroup is always a domain cgroup and can be made
|
|
threaded by writing "threaded" to the "cgroup.type" file. The
|
|
operation is single direction::
|
|
|
|
# echo threaded > cgroup.type
|
|
|
|
Once threaded, the cgroup can't be made a domain again. To enable the
|
|
thread mode, the following conditions must be met.
|
|
|
|
- As the cgroup will join the parent's resource domain. The parent
|
|
must either be a valid (threaded) domain or a threaded cgroup.
|
|
|
|
- When the parent is an unthreaded domain, it must not have any domain
|
|
controllers enabled or populated domain children. The root is
|
|
exempt from this requirement.
|
|
|
|
Topology-wise, a cgroup can be in an invalid state. Please consider
|
|
the following topology::
|
|
|
|
A (threaded domain) - B (threaded) - C (domain, just created)
|
|
|
|
C is created as a domain but isn't connected to a parent which can
|
|
host child domains. C can't be used until it is turned into a
|
|
threaded cgroup. "cgroup.type" file will report "domain (invalid)" in
|
|
these cases. Operations which fail due to invalid topology use
|
|
EOPNOTSUPP as the errno.
|
|
|
|
A domain cgroup is turned into a threaded domain when one of its child
|
|
cgroup becomes threaded or threaded controllers are enabled in the
|
|
"cgroup.subtree_control" file while there are processes in the cgroup.
|
|
A threaded domain reverts to a normal domain when the conditions
|
|
clear.
|
|
|
|
When read, "cgroup.threads" contains the list of the thread IDs of all
|
|
threads in the cgroup. Except that the operations are per-thread
|
|
instead of per-process, "cgroup.threads" has the same format and
|
|
behaves the same way as "cgroup.procs". While "cgroup.threads" can be
|
|
written to in any cgroup, as it can only move threads inside the same
|
|
threaded domain, its operations are confined inside each threaded
|
|
subtree.
|
|
|
|
The threaded domain cgroup serves as the resource domain for the whole
|
|
subtree, and, while the threads can be scattered across the subtree,
|
|
all the processes are considered to be in the threaded domain cgroup.
|
|
"cgroup.procs" in a threaded domain cgroup contains the PIDs of all
|
|
processes in the subtree and is not readable in the subtree proper.
|
|
However, "cgroup.procs" can be written to from anywhere in the subtree
|
|
to migrate all threads of the matching process to the cgroup.
|
|
|
|
Only threaded controllers can be enabled in a threaded subtree. When
|
|
a threaded controller is enabled inside a threaded subtree, it only
|
|
accounts for and controls resource consumptions associated with the
|
|
threads in the cgroup and its descendants. All consumptions which
|
|
aren't tied to a specific thread belong to the threaded domain cgroup.
|
|
|
|
Because a threaded subtree is exempt from no internal process
|
|
constraint, a threaded controller must be able to handle competition
|
|
between threads in a non-leaf cgroup and its child cgroups. Each
|
|
threaded controller defines how such competitions are handled.
|
|
|
|
Currently, the following controllers are threaded and can be enabled
|
|
in a threaded cgroup::
|
|
|
|
- cpu
|
|
- cpuset
|
|
- perf_event
|
|
- pids
|
|
|
|
[Un]populated Notification
|
|
--------------------------
|
|
|
|
Each non-root cgroup has a "cgroup.events" file which contains
|
|
"populated" field indicating whether the cgroup's sub-hierarchy has
|
|
live processes in it. Its value is 0 if there is no live process in
|
|
the cgroup and its descendants; otherwise, 1. poll and [id]notify
|
|
events are triggered when the value changes. This can be used, for
|
|
example, to start a clean-up operation after all processes of a given
|
|
sub-hierarchy have exited. The populated state updates and
|
|
notifications are recursive. Consider the following sub-hierarchy
|
|
where the numbers in the parentheses represent the numbers of processes
|
|
in each cgroup::
|
|
|
|
A(4) - B(0) - C(1)
|
|
\ D(0)
|
|
|
|
A, B and C's "populated" fields would be 1 while D's 0. After the one
|
|
process in C exits, B and C's "populated" fields would flip to "0" and
|
|
file modified events will be generated on the "cgroup.events" files of
|
|
both cgroups.
|
|
|
|
|
|
Controlling Controllers
|
|
-----------------------
|
|
|
|
Enabling and Disabling
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Each cgroup has a "cgroup.controllers" file which lists all
|
|
controllers available for the cgroup to enable::
|
|
|
|
# cat cgroup.controllers
|
|
cpu io memory
|
|
|
|
No controller is enabled by default. Controllers can be enabled and
|
|
disabled by writing to the "cgroup.subtree_control" file::
|
|
|
|
# echo "+cpu +memory -io" > cgroup.subtree_control
|
|
|
|
Only controllers which are listed in "cgroup.controllers" can be
|
|
enabled. When multiple operations are specified as above, either they
|
|
all succeed or fail. If multiple operations on the same controller
|
|
are specified, the last one is effective.
|
|
|
|
Enabling a controller in a cgroup indicates that the distribution of
|
|
the target resource across its immediate children will be controlled.
|
|
Consider the following sub-hierarchy. The enabled controllers are
|
|
listed in parentheses::
|
|
|
|
A(cpu,memory) - B(memory) - C()
|
|
\ D()
|
|
|
|
As A has "cpu" and "memory" enabled, A will control the distribution
|
|
of CPU cycles and memory to its children, in this case, B. As B has
|
|
"memory" enabled but not "CPU", C and D will compete freely on CPU
|
|
cycles but their division of memory available to B will be controlled.
|
|
|
|
As a controller regulates the distribution of the target resource to
|
|
the cgroup's children, enabling it creates the controller's interface
|
|
files in the child cgroups. In the above example, enabling "cpu" on B
|
|
would create the "cpu." prefixed controller interface files in C and
|
|
D. Likewise, disabling "memory" from B would remove the "memory."
|
|
prefixed controller interface files from C and D. This means that the
|
|
controller interface files - anything which doesn't start with
|
|
"cgroup." are owned by the parent rather than the cgroup itself.
|
|
|
|
|
|
Top-down Constraint
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
Resources are distributed top-down and a cgroup can further distribute
|
|
a resource only if the resource has been distributed to it from the
|
|
parent. This means that all non-root "cgroup.subtree_control" files
|
|
can only contain controllers which are enabled in the parent's
|
|
"cgroup.subtree_control" file. A controller can be enabled only if
|
|
the parent has the controller enabled and a controller can't be
|
|
disabled if one or more children have it enabled.
|
|
|
|
|
|
No Internal Process Constraint
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Non-root cgroups can distribute domain resources to their children
|
|
only when they don't have any processes of their own. In other words,
|
|
only domain cgroups which don't contain any processes can have domain
|
|
controllers enabled in their "cgroup.subtree_control" files.
|
|
|
|
This guarantees that, when a domain controller is looking at the part
|
|
of the hierarchy which has it enabled, processes are always only on
|
|
the leaves. This rules out situations where child cgroups compete
|
|
against internal processes of the parent.
|
|
|
|
The root cgroup is exempt from this restriction. Root contains
|
|
processes and anonymous resource consumption which can't be associated
|
|
with any other cgroups and requires special treatment from most
|
|
controllers. How resource consumption in the root cgroup is governed
|
|
is up to each controller (for more information on this topic please
|
|
refer to the Non-normative information section in the Controllers
|
|
chapter).
|
|
|
|
Note that the restriction doesn't get in the way if there is no
|
|
enabled controller in the cgroup's "cgroup.subtree_control". This is
|
|
important as otherwise it wouldn't be possible to create children of a
|
|
populated cgroup. To control resource distribution of a cgroup, the
|
|
cgroup must create children and transfer all its processes to the
|
|
children before enabling controllers in its "cgroup.subtree_control"
|
|
file.
|
|
|
|
|
|
Delegation
|
|
----------
|
|
|
|
Model of Delegation
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
A cgroup can be delegated in two ways. First, to a less privileged
|
|
user by granting write access of the directory and its "cgroup.procs",
|
|
"cgroup.threads" and "cgroup.subtree_control" files to the user.
|
|
Second, if the "nsdelegate" mount option is set, automatically to a
|
|
cgroup namespace on namespace creation.
|
|
|
|
Because the resource control interface files in a given directory
|
|
control the distribution of the parent's resources, the delegatee
|
|
shouldn't be allowed to write to them. For the first method, this is
|
|
achieved by not granting access to these files. For the second, the
|
|
kernel rejects writes to all files other than "cgroup.procs" and
|
|
"cgroup.subtree_control" on a namespace root from inside the
|
|
namespace.
|
|
|
|
The end results are equivalent for both delegation types. Once
|
|
delegated, the user can build sub-hierarchy under the directory,
|
|
organize processes inside it as it sees fit and further distribute the
|
|
resources it received from the parent. The limits and other settings
|
|
of all resource controllers are hierarchical and regardless of what
|
|
happens in the delegated sub-hierarchy, nothing can escape the
|
|
resource restrictions imposed by the parent.
|
|
|
|
Currently, cgroup doesn't impose any restrictions on the number of
|
|
cgroups in or nesting depth of a delegated sub-hierarchy; however,
|
|
this may be limited explicitly in the future.
|
|
|
|
|
|
Delegation Containment
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A delegated sub-hierarchy is contained in the sense that processes
|
|
can't be moved into or out of the sub-hierarchy by the delegatee.
|
|
|
|
For delegations to a less privileged user, this is achieved by
|
|
requiring the following conditions for a process with a non-root euid
|
|
to migrate a target process into a cgroup by writing its PID to the
|
|
"cgroup.procs" file.
|
|
|
|
- The writer must have write access to the "cgroup.procs" file.
|
|
|
|
- The writer must have write access to the "cgroup.procs" file of the
|
|
common ancestor of the source and destination cgroups.
|
|
|
|
The above two constraints ensure that while a delegatee may migrate
|
|
processes around freely in the delegated sub-hierarchy it can't pull
|
|
in from or push out to outside the sub-hierarchy.
|
|
|
|
For an example, let's assume cgroups C0 and C1 have been delegated to
|
|
user U0 who created C00, C01 under C0 and C10 under C1 as follows and
|
|
all processes under C0 and C1 belong to U0::
|
|
|
|
~~~~~~~~~~~~~ - C0 - C00
|
|
~ cgroup ~ \ C01
|
|
~ hierarchy ~
|
|
~~~~~~~~~~~~~ - C1 - C10
|
|
|
|
Let's also say U0 wants to write the PID of a process which is
|
|
currently in C10 into "C00/cgroup.procs". U0 has write access to the
|
|
file; however, the common ancestor of the source cgroup C10 and the
|
|
destination cgroup C00 is above the points of delegation and U0 would
|
|
not have write access to its "cgroup.procs" files and thus the write
|
|
will be denied with -EACCES.
|
|
|
|
For delegations to namespaces, containment is achieved by requiring
|
|
that both the source and destination cgroups are reachable from the
|
|
namespace of the process which is attempting the migration. If either
|
|
is not reachable, the migration is rejected with -ENOENT.
|
|
|
|
|
|
Guidelines
|
|
----------
|
|
|
|
Organize Once and Control
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Migrating a process across cgroups is a relatively expensive operation
|
|
and stateful resources such as memory are not moved together with the
|
|
process. This is an explicit design decision as there often exist
|
|
inherent trade-offs between migration and various hot paths in terms
|
|
of synchronization cost.
|
|
|
|
As such, migrating processes across cgroups frequently as a means to
|
|
apply different resource restrictions is discouraged. A workload
|
|
should be assigned to a cgroup according to the system's logical and
|
|
resource structure once on start-up. Dynamic adjustments to resource
|
|
distribution can be made by changing controller configuration through
|
|
the interface files.
|
|
|
|
|
|
Avoid Name Collisions
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Interface files for a cgroup and its children cgroups occupy the same
|
|
directory and it is possible to create children cgroups which collide
|
|
with interface files.
|
|
|
|
All cgroup core interface files are prefixed with "cgroup." and each
|
|
controller's interface files are prefixed with the controller name and
|
|
a dot. A controller's name is composed of lower case alphabets and
|
|
'_'s but never begins with an '_' so it can be used as the prefix
|
|
character for collision avoidance. Also, interface file names won't
|
|
start or end with terms which are often used in categorizing workloads
|
|
such as job, service, slice, unit or workload.
|
|
|
|
cgroup doesn't do anything to prevent name collisions and it's the
|
|
user's responsibility to avoid them.
|
|
|
|
|
|
Resource Distribution Models
|
|
============================
|
|
|
|
cgroup controllers implement several resource distribution schemes
|
|
depending on the resource type and expected use cases. This section
|
|
describes major schemes in use along with their expected behaviors.
|
|
|
|
|
|
Weights
|
|
-------
|
|
|
|
A parent's resource is distributed by adding up the weights of all
|
|
active children and giving each the fraction matching the ratio of its
|
|
weight against the sum. As only children which can make use of the
|
|
resource at the moment participate in the distribution, this is
|
|
work-conserving. Due to the dynamic nature, this model is usually
|
|
used for stateless resources.
|
|
|
|
All weights are in the range [1, 10000] with the default at 100. This
|
|
allows symmetric multiplicative biases in both directions at fine
|
|
enough granularity while staying in the intuitive range.
|
|
|
|
As long as the weight is in range, all configuration combinations are
|
|
valid and there is no reason to reject configuration changes or
|
|
process migrations.
|
|
|
|
"cpu.weight" proportionally distributes CPU cycles to active children
|
|
and is an example of this type.
|
|
|
|
|
|
Limits
|
|
------
|
|
|
|
A child can only consume up to the configured amount of the resource.
|
|
Limits can be over-committed - the sum of the limits of children can
|
|
exceed the amount of resource available to the parent.
|
|
|
|
Limits are in the range [0, max] and defaults to "max", which is noop.
|
|
|
|
As limits can be over-committed, all configuration combinations are
|
|
valid and there is no reason to reject configuration changes or
|
|
process migrations.
|
|
|
|
"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
|
|
on an IO device and is an example of this type.
|
|
|
|
|
|
Protections
|
|
-----------
|
|
|
|
A cgroup is protected up to the configured amount of the resource
|
|
as long as the usages of all its ancestors are under their
|
|
protected levels. Protections can be hard guarantees or best effort
|
|
soft boundaries. Protections can also be over-committed in which case
|
|
only up to the amount available to the parent is protected among
|
|
children.
|
|
|
|
Protections are in the range [0, max] and defaults to 0, which is
|
|
noop.
|
|
|
|
As protections can be over-committed, all configuration combinations
|
|
are valid and there is no reason to reject configuration changes or
|
|
process migrations.
|
|
|
|
"memory.low" implements best-effort memory protection and is an
|
|
example of this type.
|
|
|
|
|
|
Allocations
|
|
-----------
|
|
|
|
A cgroup is exclusively allocated a certain amount of a finite
|
|
resource. Allocations can't be over-committed - the sum of the
|
|
allocations of children can not exceed the amount of resource
|
|
available to the parent.
|
|
|
|
Allocations are in the range [0, max] and defaults to 0, which is no
|
|
resource.
|
|
|
|
As allocations can't be over-committed, some configuration
|
|
combinations are invalid and should be rejected. Also, if the
|
|
resource is mandatory for execution of processes, process migrations
|
|
may be rejected.
|
|
|
|
"cpu.rt.max" hard-allocates realtime slices and is an example of this
|
|
type.
|
|
|
|
|
|
Interface Files
|
|
===============
|
|
|
|
Format
|
|
------
|
|
|
|
All interface files should be in one of the following formats whenever
|
|
possible::
|
|
|
|
New-line separated values
|
|
(when only one value can be written at once)
|
|
|
|
VAL0\n
|
|
VAL1\n
|
|
...
|
|
|
|
Space separated values
|
|
(when read-only or multiple values can be written at once)
|
|
|
|
VAL0 VAL1 ...\n
|
|
|
|
Flat keyed
|
|
|
|
KEY0 VAL0\n
|
|
KEY1 VAL1\n
|
|
...
|
|
|
|
Nested keyed
|
|
|
|
KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
|
|
KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
|
|
...
|
|
|
|
For a writable file, the format for writing should generally match
|
|
reading; however, controllers may allow omitting later fields or
|
|
implement restricted shortcuts for most common use cases.
|
|
|
|
For both flat and nested keyed files, only the values for a single key
|
|
can be written at a time. For nested keyed files, the sub key pairs
|
|
may be specified in any order and not all pairs have to be specified.
|
|
|
|
|
|
Conventions
|
|
-----------
|
|
|
|
- Settings for a single feature should be contained in a single file.
|
|
|
|
- The root cgroup should be exempt from resource control and thus
|
|
shouldn't have resource control interface files.
|
|
|
|
- The default time unit is microseconds. If a different unit is ever
|
|
used, an explicit unit suffix must be present.
|
|
|
|
- A parts-per quantity should use a percentage decimal with at least
|
|
two digit fractional part - e.g. 13.40.
|
|
|
|
- If a controller implements weight based resource distribution, its
|
|
interface file should be named "weight" and have the range [1,
|
|
10000] with 100 as the default. The values are chosen to allow
|
|
enough and symmetric bias in both directions while keeping it
|
|
intuitive (the default is 100%).
|
|
|
|
- If a controller implements an absolute resource guarantee and/or
|
|
limit, the interface files should be named "min" and "max"
|
|
respectively. If a controller implements best effort resource
|
|
guarantee and/or limit, the interface files should be named "low"
|
|
and "high" respectively.
|
|
|
|
In the above four control files, the special token "max" should be
|
|
used to represent upward infinity for both reading and writing.
|
|
|
|
- If a setting has a configurable default value and keyed specific
|
|
overrides, the default entry should be keyed with "default" and
|
|
appear as the first entry in the file.
|
|
|
|
The default value can be updated by writing either "default $VAL" or
|
|
"$VAL".
|
|
|
|
When writing to update a specific override, "default" can be used as
|
|
the value to indicate removal of the override. Override entries
|
|
with "default" as the value must not appear when read.
|
|
|
|
For example, a setting which is keyed by major:minor device numbers
|
|
with integer values may look like the following::
|
|
|
|
# cat cgroup-example-interface-file
|
|
default 150
|
|
8:0 300
|
|
|
|
The default value can be updated by::
|
|
|
|
# echo 125 > cgroup-example-interface-file
|
|
|
|
or::
|
|
|
|
# echo "default 125" > cgroup-example-interface-file
|
|
|
|
An override can be set by::
|
|
|
|
# echo "8:16 170" > cgroup-example-interface-file
|
|
|
|
and cleared by::
|
|
|
|
# echo "8:0 default" > cgroup-example-interface-file
|
|
# cat cgroup-example-interface-file
|
|
default 125
|
|
8:16 170
|
|
|
|
- For events which are not very high frequency, an interface file
|
|
"events" should be created which lists event key value pairs.
|
|
Whenever a notifiable event happens, file modified event should be
|
|
generated on the file.
|
|
|
|
|
|
Core Interface Files
|
|
--------------------
|
|
|
|
All cgroup core files are prefixed with "cgroup."
|
|
|
|
cgroup.type
|
|
A read-write single value file which exists on non-root
|
|
cgroups.
|
|
|
|
When read, it indicates the current type of the cgroup, which
|
|
can be one of the following values.
|
|
|
|
- "domain" : A normal valid domain cgroup.
|
|
|
|
- "domain threaded" : A threaded domain cgroup which is
|
|
serving as the root of a threaded subtree.
|
|
|
|
- "domain invalid" : A cgroup which is in an invalid state.
|
|
It can't be populated or have controllers enabled. It may
|
|
be allowed to become a threaded cgroup.
|
|
|
|
- "threaded" : A threaded cgroup which is a member of a
|
|
threaded subtree.
|
|
|
|
A cgroup can be turned into a threaded cgroup by writing
|
|
"threaded" to this file.
|
|
|
|
cgroup.procs
|
|
A read-write new-line separated values file which exists on
|
|
all cgroups.
|
|
|
|
When read, it lists the PIDs of all processes which belong to
|
|
the cgroup one-per-line. The PIDs are not ordered and the
|
|
same PID may show up more than once if the process got moved
|
|
to another cgroup and then back or the PID got recycled while
|
|
reading.
|
|
|
|
A PID can be written to migrate the process associated with
|
|
the PID to the cgroup. The writer should match all of the
|
|
following conditions.
|
|
|
|
- It must have write access to the "cgroup.procs" file.
|
|
|
|
- It must have write access to the "cgroup.procs" file of the
|
|
common ancestor of the source and destination cgroups.
|
|
|
|
When delegating a sub-hierarchy, write access to this file
|
|
should be granted along with the containing directory.
|
|
|
|
In a threaded cgroup, reading this file fails with EOPNOTSUPP
|
|
as all the processes belong to the thread root. Writing is
|
|
supported and moves every thread of the process to the cgroup.
|
|
|
|
cgroup.threads
|
|
A read-write new-line separated values file which exists on
|
|
all cgroups.
|
|
|
|
When read, it lists the TIDs of all threads which belong to
|
|
the cgroup one-per-line. The TIDs are not ordered and the
|
|
same TID may show up more than once if the thread got moved to
|
|
another cgroup and then back or the TID got recycled while
|
|
reading.
|
|
|
|
A TID can be written to migrate the thread associated with the
|
|
TID to the cgroup. The writer should match all of the
|
|
following conditions.
|
|
|
|
- It must have write access to the "cgroup.threads" file.
|
|
|
|
- The cgroup that the thread is currently in must be in the
|
|
same resource domain as the destination cgroup.
|
|
|
|
- It must have write access to the "cgroup.procs" file of the
|
|
common ancestor of the source and destination cgroups.
|
|
|
|
When delegating a sub-hierarchy, write access to this file
|
|
should be granted along with the containing directory.
|
|
|
|
cgroup.controllers
|
|
A read-only space separated values file which exists on all
|
|
cgroups.
|
|
|
|
It shows space separated list of all controllers available to
|
|
the cgroup. The controllers are not ordered.
|
|
|
|
cgroup.subtree_control
|
|
A read-write space separated values file which exists on all
|
|
cgroups. Starts out empty.
|
|
|
|
When read, it shows space separated list of the controllers
|
|
which are enabled to control resource distribution from the
|
|
cgroup to its children.
|
|
|
|
Space separated list of controllers prefixed with '+' or '-'
|
|
can be written to enable or disable controllers. A controller
|
|
name prefixed with '+' enables the controller and '-'
|
|
disables. If a controller appears more than once on the list,
|
|
the last one is effective. When multiple enable and disable
|
|
operations are specified, either all succeed or all fail.
|
|
|
|
cgroup.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
The following entries are defined. Unless specified
|
|
otherwise, a value change in this file generates a file
|
|
modified event.
|
|
|
|
populated
|
|
1 if the cgroup or its descendants contains any live
|
|
processes; otherwise, 0.
|
|
frozen
|
|
1 if the cgroup is frozen; otherwise, 0.
|
|
|
|
cgroup.max.descendants
|
|
A read-write single value files. The default is "max".
|
|
|
|
Maximum allowed number of descent cgroups.
|
|
If the actual number of descendants is equal or larger,
|
|
an attempt to create a new cgroup in the hierarchy will fail.
|
|
|
|
cgroup.max.depth
|
|
A read-write single value files. The default is "max".
|
|
|
|
Maximum allowed descent depth below the current cgroup.
|
|
If the actual descent depth is equal or larger,
|
|
an attempt to create a new child cgroup will fail.
|
|
|
|
cgroup.stat
|
|
A read-only flat-keyed file with the following entries:
|
|
|
|
nr_descendants
|
|
Total number of visible descendant cgroups.
|
|
|
|
nr_dying_descendants
|
|
Total number of dying descendant cgroups. A cgroup becomes
|
|
dying after being deleted by a user. The cgroup will remain
|
|
in dying state for some time undefined time (which can depend
|
|
on system load) before being completely destroyed.
|
|
|
|
A process can't enter a dying cgroup under any circumstances,
|
|
a dying cgroup can't revive.
|
|
|
|
A dying cgroup can consume system resources not exceeding
|
|
limits, which were active at the moment of cgroup deletion.
|
|
|
|
nr_subsys_<cgroup_subsys>
|
|
Total number of live cgroup subsystems (e.g memory
|
|
cgroup) at and beneath the current cgroup.
|
|
|
|
nr_dying_subsys_<cgroup_subsys>
|
|
Total number of dying cgroup subsystems (e.g. memory
|
|
cgroup) at and beneath the current cgroup.
|
|
|
|
cgroup.freeze
|
|
A read-write single value file which exists on non-root cgroups.
|
|
Allowed values are "0" and "1". The default is "0".
|
|
|
|
Writing "1" to the file causes freezing of the cgroup and all
|
|
descendant cgroups. This means that all belonging processes will
|
|
be stopped and will not run until the cgroup will be explicitly
|
|
unfrozen. Freezing of the cgroup may take some time; when this action
|
|
is completed, the "frozen" value in the cgroup.events control file
|
|
will be updated to "1" and the corresponding notification will be
|
|
issued.
|
|
|
|
A cgroup can be frozen either by its own settings, or by settings
|
|
of any ancestor cgroups. If any of ancestor cgroups is frozen, the
|
|
cgroup will remain frozen.
|
|
|
|
Processes in the frozen cgroup can be killed by a fatal signal.
|
|
They also can enter and leave a frozen cgroup: either by an explicit
|
|
move by a user, or if freezing of the cgroup races with fork().
|
|
If a process is moved to a frozen cgroup, it stops. If a process is
|
|
moved out of a frozen cgroup, it becomes running.
|
|
|
|
Frozen status of a cgroup doesn't affect any cgroup tree operations:
|
|
it's possible to delete a frozen (and empty) cgroup, as well as
|
|
create new sub-cgroups.
|
|
|
|
cgroup.kill
|
|
A write-only single value file which exists in non-root cgroups.
|
|
The only allowed value is "1".
|
|
|
|
Writing "1" to the file causes the cgroup and all descendant cgroups to
|
|
be killed. This means that all processes located in the affected cgroup
|
|
tree will be killed via SIGKILL.
|
|
|
|
Killing a cgroup tree will deal with concurrent forks appropriately and
|
|
is protected against migrations.
|
|
|
|
In a threaded cgroup, writing this file fails with EOPNOTSUPP as
|
|
killing cgroups is a process directed operation, i.e. it affects
|
|
the whole thread-group.
|
|
|
|
cgroup.pressure
|
|
A read-write single value file that allowed values are "0" and "1".
|
|
The default is "1".
|
|
|
|
Writing "0" to the file will disable the cgroup PSI accounting.
|
|
Writing "1" to the file will re-enable the cgroup PSI accounting.
|
|
|
|
This control attribute is not hierarchical, so disable or enable PSI
|
|
accounting in a cgroup does not affect PSI accounting in descendants
|
|
and doesn't need pass enablement via ancestors from root.
|
|
|
|
The reason this control attribute exists is that PSI accounts stalls for
|
|
each cgroup separately and aggregates it at each level of the hierarchy.
|
|
This may cause non-negligible overhead for some workloads when under
|
|
deep level of the hierarchy, in which case this control attribute can
|
|
be used to disable PSI accounting in the non-leaf cgroups.
|
|
|
|
irq.pressure
|
|
A read-write nested-keyed file.
|
|
|
|
Shows pressure stall information for IRQ/SOFTIRQ. See
|
|
:ref:`Documentation/accounting/psi.rst <psi>` for details.
|
|
|
|
Controllers
|
|
===========
|
|
|
|
.. _cgroup-v2-cpu:
|
|
|
|
CPU
|
|
---
|
|
|
|
The "cpu" controllers regulates distribution of CPU cycles. This
|
|
controller implements weight and absolute bandwidth limit models for
|
|
normal scheduling policy and absolute bandwidth allocation model for
|
|
realtime scheduling policy.
|
|
|
|
In all the above models, cycles distribution is defined only on a temporal
|
|
base and it does not account for the frequency at which tasks are executed.
|
|
The (optional) utilization clamping support allows to hint the schedutil
|
|
cpufreq governor about the minimum desired frequency which should always be
|
|
provided by a CPU, as well as the maximum desired frequency, which should not
|
|
be exceeded by a CPU.
|
|
|
|
WARNING: cgroup2 doesn't yet support control of realtime processes and
|
|
the cpu controller can only be enabled when all RT processes are in
|
|
the root cgroup. Be aware that system management software may already
|
|
have placed RT processes into nonroot cgroups during the system boot
|
|
process, and these processes may need to be moved to the root cgroup
|
|
before the cpu controller can be enabled.
|
|
|
|
|
|
CPU Interface Files
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
All time durations are in microseconds.
|
|
|
|
cpu.stat
|
|
A read-only flat-keyed file.
|
|
This file exists whether the controller is enabled or not.
|
|
|
|
It always reports the following three stats:
|
|
|
|
- usage_usec
|
|
- user_usec
|
|
- system_usec
|
|
|
|
and the following three when the controller is enabled:
|
|
|
|
- nr_periods
|
|
- nr_throttled
|
|
- throttled_usec
|
|
- nr_bursts
|
|
- burst_usec
|
|
|
|
cpu.weight
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "100".
|
|
|
|
The weight in the range [1, 10000].
|
|
|
|
cpu.weight.nice
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
The nice value is in the range [-20, 19].
|
|
|
|
This interface file is an alternative interface for
|
|
"cpu.weight" and allows reading and setting weight using the
|
|
same values used by nice(2). Because the range is smaller and
|
|
granularity is coarser for the nice values, the read value is
|
|
the closest approximation of the current weight.
|
|
|
|
cpu.max
|
|
A read-write two value file which exists on non-root cgroups.
|
|
The default is "max 100000".
|
|
|
|
The maximum bandwidth limit. It's in the following format::
|
|
|
|
$MAX $PERIOD
|
|
|
|
which indicates that the group may consume up to $MAX in each
|
|
$PERIOD duration. "max" for $MAX indicates no limit. If only
|
|
one number is written, $MAX is updated.
|
|
|
|
cpu.max.burst
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
The burst in the range [0, $MAX].
|
|
|
|
cpu.pressure
|
|
A read-write nested-keyed file.
|
|
|
|
Shows pressure stall information for CPU. See
|
|
:ref:`Documentation/accounting/psi.rst <psi>` for details.
|
|
|
|
cpu.uclamp.min
|
|
A read-write single value file which exists on non-root cgroups.
|
|
The default is "0", i.e. no utilization boosting.
|
|
|
|
The requested minimum utilization (protection) as a percentage
|
|
rational number, e.g. 12.34 for 12.34%.
|
|
|
|
This interface allows reading and setting minimum utilization clamp
|
|
values similar to the sched_setattr(2). This minimum utilization
|
|
value is used to clamp the task specific minimum utilization clamp.
|
|
|
|
The requested minimum utilization (protection) is always capped by
|
|
the current value for the maximum utilization (limit), i.e.
|
|
`cpu.uclamp.max`.
|
|
|
|
cpu.uclamp.max
|
|
A read-write single value file which exists on non-root cgroups.
|
|
The default is "max". i.e. no utilization capping
|
|
|
|
The requested maximum utilization (limit) as a percentage rational
|
|
number, e.g. 98.76 for 98.76%.
|
|
|
|
This interface allows reading and setting maximum utilization clamp
|
|
values similar to the sched_setattr(2). This maximum utilization
|
|
value is used to clamp the task specific maximum utilization clamp.
|
|
|
|
|
|
|
|
Memory
|
|
------
|
|
|
|
The "memory" controller regulates distribution of memory. Memory is
|
|
stateful and implements both limit and protection models. Due to the
|
|
intertwining between memory usage and reclaim pressure and the
|
|
stateful nature of memory, the distribution model is relatively
|
|
complex.
|
|
|
|
While not completely water-tight, all major memory usages by a given
|
|
cgroup are tracked so that the total memory consumption can be
|
|
accounted and controlled to a reasonable extent. Currently, the
|
|
following types of memory usages are tracked.
|
|
|
|
- Userland memory - page cache and anonymous memory.
|
|
|
|
- Kernel data structures such as dentries and inodes.
|
|
|
|
- TCP socket buffers.
|
|
|
|
The above list may expand in the future for better coverage.
|
|
|
|
|
|
Memory Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
All memory amounts are in bytes. If a value which is not aligned to
|
|
PAGE_SIZE is written, the value may be rounded up to the closest
|
|
PAGE_SIZE multiple when read back.
|
|
|
|
memory.current
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The total amount of memory currently being used by the cgroup
|
|
and its descendants.
|
|
|
|
memory.min
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
Hard memory protection. If the memory usage of a cgroup
|
|
is within its effective min boundary, the cgroup's memory
|
|
won't be reclaimed under any conditions. If there is no
|
|
unprotected reclaimable memory available, OOM killer
|
|
is invoked. Above the effective min boundary (or
|
|
effective low boundary if it is higher), pages are reclaimed
|
|
proportionally to the overage, reducing reclaim pressure for
|
|
smaller overages.
|
|
|
|
Effective min boundary is limited by memory.min values of
|
|
all ancestor cgroups. If there is memory.min overcommitment
|
|
(child cgroup or cgroups are requiring more protected memory
|
|
than parent will allow), then each child cgroup will get
|
|
the part of parent's protection proportional to its
|
|
actual memory usage below memory.min.
|
|
|
|
Putting more memory than generally available under this
|
|
protection is discouraged and may lead to constant OOMs.
|
|
|
|
If a memory cgroup is not populated with processes,
|
|
its memory.min is ignored.
|
|
|
|
memory.low
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
Best-effort memory protection. If the memory usage of a
|
|
cgroup is within its effective low boundary, the cgroup's
|
|
memory won't be reclaimed unless there is no reclaimable
|
|
memory available in unprotected cgroups.
|
|
Above the effective low boundary (or
|
|
effective min boundary if it is higher), pages are reclaimed
|
|
proportionally to the overage, reducing reclaim pressure for
|
|
smaller overages.
|
|
|
|
Effective low boundary is limited by memory.low values of
|
|
all ancestor cgroups. If there is memory.low overcommitment
|
|
(child cgroup or cgroups are requiring more protected memory
|
|
than parent will allow), then each child cgroup will get
|
|
the part of parent's protection proportional to its
|
|
actual memory usage below memory.low.
|
|
|
|
Putting more memory than generally available under this
|
|
protection is discouraged.
|
|
|
|
memory.high
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Memory usage throttle limit. This is the main mechanism to
|
|
control memory usage of a cgroup. If a cgroup's usage goes
|
|
over the high boundary, the processes of the cgroup are
|
|
throttled and put under heavy reclaim pressure.
|
|
|
|
Going over the high limit never invokes the OOM killer and
|
|
under extreme conditions the limit may be breached.
|
|
|
|
memory.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Memory usage hard limit. This is the final protection
|
|
mechanism. If a cgroup's memory usage reaches this limit and
|
|
can't be reduced, the OOM killer is invoked in the cgroup.
|
|
Under certain circumstances, the usage may go over the limit
|
|
temporarily.
|
|
|
|
In default configuration regular 0-order allocations always
|
|
succeed unless OOM killer chooses current task as a victim.
|
|
|
|
Some kinds of allocations don't invoke the OOM killer.
|
|
Caller could retry them differently, return into userspace
|
|
as -ENOMEM or silently ignore in cases like disk readahead.
|
|
|
|
This is the ultimate protection mechanism. As long as the
|
|
high limit is used and monitored properly, this limit's
|
|
utility is limited to providing the final safety net.
|
|
|
|
memory.reclaim
|
|
A write-only nested-keyed file which exists for all cgroups.
|
|
|
|
This is a simple interface to trigger memory reclaim in the
|
|
target cgroup.
|
|
|
|
This file accepts a single key, the number of bytes to reclaim.
|
|
No nested keys are currently supported.
|
|
|
|
Example::
|
|
|
|
echo "1G" > memory.reclaim
|
|
|
|
The interface can be later extended with nested keys to
|
|
configure the reclaim behavior. For example, specify the
|
|
type of memory to reclaim from (anon, file, ..).
|
|
|
|
Please note that the kernel can over or under reclaim from
|
|
the target cgroup. If less bytes are reclaimed than the
|
|
specified amount, -EAGAIN is returned.
|
|
|
|
Please note that the proactive reclaim (triggered by this
|
|
interface) is not meant to indicate memory pressure on the
|
|
memory cgroup. Therefore socket memory balancing triggered by
|
|
the memory reclaim normally is not exercised in this case.
|
|
This means that the networking layer will not adapt based on
|
|
reclaim induced by memory.reclaim.
|
|
|
|
memory.peak
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The max memory usage recorded for the cgroup and its
|
|
descendants since the creation of the cgroup.
|
|
|
|
memory.oom.group
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default value is "0".
|
|
|
|
Determines whether the cgroup should be treated as
|
|
an indivisible workload by the OOM killer. If set,
|
|
all tasks belonging to the cgroup or to its descendants
|
|
(if the memory cgroup is not a leaf cgroup) are killed
|
|
together or not at all. This can be used to avoid
|
|
partial kills to guarantee workload integrity.
|
|
|
|
Tasks with the OOM protection (oom_score_adj set to -1000)
|
|
are treated as an exception and are never killed.
|
|
|
|
If the OOM killer is invoked in a cgroup, it's not going
|
|
to kill any tasks outside of this cgroup, regardless
|
|
memory.oom.group values of ancestor cgroups.
|
|
|
|
memory.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
The following entries are defined. Unless specified
|
|
otherwise, a value change in this file generates a file
|
|
modified event.
|
|
|
|
Note that all fields in this file are hierarchical and the
|
|
file modified event can be generated due to an event down the
|
|
hierarchy. For for the local events at the cgroup level see
|
|
memory.events.local.
|
|
|
|
low
|
|
The number of times the cgroup is reclaimed due to
|
|
high memory pressure even though its usage is under
|
|
the low boundary. This usually indicates that the low
|
|
boundary is over-committed.
|
|
|
|
high
|
|
The number of times processes of the cgroup are
|
|
throttled and routed to perform direct memory reclaim
|
|
because the high memory boundary was exceeded. For a
|
|
cgroup whose memory usage is capped by the high limit
|
|
rather than global memory pressure, this event's
|
|
occurrences are expected.
|
|
|
|
max
|
|
The number of times the cgroup's memory usage was
|
|
about to go over the max boundary. If direct reclaim
|
|
fails to bring it down, the cgroup goes to OOM state.
|
|
|
|
oom
|
|
The number of time the cgroup's memory usage was
|
|
reached the limit and allocation was about to fail.
|
|
|
|
This event is not raised if the OOM killer is not
|
|
considered as an option, e.g. for failed high-order
|
|
allocations or if caller asked to not retry attempts.
|
|
|
|
oom_kill
|
|
The number of processes belonging to this cgroup
|
|
killed by any kind of OOM killer.
|
|
|
|
oom_group_kill
|
|
The number of times a group OOM has occurred.
|
|
|
|
memory.events.local
|
|
Similar to memory.events but the fields in the file are local
|
|
to the cgroup i.e. not hierarchical. The file modified event
|
|
generated on this file reflects only the local events.
|
|
|
|
memory.stat
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
|
|
This breaks down the cgroup's memory footprint into different
|
|
types of memory, type-specific details, and other information
|
|
on the state and past events of the memory management system.
|
|
|
|
All memory amounts are in bytes.
|
|
|
|
The entries are ordered to be human readable, and new entries
|
|
can show up in the middle. Don't rely on items remaining in a
|
|
fixed position; use the keys to look up specific values!
|
|
|
|
If the entry has no per-node counter (or not show in the
|
|
memory.numa_stat). We use 'npn' (non-per-node) as the tag
|
|
to indicate that it will not show in the memory.numa_stat.
|
|
|
|
anon
|
|
Amount of memory used in anonymous mappings such as
|
|
brk(), sbrk(), and mmap(MAP_ANONYMOUS)
|
|
|
|
file
|
|
Amount of memory used to cache filesystem data,
|
|
including tmpfs and shared memory.
|
|
|
|
kernel (npn)
|
|
Amount of total kernel memory, including
|
|
(kernel_stack, pagetables, percpu, vmalloc, slab) in
|
|
addition to other kernel memory use cases.
|
|
|
|
kernel_stack
|
|
Amount of memory allocated to kernel stacks.
|
|
|
|
pagetables
|
|
Amount of memory allocated for page tables.
|
|
|
|
sec_pagetables
|
|
Amount of memory allocated for secondary page tables,
|
|
this currently includes KVM mmu allocations on x86
|
|
and arm64 and IOMMU page tables.
|
|
|
|
percpu (npn)
|
|
Amount of memory used for storing per-cpu kernel
|
|
data structures.
|
|
|
|
sock (npn)
|
|
Amount of memory used in network transmission buffers
|
|
|
|
vmalloc (npn)
|
|
Amount of memory used for vmap backed memory.
|
|
|
|
shmem
|
|
Amount of cached filesystem data that is swap-backed,
|
|
such as tmpfs, shm segments, shared anonymous mmap()s
|
|
|
|
zswap
|
|
Amount of memory consumed by the zswap compression backend.
|
|
|
|
zswapped
|
|
Amount of application memory swapped out to zswap.
|
|
|
|
file_mapped
|
|
Amount of cached filesystem data mapped with mmap()
|
|
|
|
file_dirty
|
|
Amount of cached filesystem data that was modified but
|
|
not yet written back to disk
|
|
|
|
file_writeback
|
|
Amount of cached filesystem data that was modified and
|
|
is currently being written back to disk
|
|
|
|
swapcached
|
|
Amount of swap cached in memory. The swapcache is accounted
|
|
against both memory and swap usage.
|
|
|
|
anon_thp
|
|
Amount of memory used in anonymous mappings backed by
|
|
transparent hugepages
|
|
|
|
file_thp
|
|
Amount of cached filesystem data backed by transparent
|
|
hugepages
|
|
|
|
shmem_thp
|
|
Amount of shm, tmpfs, shared anonymous mmap()s backed by
|
|
transparent hugepages
|
|
|
|
inactive_anon, active_anon, inactive_file, active_file, unevictable
|
|
Amount of memory, swap-backed and filesystem-backed,
|
|
on the internal memory management lists used by the
|
|
page reclaim algorithm.
|
|
|
|
As these represent internal list state (eg. shmem pages are on anon
|
|
memory management lists), inactive_foo + active_foo may not be equal to
|
|
the value for the foo counter, since the foo counter is type-based, not
|
|
list-based.
|
|
|
|
slab_reclaimable
|
|
Part of "slab" that might be reclaimed, such as
|
|
dentries and inodes.
|
|
|
|
slab_unreclaimable
|
|
Part of "slab" that cannot be reclaimed on memory
|
|
pressure.
|
|
|
|
slab (npn)
|
|
Amount of memory used for storing in-kernel data
|
|
structures.
|
|
|
|
workingset_refault_anon
|
|
Number of refaults of previously evicted anonymous pages.
|
|
|
|
workingset_refault_file
|
|
Number of refaults of previously evicted file pages.
|
|
|
|
workingset_activate_anon
|
|
Number of refaulted anonymous pages that were immediately
|
|
activated.
|
|
|
|
workingset_activate_file
|
|
Number of refaulted file pages that were immediately activated.
|
|
|
|
workingset_restore_anon
|
|
Number of restored anonymous pages which have been detected as
|
|
an active workingset before they got reclaimed.
|
|
|
|
workingset_restore_file
|
|
Number of restored file pages which have been detected as an
|
|
active workingset before they got reclaimed.
|
|
|
|
workingset_nodereclaim
|
|
Number of times a shadow node has been reclaimed
|
|
|
|
pgscan (npn)
|
|
Amount of scanned pages (in an inactive LRU list)
|
|
|
|
pgsteal (npn)
|
|
Amount of reclaimed pages
|
|
|
|
pgscan_kswapd (npn)
|
|
Amount of scanned pages by kswapd (in an inactive LRU list)
|
|
|
|
pgscan_direct (npn)
|
|
Amount of scanned pages directly (in an inactive LRU list)
|
|
|
|
pgscan_khugepaged (npn)
|
|
Amount of scanned pages by khugepaged (in an inactive LRU list)
|
|
|
|
pgsteal_kswapd (npn)
|
|
Amount of reclaimed pages by kswapd
|
|
|
|
pgsteal_direct (npn)
|
|
Amount of reclaimed pages directly
|
|
|
|
pgsteal_khugepaged (npn)
|
|
Amount of reclaimed pages by khugepaged
|
|
|
|
pgfault (npn)
|
|
Total number of page faults incurred
|
|
|
|
pgmajfault (npn)
|
|
Number of major page faults incurred
|
|
|
|
pgrefill (npn)
|
|
Amount of scanned pages (in an active LRU list)
|
|
|
|
pgactivate (npn)
|
|
Amount of pages moved to the active LRU list
|
|
|
|
pgdeactivate (npn)
|
|
Amount of pages moved to the inactive LRU list
|
|
|
|
pglazyfree (npn)
|
|
Amount of pages postponed to be freed under memory pressure
|
|
|
|
pglazyfreed (npn)
|
|
Amount of reclaimed lazyfree pages
|
|
|
|
thp_fault_alloc (npn)
|
|
Number of transparent hugepages which were allocated to satisfy
|
|
a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE
|
|
is not set.
|
|
|
|
thp_collapse_alloc (npn)
|
|
Number of transparent hugepages which were allocated to allow
|
|
collapsing an existing range of pages. This counter is not
|
|
present when CONFIG_TRANSPARENT_HUGEPAGE is not set.
|
|
|
|
thp_swpout (npn)
|
|
Number of transparent hugepages which are swapout in one piece
|
|
without splitting.
|
|
|
|
thp_swpout_fallback (npn)
|
|
Number of transparent hugepages which were split before swapout.
|
|
Usually because failed to allocate some continuous swap space
|
|
for the huge page.
|
|
|
|
memory.numa_stat
|
|
A read-only nested-keyed file which exists on non-root cgroups.
|
|
|
|
This breaks down the cgroup's memory footprint into different
|
|
types of memory, type-specific details, and other information
|
|
per node on the state of the memory management system.
|
|
|
|
This is useful for providing visibility into the NUMA locality
|
|
information within an memcg since the pages are allowed to be
|
|
allocated from any physical node. One of the use case is evaluating
|
|
application performance by combining this information with the
|
|
application's CPU allocation.
|
|
|
|
All memory amounts are in bytes.
|
|
|
|
The output format of memory.numa_stat is::
|
|
|
|
type N0=<bytes in node 0> N1=<bytes in node 1> ...
|
|
|
|
The entries are ordered to be human readable, and new entries
|
|
can show up in the middle. Don't rely on items remaining in a
|
|
fixed position; use the keys to look up specific values!
|
|
|
|
The entries can refer to the memory.stat.
|
|
|
|
memory.swap.current
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The total amount of swap currently being used by the cgroup
|
|
and its descendants.
|
|
|
|
memory.swap.high
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Swap usage throttle limit. If a cgroup's swap usage exceeds
|
|
this limit, all its further allocations will be throttled to
|
|
allow userspace to implement custom out-of-memory procedures.
|
|
|
|
This limit marks a point of no return for the cgroup. It is NOT
|
|
designed to manage the amount of swapping a workload does
|
|
during regular operation. Compare to memory.swap.max, which
|
|
prohibits swapping past a set amount, but lets the cgroup
|
|
continue unimpeded as long as other memory can be reclaimed.
|
|
|
|
Healthy workloads are not expected to reach this limit.
|
|
|
|
memory.swap.peak
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The max swap usage recorded for the cgroup and its
|
|
descendants since the creation of the cgroup.
|
|
|
|
memory.swap.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Swap usage hard limit. If a cgroup's swap usage reaches this
|
|
limit, anonymous memory of the cgroup will not be swapped out.
|
|
|
|
memory.swap.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
The following entries are defined. Unless specified
|
|
otherwise, a value change in this file generates a file
|
|
modified event.
|
|
|
|
high
|
|
The number of times the cgroup's swap usage was over
|
|
the high threshold.
|
|
|
|
max
|
|
The number of times the cgroup's swap usage was about
|
|
to go over the max boundary and swap allocation
|
|
failed.
|
|
|
|
fail
|
|
The number of times swap allocation failed either
|
|
because of running out of swap system-wide or max
|
|
limit.
|
|
|
|
When reduced under the current usage, the existing swap
|
|
entries are reclaimed gradually and the swap usage may stay
|
|
higher than the limit for an extended period of time. This
|
|
reduces the impact on the workload and memory management.
|
|
|
|
memory.zswap.current
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The total amount of memory consumed by the zswap compression
|
|
backend.
|
|
|
|
memory.zswap.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Zswap usage hard limit. If a cgroup's zswap pool reaches this
|
|
limit, it will refuse to take any more stores before existing
|
|
entries fault back in or are written out to disk.
|
|
|
|
memory.zswap.writeback
|
|
A read-write single value file. The default value is "1".
|
|
Note that this setting is hierarchical, i.e. the writeback would be
|
|
implicitly disabled for child cgroups if the upper hierarchy
|
|
does so.
|
|
|
|
When this is set to 0, all swapping attempts to swapping devices
|
|
are disabled. This included both zswap writebacks, and swapping due
|
|
to zswap store failures. If the zswap store failures are recurring
|
|
(for e.g if the pages are incompressible), users can observe
|
|
reclaim inefficiency after disabling writeback (because the same
|
|
pages might be rejected again and again).
|
|
|
|
Note that this is subtly different from setting memory.swap.max to
|
|
0, as it still allows for pages to be written to the zswap pool.
|
|
|
|
memory.pressure
|
|
A read-only nested-keyed file.
|
|
|
|
Shows pressure stall information for memory. See
|
|
:ref:`Documentation/accounting/psi.rst <psi>` for details.
|
|
|
|
|
|
Usage Guidelines
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
"memory.high" is the main mechanism to control memory usage.
|
|
Over-committing on high limit (sum of high limits > available memory)
|
|
and letting global memory pressure to distribute memory according to
|
|
usage is a viable strategy.
|
|
|
|
Because breach of the high limit doesn't trigger the OOM killer but
|
|
throttles the offending cgroup, a management agent has ample
|
|
opportunities to monitor and take appropriate actions such as granting
|
|
more memory or terminating the workload.
|
|
|
|
Determining whether a cgroup has enough memory is not trivial as
|
|
memory usage doesn't indicate whether the workload can benefit from
|
|
more memory. For example, a workload which writes data received from
|
|
network to a file can use all available memory but can also operate as
|
|
performant with a small amount of memory. A measure of memory
|
|
pressure - how much the workload is being impacted due to lack of
|
|
memory - is necessary to determine whether a workload needs more
|
|
memory; unfortunately, memory pressure monitoring mechanism isn't
|
|
implemented yet.
|
|
|
|
|
|
Memory Ownership
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
A memory area is charged to the cgroup which instantiated it and stays
|
|
charged to the cgroup until the area is released. Migrating a process
|
|
to a different cgroup doesn't move the memory usages that it
|
|
instantiated while in the previous cgroup to the new cgroup.
|
|
|
|
A memory area may be used by processes belonging to different cgroups.
|
|
To which cgroup the area will be charged is in-deterministic; however,
|
|
over time, the memory area is likely to end up in a cgroup which has
|
|
enough memory allowance to avoid high reclaim pressure.
|
|
|
|
If a cgroup sweeps a considerable amount of memory which is expected
|
|
to be accessed repeatedly by other cgroups, it may make sense to use
|
|
POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
|
|
belonging to the affected files to ensure correct memory ownership.
|
|
|
|
|
|
IO
|
|
--
|
|
|
|
The "io" controller regulates the distribution of IO resources. This
|
|
controller implements both weight based and absolute bandwidth or IOPS
|
|
limit distribution; however, weight based distribution is available
|
|
only if cfq-iosched is in use and neither scheme is available for
|
|
blk-mq devices.
|
|
|
|
|
|
IO Interface Files
|
|
~~~~~~~~~~~~~~~~~~
|
|
|
|
io.stat
|
|
A read-only nested-keyed file.
|
|
|
|
Lines are keyed by $MAJ:$MIN device numbers and not ordered.
|
|
The following nested keys are defined.
|
|
|
|
====== =====================
|
|
rbytes Bytes read
|
|
wbytes Bytes written
|
|
rios Number of read IOs
|
|
wios Number of write IOs
|
|
dbytes Bytes discarded
|
|
dios Number of discard IOs
|
|
====== =====================
|
|
|
|
An example read output follows::
|
|
|
|
8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0
|
|
8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021
|
|
|
|
io.cost.qos
|
|
A read-write nested-keyed file which exists only on the root
|
|
cgroup.
|
|
|
|
This file configures the Quality of Service of the IO cost
|
|
model based controller (CONFIG_BLK_CGROUP_IOCOST) which
|
|
currently implements "io.weight" proportional control. Lines
|
|
are keyed by $MAJ:$MIN device numbers and not ordered. The
|
|
line for a given device is populated on the first write for
|
|
the device on "io.cost.qos" or "io.cost.model". The following
|
|
nested keys are defined.
|
|
|
|
====== =====================================
|
|
enable Weight-based control enable
|
|
ctrl "auto" or "user"
|
|
rpct Read latency percentile [0, 100]
|
|
rlat Read latency threshold
|
|
wpct Write latency percentile [0, 100]
|
|
wlat Write latency threshold
|
|
min Minimum scaling percentage [1, 10000]
|
|
max Maximum scaling percentage [1, 10000]
|
|
====== =====================================
|
|
|
|
The controller is disabled by default and can be enabled by
|
|
setting "enable" to 1. "rpct" and "wpct" parameters default
|
|
to zero and the controller uses internal device saturation
|
|
state to adjust the overall IO rate between "min" and "max".
|
|
|
|
When a better control quality is needed, latency QoS
|
|
parameters can be configured. For example::
|
|
|
|
8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0
|
|
|
|
shows that on sdb, the controller is enabled, will consider
|
|
the device saturated if the 95th percentile of read completion
|
|
latencies is above 75ms or write 150ms, and adjust the overall
|
|
IO issue rate between 50% and 150% accordingly.
|
|
|
|
The lower the saturation point, the better the latency QoS at
|
|
the cost of aggregate bandwidth. The narrower the allowed
|
|
adjustment range between "min" and "max", the more conformant
|
|
to the cost model the IO behavior. Note that the IO issue
|
|
base rate may be far off from 100% and setting "min" and "max"
|
|
blindly can lead to a significant loss of device capacity or
|
|
control quality. "min" and "max" are useful for regulating
|
|
devices which show wide temporary behavior changes - e.g. a
|
|
ssd which accepts writes at the line speed for a while and
|
|
then completely stalls for multiple seconds.
|
|
|
|
When "ctrl" is "auto", the parameters are controlled by the
|
|
kernel and may change automatically. Setting "ctrl" to "user"
|
|
or setting any of the percentile and latency parameters puts
|
|
it into "user" mode and disables the automatic changes. The
|
|
automatic mode can be restored by setting "ctrl" to "auto".
|
|
|
|
io.cost.model
|
|
A read-write nested-keyed file which exists only on the root
|
|
cgroup.
|
|
|
|
This file configures the cost model of the IO cost model based
|
|
controller (CONFIG_BLK_CGROUP_IOCOST) which currently
|
|
implements "io.weight" proportional control. Lines are keyed
|
|
by $MAJ:$MIN device numbers and not ordered. The line for a
|
|
given device is populated on the first write for the device on
|
|
"io.cost.qos" or "io.cost.model". The following nested keys
|
|
are defined.
|
|
|
|
===== ================================
|
|
ctrl "auto" or "user"
|
|
model The cost model in use - "linear"
|
|
===== ================================
|
|
|
|
When "ctrl" is "auto", the kernel may change all parameters
|
|
dynamically. When "ctrl" is set to "user" or any other
|
|
parameters are written to, "ctrl" become "user" and the
|
|
automatic changes are disabled.
|
|
|
|
When "model" is "linear", the following model parameters are
|
|
defined.
|
|
|
|
============= ========================================
|
|
[r|w]bps The maximum sequential IO throughput
|
|
[r|w]seqiops The maximum 4k sequential IOs per second
|
|
[r|w]randiops The maximum 4k random IOs per second
|
|
============= ========================================
|
|
|
|
From the above, the builtin linear model determines the base
|
|
costs of a sequential and random IO and the cost coefficient
|
|
for the IO size. While simple, this model can cover most
|
|
common device classes acceptably.
|
|
|
|
The IO cost model isn't expected to be accurate in absolute
|
|
sense and is scaled to the device behavior dynamically.
|
|
|
|
If needed, tools/cgroup/iocost_coef_gen.py can be used to
|
|
generate device-specific coefficients.
|
|
|
|
io.weight
|
|
A read-write flat-keyed file which exists on non-root cgroups.
|
|
The default is "default 100".
|
|
|
|
The first line is the default weight applied to devices
|
|
without specific override. The rest are overrides keyed by
|
|
$MAJ:$MIN device numbers and not ordered. The weights are in
|
|
the range [1, 10000] and specifies the relative amount IO time
|
|
the cgroup can use in relation to its siblings.
|
|
|
|
The default weight can be updated by writing either "default
|
|
$WEIGHT" or simply "$WEIGHT". Overrides can be set by writing
|
|
"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
|
|
|
|
An example read output follows::
|
|
|
|
default 100
|
|
8:16 200
|
|
8:0 50
|
|
|
|
io.max
|
|
A read-write nested-keyed file which exists on non-root
|
|
cgroups.
|
|
|
|
BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN
|
|
device numbers and not ordered. The following nested keys are
|
|
defined.
|
|
|
|
===== ==================================
|
|
rbps Max read bytes per second
|
|
wbps Max write bytes per second
|
|
riops Max read IO operations per second
|
|
wiops Max write IO operations per second
|
|
===== ==================================
|
|
|
|
When writing, any number of nested key-value pairs can be
|
|
specified in any order. "max" can be specified as the value
|
|
to remove a specific limit. If the same key is specified
|
|
multiple times, the outcome is undefined.
|
|
|
|
BPS and IOPS are measured in each IO direction and IOs are
|
|
delayed if limit is reached. Temporary bursts are allowed.
|
|
|
|
Setting read limit at 2M BPS and write at 120 IOPS for 8:16::
|
|
|
|
echo "8:16 rbps=2097152 wiops=120" > io.max
|
|
|
|
Reading returns the following::
|
|
|
|
8:16 rbps=2097152 wbps=max riops=max wiops=120
|
|
|
|
Write IOPS limit can be removed by writing the following::
|
|
|
|
echo "8:16 wiops=max" > io.max
|
|
|
|
Reading now returns the following::
|
|
|
|
8:16 rbps=2097152 wbps=max riops=max wiops=max
|
|
|
|
io.pressure
|
|
A read-only nested-keyed file.
|
|
|
|
Shows pressure stall information for IO. See
|
|
:ref:`Documentation/accounting/psi.rst <psi>` for details.
|
|
|
|
|
|
Writeback
|
|
~~~~~~~~~
|
|
|
|
Page cache is dirtied through buffered writes and shared mmaps and
|
|
written asynchronously to the backing filesystem by the writeback
|
|
mechanism. Writeback sits between the memory and IO domains and
|
|
regulates the proportion of dirty memory by balancing dirtying and
|
|
write IOs.
|
|
|
|
The io controller, in conjunction with the memory controller,
|
|
implements control of page cache writeback IOs. The memory controller
|
|
defines the memory domain that dirty memory ratio is calculated and
|
|
maintained for and the io controller defines the io domain which
|
|
writes out dirty pages for the memory domain. Both system-wide and
|
|
per-cgroup dirty memory states are examined and the more restrictive
|
|
of the two is enforced.
|
|
|
|
cgroup writeback requires explicit support from the underlying
|
|
filesystem. Currently, cgroup writeback is implemented on ext2, ext4,
|
|
btrfs, f2fs, and xfs. On other filesystems, all writeback IOs are
|
|
attributed to the root cgroup.
|
|
|
|
There are inherent differences in memory and writeback management
|
|
which affects how cgroup ownership is tracked. Memory is tracked per
|
|
page while writeback per inode. For the purpose of writeback, an
|
|
inode is assigned to a cgroup and all IO requests to write dirty pages
|
|
from the inode are attributed to that cgroup.
|
|
|
|
As cgroup ownership for memory is tracked per page, there can be pages
|
|
which are associated with different cgroups than the one the inode is
|
|
associated with. These are called foreign pages. The writeback
|
|
constantly keeps track of foreign pages and, if a particular foreign
|
|
cgroup becomes the majority over a certain period of time, switches
|
|
the ownership of the inode to that cgroup.
|
|
|
|
While this model is enough for most use cases where a given inode is
|
|
mostly dirtied by a single cgroup even when the main writing cgroup
|
|
changes over time, use cases where multiple cgroups write to a single
|
|
inode simultaneously are not supported well. In such circumstances, a
|
|
significant portion of IOs are likely to be attributed incorrectly.
|
|
As memory controller assigns page ownership on the first use and
|
|
doesn't update it until the page is released, even if writeback
|
|
strictly follows page ownership, multiple cgroups dirtying overlapping
|
|
areas wouldn't work as expected. It's recommended to avoid such usage
|
|
patterns.
|
|
|
|
The sysctl knobs which affect writeback behavior are applied to cgroup
|
|
writeback as follows.
|
|
|
|
vm.dirty_background_ratio, vm.dirty_ratio
|
|
These ratios apply the same to cgroup writeback with the
|
|
amount of available memory capped by limits imposed by the
|
|
memory controller and system-wide clean memory.
|
|
|
|
vm.dirty_background_bytes, vm.dirty_bytes
|
|
For cgroup writeback, this is calculated into ratio against
|
|
total available memory and applied the same way as
|
|
vm.dirty[_background]_ratio.
|
|
|
|
|
|
IO Latency
|
|
~~~~~~~~~~
|
|
|
|
This is a cgroup v2 controller for IO workload protection. You provide a group
|
|
with a latency target, and if the average latency exceeds that target the
|
|
controller will throttle any peers that have a lower latency target than the
|
|
protected workload.
|
|
|
|
The limits are only applied at the peer level in the hierarchy. This means that
|
|
in the diagram below, only groups A, B, and C will influence each other, and
|
|
groups D and F will influence each other. Group G will influence nobody::
|
|
|
|
[root]
|
|
/ | \
|
|
A B C
|
|
/ \ |
|
|
D F G
|
|
|
|
|
|
So the ideal way to configure this is to set io.latency in groups A, B, and C.
|
|
Generally you do not want to set a value lower than the latency your device
|
|
supports. Experiment to find the value that works best for your workload.
|
|
Start at higher than the expected latency for your device and watch the
|
|
avg_lat value in io.stat for your workload group to get an idea of the
|
|
latency you see during normal operation. Use the avg_lat value as a basis for
|
|
your real setting, setting at 10-15% higher than the value in io.stat.
|
|
|
|
How IO Latency Throttling Works
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
io.latency is work conserving; so as long as everybody is meeting their latency
|
|
target the controller doesn't do anything. Once a group starts missing its
|
|
target it begins throttling any peer group that has a higher target than itself.
|
|
This throttling takes 2 forms:
|
|
|
|
- Queue depth throttling. This is the number of outstanding IO's a group is
|
|
allowed to have. We will clamp down relatively quickly, starting at no limit
|
|
and going all the way down to 1 IO at a time.
|
|
|
|
- Artificial delay induction. There are certain types of IO that cannot be
|
|
throttled without possibly adversely affecting higher priority groups. This
|
|
includes swapping and metadata IO. These types of IO are allowed to occur
|
|
normally, however they are "charged" to the originating group. If the
|
|
originating group is being throttled you will see the use_delay and delay
|
|
fields in io.stat increase. The delay value is how many microseconds that are
|
|
being added to any process that runs in this group. Because this number can
|
|
grow quite large if there is a lot of swapping or metadata IO occurring we
|
|
limit the individual delay events to 1 second at a time.
|
|
|
|
Once the victimized group starts meeting its latency target again it will start
|
|
unthrottling any peer groups that were throttled previously. If the victimized
|
|
group simply stops doing IO the global counter will unthrottle appropriately.
|
|
|
|
IO Latency Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
io.latency
|
|
This takes a similar format as the other controllers.
|
|
|
|
"MAJOR:MINOR target=<target time in microseconds"
|
|
|
|
io.stat
|
|
If the controller is enabled you will see extra stats in io.stat in
|
|
addition to the normal ones.
|
|
|
|
depth
|
|
This is the current queue depth for the group.
|
|
|
|
avg_lat
|
|
This is an exponential moving average with a decay rate of 1/exp
|
|
bound by the sampling interval. The decay rate interval can be
|
|
calculated by multiplying the win value in io.stat by the
|
|
corresponding number of samples based on the win value.
|
|
|
|
win
|
|
The sampling window size in milliseconds. This is the minimum
|
|
duration of time between evaluation events. Windows only elapse
|
|
with IO activity. Idle periods extend the most recent window.
|
|
|
|
IO Priority
|
|
~~~~~~~~~~~
|
|
|
|
A single attribute controls the behavior of the I/O priority cgroup policy,
|
|
namely the blkio.prio.class attribute. The following values are accepted for
|
|
that attribute:
|
|
|
|
no-change
|
|
Do not modify the I/O priority class.
|
|
|
|
promote-to-rt
|
|
For requests that have a non-RT I/O priority class, change it into RT.
|
|
Also change the priority level of these requests to 4. Do not modify
|
|
the I/O priority of requests that have priority class RT.
|
|
|
|
restrict-to-be
|
|
For requests that do not have an I/O priority class or that have I/O
|
|
priority class RT, change it into BE. Also change the priority level
|
|
of these requests to 0. Do not modify the I/O priority class of
|
|
requests that have priority class IDLE.
|
|
|
|
idle
|
|
Change the I/O priority class of all requests into IDLE, the lowest
|
|
I/O priority class.
|
|
|
|
none-to-rt
|
|
Deprecated. Just an alias for promote-to-rt.
|
|
|
|
The following numerical values are associated with the I/O priority policies:
|
|
|
|
+----------------+---+
|
|
| no-change | 0 |
|
|
+----------------+---+
|
|
| rt-to-be | 2 |
|
|
+----------------+---+
|
|
| all-to-idle | 3 |
|
|
+----------------+---+
|
|
|
|
The numerical value that corresponds to each I/O priority class is as follows:
|
|
|
|
+-------------------------------+---+
|
|
| IOPRIO_CLASS_NONE | 0 |
|
|
+-------------------------------+---+
|
|
| IOPRIO_CLASS_RT (real-time) | 1 |
|
|
+-------------------------------+---+
|
|
| IOPRIO_CLASS_BE (best effort) | 2 |
|
|
+-------------------------------+---+
|
|
| IOPRIO_CLASS_IDLE | 3 |
|
|
+-------------------------------+---+
|
|
|
|
The algorithm to set the I/O priority class for a request is as follows:
|
|
|
|
- If I/O priority class policy is promote-to-rt, change the request I/O
|
|
priority class to IOPRIO_CLASS_RT and change the request I/O priority
|
|
level to 4.
|
|
- If I/O priorityt class is not promote-to-rt, translate the I/O priority
|
|
class policy into a number, then change the request I/O priority class
|
|
into the maximum of the I/O priority class policy number and the numerical
|
|
I/O priority class.
|
|
|
|
PID
|
|
---
|
|
|
|
The process number controller is used to allow a cgroup to stop any
|
|
new tasks from being fork()'d or clone()'d after a specified limit is
|
|
reached.
|
|
|
|
The number of tasks in a cgroup can be exhausted in ways which other
|
|
controllers cannot prevent, thus warranting its own controller. For
|
|
example, a fork bomb is likely to exhaust the number of tasks before
|
|
hitting memory restrictions.
|
|
|
|
Note that PIDs used in this controller refer to TIDs, process IDs as
|
|
used by the kernel.
|
|
|
|
|
|
PID Interface Files
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
pids.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Hard limit of number of processes.
|
|
|
|
pids.current
|
|
A read-only single value file which exists on non-root cgroups.
|
|
|
|
The number of processes currently in the cgroup and its
|
|
descendants.
|
|
|
|
pids.peak
|
|
A read-only single value file which exists on non-root cgroups.
|
|
|
|
The maximum value that the number of processes in the cgroup and its
|
|
descendants has ever reached.
|
|
|
|
pids.events
|
|
A read-only flat-keyed file which exists on non-root cgroups. Unless
|
|
specified otherwise, a value change in this file generates a file
|
|
modified event. The following entries are defined.
|
|
|
|
max
|
|
The number of times the cgroup's total number of processes hit the pids.max
|
|
limit (see also pids_localevents).
|
|
|
|
pids.events.local
|
|
Similar to pids.events but the fields in the file are local
|
|
to the cgroup i.e. not hierarchical. The file modified event
|
|
generated on this file reflects only the local events.
|
|
|
|
Organisational operations are not blocked by cgroup policies, so it is
|
|
possible to have pids.current > pids.max. This can be done by either
|
|
setting the limit to be smaller than pids.current, or attaching enough
|
|
processes to the cgroup such that pids.current is larger than
|
|
pids.max. However, it is not possible to violate a cgroup PID policy
|
|
through fork() or clone(). These will return -EAGAIN if the creation
|
|
of a new process would cause a cgroup policy to be violated.
|
|
|
|
|
|
Cpuset
|
|
------
|
|
|
|
The "cpuset" controller provides a mechanism for constraining
|
|
the CPU and memory node placement of tasks to only the resources
|
|
specified in the cpuset interface files in a task's current cgroup.
|
|
This is especially valuable on large NUMA systems where placing jobs
|
|
on properly sized subsets of the systems with careful processor and
|
|
memory placement to reduce cross-node memory access and contention
|
|
can improve overall system performance.
|
|
|
|
The "cpuset" controller is hierarchical. That means the controller
|
|
cannot use CPUs or memory nodes not allowed in its parent.
|
|
|
|
|
|
Cpuset Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
cpuset.cpus
|
|
A read-write multiple values file which exists on non-root
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the requested CPUs to be used by tasks within this
|
|
cgroup. The actual list of CPUs to be granted, however, is
|
|
subjected to constraints imposed by its parent and can differ
|
|
from the requested CPUs.
|
|
|
|
The CPU numbers are comma-separated numbers or ranges.
|
|
For example::
|
|
|
|
# cat cpuset.cpus
|
|
0-4,6,8-10
|
|
|
|
An empty value indicates that the cgroup is using the same
|
|
setting as the nearest cgroup ancestor with a non-empty
|
|
"cpuset.cpus" or all the available CPUs if none is found.
|
|
|
|
The value of "cpuset.cpus" stays constant until the next update
|
|
and won't be affected by any CPU hotplug events.
|
|
|
|
cpuset.cpus.effective
|
|
A read-only multiple values file which exists on all
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the onlined CPUs that are actually granted to this
|
|
cgroup by its parent. These CPUs are allowed to be used by
|
|
tasks within the current cgroup.
|
|
|
|
If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows
|
|
all the CPUs from the parent cgroup that can be available to
|
|
be used by this cgroup. Otherwise, it should be a subset of
|
|
"cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus"
|
|
can be granted. In this case, it will be treated just like an
|
|
empty "cpuset.cpus".
|
|
|
|
Its value will be affected by CPU hotplug events.
|
|
|
|
cpuset.mems
|
|
A read-write multiple values file which exists on non-root
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the requested memory nodes to be used by tasks within
|
|
this cgroup. The actual list of memory nodes granted, however,
|
|
is subjected to constraints imposed by its parent and can differ
|
|
from the requested memory nodes.
|
|
|
|
The memory node numbers are comma-separated numbers or ranges.
|
|
For example::
|
|
|
|
# cat cpuset.mems
|
|
0-1,3
|
|
|
|
An empty value indicates that the cgroup is using the same
|
|
setting as the nearest cgroup ancestor with a non-empty
|
|
"cpuset.mems" or all the available memory nodes if none
|
|
is found.
|
|
|
|
The value of "cpuset.mems" stays constant until the next update
|
|
and won't be affected by any memory nodes hotplug events.
|
|
|
|
Setting a non-empty value to "cpuset.mems" causes memory of
|
|
tasks within the cgroup to be migrated to the designated nodes if
|
|
they are currently using memory outside of the designated nodes.
|
|
|
|
There is a cost for this memory migration. The migration
|
|
may not be complete and some memory pages may be left behind.
|
|
So it is recommended that "cpuset.mems" should be set properly
|
|
before spawning new tasks into the cpuset. Even if there is
|
|
a need to change "cpuset.mems" with active tasks, it shouldn't
|
|
be done frequently.
|
|
|
|
cpuset.mems.effective
|
|
A read-only multiple values file which exists on all
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the onlined memory nodes that are actually granted to
|
|
this cgroup by its parent. These memory nodes are allowed to
|
|
be used by tasks within the current cgroup.
|
|
|
|
If "cpuset.mems" is empty, it shows all the memory nodes from the
|
|
parent cgroup that will be available to be used by this cgroup.
|
|
Otherwise, it should be a subset of "cpuset.mems" unless none of
|
|
the memory nodes listed in "cpuset.mems" can be granted. In this
|
|
case, it will be treated just like an empty "cpuset.mems".
|
|
|
|
Its value will be affected by memory nodes hotplug events.
|
|
|
|
cpuset.cpus.exclusive
|
|
A read-write multiple values file which exists on non-root
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists all the exclusive CPUs that are allowed to be used
|
|
to create a new cpuset partition. Its value is not used
|
|
unless the cgroup becomes a valid partition root. See the
|
|
"cpuset.cpus.partition" section below for a description of what
|
|
a cpuset partition is.
|
|
|
|
When the cgroup becomes a partition root, the actual exclusive
|
|
CPUs that are allocated to that partition are listed in
|
|
"cpuset.cpus.exclusive.effective" which may be different
|
|
from "cpuset.cpus.exclusive". If "cpuset.cpus.exclusive"
|
|
has previously been set, "cpuset.cpus.exclusive.effective"
|
|
is always a subset of it.
|
|
|
|
Users can manually set it to a value that is different from
|
|
"cpuset.cpus". One constraint in setting it is that the list of
|
|
CPUs must be exclusive with respect to "cpuset.cpus.exclusive"
|
|
of its sibling. If "cpuset.cpus.exclusive" of a sibling cgroup
|
|
isn't set, its "cpuset.cpus" value, if set, cannot be a subset
|
|
of it to leave at least one CPU available when the exclusive
|
|
CPUs are taken away.
|
|
|
|
For a parent cgroup, any one of its exclusive CPUs can only
|
|
be distributed to at most one of its child cgroups. Having an
|
|
exclusive CPU appearing in two or more of its child cgroups is
|
|
not allowed (the exclusivity rule). A value that violates the
|
|
exclusivity rule will be rejected with a write error.
|
|
|
|
The root cgroup is a partition root and all its available CPUs
|
|
are in its exclusive CPU set.
|
|
|
|
cpuset.cpus.exclusive.effective
|
|
A read-only multiple values file which exists on all non-root
|
|
cpuset-enabled cgroups.
|
|
|
|
This file shows the effective set of exclusive CPUs that
|
|
can be used to create a partition root. The content
|
|
of this file will always be a subset of its parent's
|
|
"cpuset.cpus.exclusive.effective" if its parent is not the root
|
|
cgroup. It will also be a subset of "cpuset.cpus.exclusive"
|
|
if it is set. If "cpuset.cpus.exclusive" is not set, it is
|
|
treated to have an implicit value of "cpuset.cpus" in the
|
|
formation of local partition.
|
|
|
|
cpuset.cpus.isolated
|
|
A read-only and root cgroup only multiple values file.
|
|
|
|
This file shows the set of all isolated CPUs used in existing
|
|
isolated partitions. It will be empty if no isolated partition
|
|
is created.
|
|
|
|
cpuset.cpus.partition
|
|
A read-write single value file which exists on non-root
|
|
cpuset-enabled cgroups. This flag is owned by the parent cgroup
|
|
and is not delegatable.
|
|
|
|
It accepts only the following input values when written to.
|
|
|
|
========== =====================================
|
|
"member" Non-root member of a partition
|
|
"root" Partition root
|
|
"isolated" Partition root without load balancing
|
|
========== =====================================
|
|
|
|
A cpuset partition is a collection of cpuset-enabled cgroups with
|
|
a partition root at the top of the hierarchy and its descendants
|
|
except those that are separate partition roots themselves and
|
|
their descendants. A partition has exclusive access to the
|
|
set of exclusive CPUs allocated to it. Other cgroups outside
|
|
of that partition cannot use any CPUs in that set.
|
|
|
|
There are two types of partitions - local and remote. A local
|
|
partition is one whose parent cgroup is also a valid partition
|
|
root. A remote partition is one whose parent cgroup is not a
|
|
valid partition root itself. Writing to "cpuset.cpus.exclusive"
|
|
is optional for the creation of a local partition as its
|
|
"cpuset.cpus.exclusive" file will assume an implicit value that
|
|
is the same as "cpuset.cpus" if it is not set. Writing the
|
|
proper "cpuset.cpus.exclusive" values down the cgroup hierarchy
|
|
before the target partition root is mandatory for the creation
|
|
of a remote partition.
|
|
|
|
Currently, a remote partition cannot be created under a local
|
|
partition. All the ancestors of a remote partition root except
|
|
the root cgroup cannot be a partition root.
|
|
|
|
The root cgroup is always a partition root and its state cannot
|
|
be changed. All other non-root cgroups start out as "member".
|
|
|
|
When set to "root", the current cgroup is the root of a new
|
|
partition or scheduling domain. The set of exclusive CPUs is
|
|
determined by the value of its "cpuset.cpus.exclusive.effective".
|
|
|
|
When set to "isolated", the CPUs in that partition will be in
|
|
an isolated state without any load balancing from the scheduler
|
|
and excluded from the unbound workqueues. Tasks placed in such
|
|
a partition with multiple CPUs should be carefully distributed
|
|
and bound to each of the individual CPUs for optimal performance.
|
|
|
|
A partition root ("root" or "isolated") can be in one of the
|
|
two possible states - valid or invalid. An invalid partition
|
|
root is in a degraded state where some state information may
|
|
be retained, but behaves more like a "member".
|
|
|
|
All possible state transitions among "member", "root" and
|
|
"isolated" are allowed.
|
|
|
|
On read, the "cpuset.cpus.partition" file can show the following
|
|
values.
|
|
|
|
============================= =====================================
|
|
"member" Non-root member of a partition
|
|
"root" Partition root
|
|
"isolated" Partition root without load balancing
|
|
"root invalid (<reason>)" Invalid partition root
|
|
"isolated invalid (<reason>)" Invalid isolated partition root
|
|
============================= =====================================
|
|
|
|
In the case of an invalid partition root, a descriptive string on
|
|
why the partition is invalid is included within parentheses.
|
|
|
|
For a local partition root to be valid, the following conditions
|
|
must be met.
|
|
|
|
1) The parent cgroup is a valid partition root.
|
|
2) The "cpuset.cpus.exclusive.effective" file cannot be empty,
|
|
though it may contain offline CPUs.
|
|
3) The "cpuset.cpus.effective" cannot be empty unless there is
|
|
no task associated with this partition.
|
|
|
|
For a remote partition root to be valid, all the above conditions
|
|
except the first one must be met.
|
|
|
|
External events like hotplug or changes to "cpuset.cpus" or
|
|
"cpuset.cpus.exclusive" can cause a valid partition root to
|
|
become invalid and vice versa. Note that a task cannot be
|
|
moved to a cgroup with empty "cpuset.cpus.effective".
|
|
|
|
A valid non-root parent partition may distribute out all its CPUs
|
|
to its child local partitions when there is no task associated
|
|
with it.
|
|
|
|
Care must be taken to change a valid partition root to "member"
|
|
as all its child local partitions, if present, will become
|
|
invalid causing disruption to tasks running in those child
|
|
partitions. These inactivated partitions could be recovered if
|
|
their parent is switched back to a partition root with a proper
|
|
value in "cpuset.cpus" or "cpuset.cpus.exclusive".
|
|
|
|
Poll and inotify events are triggered whenever the state of
|
|
"cpuset.cpus.partition" changes. That includes changes caused
|
|
by write to "cpuset.cpus.partition", cpu hotplug or other
|
|
changes that modify the validity status of the partition.
|
|
This will allow user space agents to monitor unexpected changes
|
|
to "cpuset.cpus.partition" without the need to do continuous
|
|
polling.
|
|
|
|
A user can pre-configure certain CPUs to an isolated state
|
|
with load balancing disabled at boot time with the "isolcpus"
|
|
kernel boot command line option. If those CPUs are to be put
|
|
into a partition, they have to be used in an isolated partition.
|
|
|
|
|
|
Device controller
|
|
-----------------
|
|
|
|
Device controller manages access to device files. It includes both
|
|
creation of new device files (using mknod), and access to the
|
|
existing device files.
|
|
|
|
Cgroup v2 device controller has no interface files and is implemented
|
|
on top of cgroup BPF. To control access to device files, a user may
|
|
create bpf programs of the BPF_CGROUP_DEVICE type and attach them
|
|
to cgroups. On an attempt to access a device file, corresponding
|
|
BPF programs will be executed, and depending on the return value
|
|
the attempt will succeed or fail with -EPERM.
|
|
|
|
A BPF_CGROUP_DEVICE program takes a pointer to the bpf_cgroup_dev_ctx
|
|
structure, which describes the device access attempt: access type
|
|
(mknod/read/write) and device (type, major and minor numbers).
|
|
If the program returns 0, the attempt fails with -EPERM, otherwise
|
|
it succeeds.
|
|
|
|
An example of BPF_CGROUP_DEVICE program may be found in the kernel
|
|
source tree in the tools/testing/selftests/bpf/progs/dev_cgroup.c file.
|
|
|
|
|
|
RDMA
|
|
----
|
|
|
|
The "rdma" controller regulates the distribution and accounting of
|
|
RDMA resources.
|
|
|
|
RDMA Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
rdma.max
|
|
A readwrite nested-keyed file that exists for all the cgroups
|
|
except root that describes current configured resource limit
|
|
for a RDMA/IB device.
|
|
|
|
Lines are keyed by device name and are not ordered.
|
|
Each line contains space separated resource name and its configured
|
|
limit that can be distributed.
|
|
|
|
The following nested keys are defined.
|
|
|
|
========== =============================
|
|
hca_handle Maximum number of HCA Handles
|
|
hca_object Maximum number of HCA Objects
|
|
========== =============================
|
|
|
|
An example for mlx4 and ocrdma device follows::
|
|
|
|
mlx4_0 hca_handle=2 hca_object=2000
|
|
ocrdma1 hca_handle=3 hca_object=max
|
|
|
|
rdma.current
|
|
A read-only file that describes current resource usage.
|
|
It exists for all the cgroup except root.
|
|
|
|
An example for mlx4 and ocrdma device follows::
|
|
|
|
mlx4_0 hca_handle=1 hca_object=20
|
|
ocrdma1 hca_handle=1 hca_object=23
|
|
|
|
HugeTLB
|
|
-------
|
|
|
|
The HugeTLB controller allows to limit the HugeTLB usage per control group and
|
|
enforces the controller limit during page fault.
|
|
|
|
HugeTLB Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
hugetlb.<hugepagesize>.current
|
|
Show current usage for "hugepagesize" hugetlb. It exists for all
|
|
the cgroup except root.
|
|
|
|
hugetlb.<hugepagesize>.max
|
|
Set/show the hard limit of "hugepagesize" hugetlb usage.
|
|
The default value is "max". It exists for all the cgroup except root.
|
|
|
|
hugetlb.<hugepagesize>.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
|
|
max
|
|
The number of allocation failure due to HugeTLB limit
|
|
|
|
hugetlb.<hugepagesize>.events.local
|
|
Similar to hugetlb.<hugepagesize>.events but the fields in the file
|
|
are local to the cgroup i.e. not hierarchical. The file modified event
|
|
generated on this file reflects only the local events.
|
|
|
|
hugetlb.<hugepagesize>.numa_stat
|
|
Similar to memory.numa_stat, it shows the numa information of the
|
|
hugetlb pages of <hugepagesize> in this cgroup. Only active in
|
|
use hugetlb pages are included. The per-node values are in bytes.
|
|
|
|
Misc
|
|
----
|
|
|
|
The Miscellaneous cgroup provides the resource limiting and tracking
|
|
mechanism for the scalar resources which cannot be abstracted like the other
|
|
cgroup resources. Controller is enabled by the CONFIG_CGROUP_MISC config
|
|
option.
|
|
|
|
A resource can be added to the controller via enum misc_res_type{} in the
|
|
include/linux/misc_cgroup.h file and the corresponding name via misc_res_name[]
|
|
in the kernel/cgroup/misc.c file. Provider of the resource must set its
|
|
capacity prior to using the resource by calling misc_cg_set_capacity().
|
|
|
|
Once a capacity is set then the resource usage can be updated using charge and
|
|
uncharge APIs. All of the APIs to interact with misc controller are in
|
|
include/linux/misc_cgroup.h.
|
|
|
|
Misc Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Miscellaneous controller provides 3 interface files. If two misc resources (res_a and res_b) are registered then:
|
|
|
|
misc.capacity
|
|
A read-only flat-keyed file shown only in the root cgroup. It shows
|
|
miscellaneous scalar resources available on the platform along with
|
|
their quantities::
|
|
|
|
$ cat misc.capacity
|
|
res_a 50
|
|
res_b 10
|
|
|
|
misc.current
|
|
A read-only flat-keyed file shown in the non-root cgroups. It shows
|
|
the current usage of the resources in the cgroup and its children.::
|
|
|
|
$ cat misc.current
|
|
res_a 3
|
|
res_b 0
|
|
|
|
misc.peak
|
|
A read-only flat-keyed file shown in all cgroups. It shows the
|
|
historical maximum usage of the resources in the cgroup and its
|
|
children.::
|
|
|
|
$ cat misc.peak
|
|
res_a 10
|
|
res_b 8
|
|
|
|
misc.max
|
|
A read-write flat-keyed file shown in the non root cgroups. Allowed
|
|
maximum usage of the resources in the cgroup and its children.::
|
|
|
|
$ cat misc.max
|
|
res_a max
|
|
res_b 4
|
|
|
|
Limit can be set by::
|
|
|
|
# echo res_a 1 > misc.max
|
|
|
|
Limit can be set to max by::
|
|
|
|
# echo res_a max > misc.max
|
|
|
|
Limits can be set higher than the capacity value in the misc.capacity
|
|
file.
|
|
|
|
misc.events
|
|
A read-only flat-keyed file which exists on non-root cgroups. The
|
|
following entries are defined. Unless specified otherwise, a value
|
|
change in this file generates a file modified event. All fields in
|
|
this file are hierarchical.
|
|
|
|
max
|
|
The number of times the cgroup's resource usage was
|
|
about to go over the max boundary.
|
|
|
|
misc.events.local
|
|
Similar to misc.events but the fields in the file are local to the
|
|
cgroup i.e. not hierarchical. The file modified event generated on
|
|
this file reflects only the local events.
|
|
|
|
Migration and Ownership
|
|
~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A miscellaneous scalar resource is charged to the cgroup in which it is used
|
|
first, and stays charged to that cgroup until that resource is freed. Migrating
|
|
a process to a different cgroup does not move the charge to the destination
|
|
cgroup where the process has moved.
|
|
|
|
Others
|
|
------
|
|
|
|
perf_event
|
|
~~~~~~~~~~
|
|
|
|
perf_event controller, if not mounted on a legacy hierarchy, is
|
|
automatically enabled on the v2 hierarchy so that perf events can
|
|
always be filtered by cgroup v2 path. The controller can still be
|
|
moved to a legacy hierarchy after v2 hierarchy is populated.
|
|
|
|
|
|
Non-normative information
|
|
-------------------------
|
|
|
|
This section contains information that isn't considered to be a part of
|
|
the stable kernel API and so is subject to change.
|
|
|
|
|
|
CPU controller root cgroup process behaviour
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
When distributing CPU cycles in the root cgroup each thread in this
|
|
cgroup is treated as if it was hosted in a separate child cgroup of the
|
|
root cgroup. This child cgroup weight is dependent on its thread nice
|
|
level.
|
|
|
|
For details of this mapping see sched_prio_to_weight array in
|
|
kernel/sched/core.c file (values from this array should be scaled
|
|
appropriately so the neutral - nice 0 - value is 100 instead of 1024).
|
|
|
|
|
|
IO controller root cgroup process behaviour
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Root cgroup processes are hosted in an implicit leaf child node.
|
|
When distributing IO resources this implicit child node is taken into
|
|
account as if it was a normal child cgroup of the root cgroup with a
|
|
weight value of 200.
|
|
|
|
|
|
Namespace
|
|
=========
|
|
|
|
Basics
|
|
------
|
|
|
|
cgroup namespace provides a mechanism to virtualize the view of the
|
|
"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone
|
|
flag can be used with clone(2) and unshare(2) to create a new cgroup
|
|
namespace. The process running inside the cgroup namespace will have
|
|
its "/proc/$PID/cgroup" output restricted to cgroupns root. The
|
|
cgroupns root is the cgroup of the process at the time of creation of
|
|
the cgroup namespace.
|
|
|
|
Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
|
|
complete path of the cgroup of a process. In a container setup where
|
|
a set of cgroups and namespaces are intended to isolate processes the
|
|
"/proc/$PID/cgroup" file may leak potential system level information
|
|
to the isolated processes. For example::
|
|
|
|
# cat /proc/self/cgroup
|
|
0::/batchjobs/container_id1
|
|
|
|
The path '/batchjobs/container_id1' can be considered as system-data
|
|
and undesirable to expose to the isolated processes. cgroup namespace
|
|
can be used to restrict visibility of this path. For example, before
|
|
creating a cgroup namespace, one would see::
|
|
|
|
# ls -l /proc/self/ns/cgroup
|
|
lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
|
|
# cat /proc/self/cgroup
|
|
0::/batchjobs/container_id1
|
|
|
|
After unsharing a new namespace, the view changes::
|
|
|
|
# ls -l /proc/self/ns/cgroup
|
|
lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
|
|
# cat /proc/self/cgroup
|
|
0::/
|
|
|
|
When some thread from a multi-threaded process unshares its cgroup
|
|
namespace, the new cgroupns gets applied to the entire process (all
|
|
the threads). This is natural for the v2 hierarchy; however, for the
|
|
legacy hierarchies, this may be unexpected.
|
|
|
|
A cgroup namespace is alive as long as there are processes inside or
|
|
mounts pinning it. When the last usage goes away, the cgroup
|
|
namespace is destroyed. The cgroupns root and the actual cgroups
|
|
remain.
|
|
|
|
|
|
The Root and Views
|
|
------------------
|
|
|
|
The 'cgroupns root' for a cgroup namespace is the cgroup in which the
|
|
process calling unshare(2) is running. For example, if a process in
|
|
/batchjobs/container_id1 cgroup calls unshare, cgroup
|
|
/batchjobs/container_id1 becomes the cgroupns root. For the
|
|
init_cgroup_ns, this is the real root ('/') cgroup.
|
|
|
|
The cgroupns root cgroup does not change even if the namespace creator
|
|
process later moves to a different cgroup::
|
|
|
|
# ~/unshare -c # unshare cgroupns in some cgroup
|
|
# cat /proc/self/cgroup
|
|
0::/
|
|
# mkdir sub_cgrp_1
|
|
# echo 0 > sub_cgrp_1/cgroup.procs
|
|
# cat /proc/self/cgroup
|
|
0::/sub_cgrp_1
|
|
|
|
Each process gets its namespace-specific view of "/proc/$PID/cgroup"
|
|
|
|
Processes running inside the cgroup namespace will be able to see
|
|
cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
|
|
From within an unshared cgroupns::
|
|
|
|
# sleep 100000 &
|
|
[1] 7353
|
|
# echo 7353 > sub_cgrp_1/cgroup.procs
|
|
# cat /proc/7353/cgroup
|
|
0::/sub_cgrp_1
|
|
|
|
From the initial cgroup namespace, the real cgroup path will be
|
|
visible::
|
|
|
|
$ cat /proc/7353/cgroup
|
|
0::/batchjobs/container_id1/sub_cgrp_1
|
|
|
|
From a sibling cgroup namespace (that is, a namespace rooted at a
|
|
different cgroup), the cgroup path relative to its own cgroup
|
|
namespace root will be shown. For instance, if PID 7353's cgroup
|
|
namespace root is at '/batchjobs/container_id2', then it will see::
|
|
|
|
# cat /proc/7353/cgroup
|
|
0::/../container_id2/sub_cgrp_1
|
|
|
|
Note that the relative path always starts with '/' to indicate that
|
|
its relative to the cgroup namespace root of the caller.
|
|
|
|
|
|
Migration and setns(2)
|
|
----------------------
|
|
|
|
Processes inside a cgroup namespace can move into and out of the
|
|
namespace root if they have proper access to external cgroups. For
|
|
example, from inside a namespace with cgroupns root at
|
|
/batchjobs/container_id1, and assuming that the global hierarchy is
|
|
still accessible inside cgroupns::
|
|
|
|
# cat /proc/7353/cgroup
|
|
0::/sub_cgrp_1
|
|
# echo 7353 > batchjobs/container_id2/cgroup.procs
|
|
# cat /proc/7353/cgroup
|
|
0::/../container_id2
|
|
|
|
Note that this kind of setup is not encouraged. A task inside cgroup
|
|
namespace should only be exposed to its own cgroupns hierarchy.
|
|
|
|
setns(2) to another cgroup namespace is allowed when:
|
|
|
|
(a) the process has CAP_SYS_ADMIN against its current user namespace
|
|
(b) the process has CAP_SYS_ADMIN against the target cgroup
|
|
namespace's userns
|
|
|
|
No implicit cgroup changes happen with attaching to another cgroup
|
|
namespace. It is expected that the someone moves the attaching
|
|
process under the target cgroup namespace root.
|
|
|
|
|
|
Interaction with Other Namespaces
|
|
---------------------------------
|
|
|
|
Namespace specific cgroup hierarchy can be mounted by a process
|
|
running inside a non-init cgroup namespace::
|
|
|
|
# mount -t cgroup2 none $MOUNT_POINT
|
|
|
|
This will mount the unified cgroup hierarchy with cgroupns root as the
|
|
filesystem root. The process needs CAP_SYS_ADMIN against its user and
|
|
mount namespaces.
|
|
|
|
The virtualization of /proc/self/cgroup file combined with restricting
|
|
the view of cgroup hierarchy by namespace-private cgroupfs mount
|
|
provides a properly isolated cgroup view inside the container.
|
|
|
|
|
|
Information on Kernel Programming
|
|
=================================
|
|
|
|
This section contains kernel programming information in the areas
|
|
where interacting with cgroup is necessary. cgroup core and
|
|
controllers are not covered.
|
|
|
|
|
|
Filesystem Support for Writeback
|
|
--------------------------------
|
|
|
|
A filesystem can support cgroup writeback by updating
|
|
address_space_operations->writepage[s]() to annotate bio's using the
|
|
following two functions.
|
|
|
|
wbc_init_bio(@wbc, @bio)
|
|
Should be called for each bio carrying writeback data and
|
|
associates the bio with the inode's owner cgroup and the
|
|
corresponding request queue. This must be called after
|
|
a queue (device) has been associated with the bio and
|
|
before submission.
|
|
|
|
wbc_account_cgroup_owner(@wbc, @page, @bytes)
|
|
Should be called for each data segment being written out.
|
|
While this function doesn't care exactly when it's called
|
|
during the writeback session, it's the easiest and most
|
|
natural to call it as data segments are added to a bio.
|
|
|
|
With writeback bio's annotated, cgroup support can be enabled per
|
|
super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for
|
|
selective disabling of cgroup writeback support which is helpful when
|
|
certain filesystem features, e.g. journaled data mode, are
|
|
incompatible.
|
|
|
|
wbc_init_bio() binds the specified bio to its cgroup. Depending on
|
|
the configuration, the bio may be executed at a lower priority and if
|
|
the writeback session is holding shared resources, e.g. a journal
|
|
entry, may lead to priority inversion. There is no one easy solution
|
|
for the problem. Filesystems can try to work around specific problem
|
|
cases by skipping wbc_init_bio() and using bio_associate_blkg()
|
|
directly.
|
|
|
|
|
|
Deprecated v1 Core Features
|
|
===========================
|
|
|
|
- Multiple hierarchies including named ones are not supported.
|
|
|
|
- All v1 mount options are not supported.
|
|
|
|
- The "tasks" file is removed and "cgroup.procs" is not sorted.
|
|
|
|
- "cgroup.clone_children" is removed.
|
|
|
|
- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" or
|
|
"cgroup.stat" files at the root instead.
|
|
|
|
|
|
Issues with v1 and Rationales for v2
|
|
====================================
|
|
|
|
Multiple Hierarchies
|
|
--------------------
|
|
|
|
cgroup v1 allowed an arbitrary number of hierarchies and each
|
|
hierarchy could host any number of controllers. While this seemed to
|
|
provide a high level of flexibility, it wasn't useful in practice.
|
|
|
|
For example, as there is only one instance of each controller, utility
|
|
type controllers such as freezer which can be useful in all
|
|
hierarchies could only be used in one. The issue is exacerbated by
|
|
the fact that controllers couldn't be moved to another hierarchy once
|
|
hierarchies were populated. Another issue was that all controllers
|
|
bound to a hierarchy were forced to have exactly the same view of the
|
|
hierarchy. It wasn't possible to vary the granularity depending on
|
|
the specific controller.
|
|
|
|
In practice, these issues heavily limited which controllers could be
|
|
put on the same hierarchy and most configurations resorted to putting
|
|
each controller on its own hierarchy. Only closely related ones, such
|
|
as the cpu and cpuacct controllers, made sense to be put on the same
|
|
hierarchy. This often meant that userland ended up managing multiple
|
|
similar hierarchies repeating the same steps on each hierarchy
|
|
whenever a hierarchy management operation was necessary.
|
|
|
|
Furthermore, support for multiple hierarchies came at a steep cost.
|
|
It greatly complicated cgroup core implementation but more importantly
|
|
the support for multiple hierarchies restricted how cgroup could be
|
|
used in general and what controllers was able to do.
|
|
|
|
There was no limit on how many hierarchies there might be, which meant
|
|
that a thread's cgroup membership couldn't be described in finite
|
|
length. The key might contain any number of entries and was unlimited
|
|
in length, which made it highly awkward to manipulate and led to
|
|
addition of controllers which existed only to identify membership,
|
|
which in turn exacerbated the original problem of proliferating number
|
|
of hierarchies.
|
|
|
|
Also, as a controller couldn't have any expectation regarding the
|
|
topologies of hierarchies other controllers might be on, each
|
|
controller had to assume that all other controllers were attached to
|
|
completely orthogonal hierarchies. This made it impossible, or at
|
|
least very cumbersome, for controllers to cooperate with each other.
|
|
|
|
In most use cases, putting controllers on hierarchies which are
|
|
completely orthogonal to each other isn't necessary. What usually is
|
|
called for is the ability to have differing levels of granularity
|
|
depending on the specific controller. In other words, hierarchy may
|
|
be collapsed from leaf towards root when viewed from specific
|
|
controllers. For example, a given configuration might not care about
|
|
how memory is distributed beyond a certain level while still wanting
|
|
to control how CPU cycles are distributed.
|
|
|
|
|
|
Thread Granularity
|
|
------------------
|
|
|
|
cgroup v1 allowed threads of a process to belong to different cgroups.
|
|
This didn't make sense for some controllers and those controllers
|
|
ended up implementing different ways to ignore such situations but
|
|
much more importantly it blurred the line between API exposed to
|
|
individual applications and system management interface.
|
|
|
|
Generally, in-process knowledge is available only to the process
|
|
itself; thus, unlike service-level organization of processes,
|
|
categorizing threads of a process requires active participation from
|
|
the application which owns the target process.
|
|
|
|
cgroup v1 had an ambiguously defined delegation model which got abused
|
|
in combination with thread granularity. cgroups were delegated to
|
|
individual applications so that they can create and manage their own
|
|
sub-hierarchies and control resource distributions along them. This
|
|
effectively raised cgroup to the status of a syscall-like API exposed
|
|
to lay programs.
|
|
|
|
First of all, cgroup has a fundamentally inadequate interface to be
|
|
exposed this way. For a process to access its own knobs, it has to
|
|
extract the path on the target hierarchy from /proc/self/cgroup,
|
|
construct the path by appending the name of the knob to the path, open
|
|
and then read and/or write to it. This is not only extremely clunky
|
|
and unusual but also inherently racy. There is no conventional way to
|
|
define transaction across the required steps and nothing can guarantee
|
|
that the process would actually be operating on its own sub-hierarchy.
|
|
|
|
cgroup controllers implemented a number of knobs which would never be
|
|
accepted as public APIs because they were just adding control knobs to
|
|
system-management pseudo filesystem. cgroup ended up with interface
|
|
knobs which were not properly abstracted or refined and directly
|
|
revealed kernel internal details. These knobs got exposed to
|
|
individual applications through the ill-defined delegation mechanism
|
|
effectively abusing cgroup as a shortcut to implementing public APIs
|
|
without going through the required scrutiny.
|
|
|
|
This was painful for both userland and kernel. Userland ended up with
|
|
misbehaving and poorly abstracted interfaces and kernel exposing and
|
|
locked into constructs inadvertently.
|
|
|
|
|
|
Competition Between Inner Nodes and Threads
|
|
-------------------------------------------
|
|
|
|
cgroup v1 allowed threads to be in any cgroups which created an
|
|
interesting problem where threads belonging to a parent cgroup and its
|
|
children cgroups competed for resources. This was nasty as two
|
|
different types of entities competed and there was no obvious way to
|
|
settle it. Different controllers did different things.
|
|
|
|
The cpu controller considered threads and cgroups as equivalents and
|
|
mapped nice levels to cgroup weights. This worked for some cases but
|
|
fell flat when children wanted to be allocated specific ratios of CPU
|
|
cycles and the number of internal threads fluctuated - the ratios
|
|
constantly changed as the number of competing entities fluctuated.
|
|
There also were other issues. The mapping from nice level to weight
|
|
wasn't obvious or universal, and there were various other knobs which
|
|
simply weren't available for threads.
|
|
|
|
The io controller implicitly created a hidden leaf node for each
|
|
cgroup to host the threads. The hidden leaf had its own copies of all
|
|
the knobs with ``leaf_`` prefixed. While this allowed equivalent
|
|
control over internal threads, it was with serious drawbacks. It
|
|
always added an extra layer of nesting which wouldn't be necessary
|
|
otherwise, made the interface messy and significantly complicated the
|
|
implementation.
|
|
|
|
The memory controller didn't have a way to control what happened
|
|
between internal tasks and child cgroups and the behavior was not
|
|
clearly defined. There were attempts to add ad-hoc behaviors and
|
|
knobs to tailor the behavior to specific workloads which would have
|
|
led to problems extremely difficult to resolve in the long term.
|
|
|
|
Multiple controllers struggled with internal tasks and came up with
|
|
different ways to deal with it; unfortunately, all the approaches were
|
|
severely flawed and, furthermore, the widely different behaviors
|
|
made cgroup as a whole highly inconsistent.
|
|
|
|
This clearly is a problem which needs to be addressed from cgroup core
|
|
in a uniform way.
|
|
|
|
|
|
Other Interface Issues
|
|
----------------------
|
|
|
|
cgroup v1 grew without oversight and developed a large number of
|
|
idiosyncrasies and inconsistencies. One issue on the cgroup core side
|
|
was how an empty cgroup was notified - a userland helper binary was
|
|
forked and executed for each event. The event delivery wasn't
|
|
recursive or delegatable. The limitations of the mechanism also led
|
|
to in-kernel event delivery filtering mechanism further complicating
|
|
the interface.
|
|
|
|
Controller interfaces were problematic too. An extreme example is
|
|
controllers completely ignoring hierarchical organization and treating
|
|
all cgroups as if they were all located directly under the root
|
|
cgroup. Some controllers exposed a large amount of inconsistent
|
|
implementation details to userland.
|
|
|
|
There also was no consistency across controllers. When a new cgroup
|
|
was created, some controllers defaulted to not imposing extra
|
|
restrictions while others disallowed any resource usage until
|
|
explicitly configured. Configuration knobs for the same type of
|
|
control used widely differing naming schemes and formats. Statistics
|
|
and information knobs were named arbitrarily and used different
|
|
formats and units even in the same controller.
|
|
|
|
cgroup v2 establishes common conventions where appropriate and updates
|
|
controllers so that they expose minimal and consistent interfaces.
|
|
|
|
|
|
Controller Issues and Remedies
|
|
------------------------------
|
|
|
|
Memory
|
|
~~~~~~
|
|
|
|
The original lower boundary, the soft limit, is defined as a limit
|
|
that is per default unset. As a result, the set of cgroups that
|
|
global reclaim prefers is opt-in, rather than opt-out. The costs for
|
|
optimizing these mostly negative lookups are so high that the
|
|
implementation, despite its enormous size, does not even provide the
|
|
basic desirable behavior. First off, the soft limit has no
|
|
hierarchical meaning. All configured groups are organized in a global
|
|
rbtree and treated like equal peers, regardless where they are located
|
|
in the hierarchy. This makes subtree delegation impossible. Second,
|
|
the soft limit reclaim pass is so aggressive that it not just
|
|
introduces high allocation latencies into the system, but also impacts
|
|
system performance due to overreclaim, to the point where the feature
|
|
becomes self-defeating.
|
|
|
|
The memory.low boundary on the other hand is a top-down allocated
|
|
reserve. A cgroup enjoys reclaim protection when it's within its
|
|
effective low, which makes delegation of subtrees possible. It also
|
|
enjoys having reclaim pressure proportional to its overage when
|
|
above its effective low.
|
|
|
|
The original high boundary, the hard limit, is defined as a strict
|
|
limit that can not budge, even if the OOM killer has to be called.
|
|
But this generally goes against the goal of making the most out of the
|
|
available memory. The memory consumption of workloads varies during
|
|
runtime, and that requires users to overcommit. But doing that with a
|
|
strict upper limit requires either a fairly accurate prediction of the
|
|
working set size or adding slack to the limit. Since working set size
|
|
estimation is hard and error prone, and getting it wrong results in
|
|
OOM kills, most users tend to err on the side of a looser limit and
|
|
end up wasting precious resources.
|
|
|
|
The memory.high boundary on the other hand can be set much more
|
|
conservatively. When hit, it throttles allocations by forcing them
|
|
into direct reclaim to work off the excess, but it never invokes the
|
|
OOM killer. As a result, a high boundary that is chosen too
|
|
aggressively will not terminate the processes, but instead it will
|
|
lead to gradual performance degradation. The user can monitor this
|
|
and make corrections until the minimal memory footprint that still
|
|
gives acceptable performance is found.
|
|
|
|
In extreme cases, with many concurrent allocations and a complete
|
|
breakdown of reclaim progress within the group, the high boundary can
|
|
be exceeded. But even then it's mostly better to satisfy the
|
|
allocation from the slack available in other groups or the rest of the
|
|
system than killing the group. Otherwise, memory.max is there to
|
|
limit this type of spillover and ultimately contain buggy or even
|
|
malicious applications.
|
|
|
|
Setting the original memory.limit_in_bytes below the current usage was
|
|
subject to a race condition, where concurrent charges could cause the
|
|
limit setting to fail. memory.max on the other hand will first set the
|
|
limit to prevent new charges, and then reclaim and OOM kill until the
|
|
new limit is met - or the task writing to memory.max is killed.
|
|
|
|
The combined memory+swap accounting and limiting is replaced by real
|
|
control over swap space.
|
|
|
|
The main argument for a combined memory+swap facility in the original
|
|
cgroup design was that global or parental pressure would always be
|
|
able to swap all anonymous memory of a child group, regardless of the
|
|
child's own (possibly untrusted) configuration. However, untrusted
|
|
groups can sabotage swapping by other means - such as referencing its
|
|
anonymous memory in a tight loop - and an admin can not assume full
|
|
swappability when overcommitting untrusted jobs.
|
|
|
|
For trusted jobs, on the other hand, a combined counter is not an
|
|
intuitive userspace interface, and it flies in the face of the idea
|
|
that cgroup controllers should account and limit specific physical
|
|
resources. Swap space is a resource like all others in the system,
|
|
and that's why unified hierarchy allows distributing it separately.
|