118 lines
3.0 KiB
ReStructuredText
118 lines
3.0 KiB
ReStructuredText
========
|
|
CPU load
|
|
========
|
|
|
|
Linux exports various bits of information via ``/proc/stat`` and
|
|
``/proc/uptime`` that userland tools, such as top(1), use to calculate
|
|
the average time system spent in a particular state, for example::
|
|
|
|
$ iostat
|
|
Linux 2.6.18.3-exp (linmac) 02/20/2007
|
|
|
|
avg-cpu: %user %nice %system %iowait %steal %idle
|
|
10.01 0.00 2.92 5.44 0.00 81.63
|
|
|
|
...
|
|
|
|
Here the system thinks that over the default sampling period the
|
|
system spent 10.01% of the time doing work in user space, 2.92% in the
|
|
kernel, and was overall 81.63% of the time idle.
|
|
|
|
In most cases the ``/proc/stat`` information reflects the reality quite
|
|
closely, however due to the nature of how/when the kernel collects
|
|
this data sometimes it can not be trusted at all.
|
|
|
|
So how is this information collected? Whenever timer interrupt is
|
|
signalled the kernel looks what kind of task was running at this
|
|
moment and increments the counter that corresponds to this tasks
|
|
kind/state. The problem with this is that the system could have
|
|
switched between various states multiple times between two timer
|
|
interrupts yet the counter is incremented only for the last state.
|
|
|
|
|
|
Example
|
|
-------
|
|
|
|
If we imagine the system with one task that periodically burns cycles
|
|
in the following manner::
|
|
|
|
time line between two timer interrupts
|
|
|--------------------------------------|
|
|
^ ^
|
|
|_ something begins working |
|
|
|_ something goes to sleep
|
|
(only to be awaken quite soon)
|
|
|
|
In the above situation the system will be 0% loaded according to the
|
|
``/proc/stat`` (since the timer interrupt will always happen when the
|
|
system is executing the idle handler), but in reality the load is
|
|
closer to 99%.
|
|
|
|
One can imagine many more situations where this behavior of the kernel
|
|
will lead to quite erratic information inside ``/proc/stat``::
|
|
|
|
|
|
/* gcc -o hog smallhog.c */
|
|
#include <time.h>
|
|
#include <limits.h>
|
|
#include <signal.h>
|
|
#include <sys/time.h>
|
|
#define HIST 10
|
|
|
|
static volatile sig_atomic_t stop;
|
|
|
|
static void sighandler(int signr)
|
|
{
|
|
(void) signr;
|
|
stop = 1;
|
|
}
|
|
|
|
static unsigned long hog (unsigned long niters)
|
|
{
|
|
stop = 0;
|
|
while (!stop && --niters);
|
|
return niters;
|
|
}
|
|
|
|
int main (void)
|
|
{
|
|
int i;
|
|
struct itimerval it = {
|
|
.it_interval = { .tv_sec = 0, .tv_usec = 1 },
|
|
.it_value = { .tv_sec = 0, .tv_usec = 1 } };
|
|
sigset_t set;
|
|
unsigned long v[HIST];
|
|
double tmp = 0.0;
|
|
unsigned long n;
|
|
signal(SIGALRM, &sighandler);
|
|
setitimer(ITIMER_REAL, &it, NULL);
|
|
|
|
hog (ULONG_MAX);
|
|
for (i = 0; i < HIST; ++i) v[i] = ULONG_MAX - hog(ULONG_MAX);
|
|
for (i = 0; i < HIST; ++i) tmp += v[i];
|
|
tmp /= HIST;
|
|
n = tmp - (tmp / 3.0);
|
|
|
|
sigemptyset(&set);
|
|
sigaddset(&set, SIGALRM);
|
|
|
|
for (;;) {
|
|
hog(n);
|
|
sigwait(&set, &i);
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
|
|
References
|
|
----------
|
|
|
|
- https://lore.kernel.org/r/loom.20070212T063225-663@post.gmane.org
|
|
- Documentation/filesystems/proc.rst (1.8)
|
|
|
|
|
|
Thanks
|
|
------
|
|
|
|
Con Kolivas, Pavel Machek
|