80 lines
3.5 KiB
Diff
80 lines
3.5 KiB
Diff
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From b436ca6c69ef7d8d826be609820027f10134274d Mon Sep 17 00:00:00 2001
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From: "Richard W.M. Jones" <rjones@redhat.com>
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Date: Tue, 27 Jul 2021 21:28:48 +0100
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Subject: [PATCH] cache: Fix misleading LRU diagram and comment
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Only comment changes.
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(cherry picked from commit 7b33c86e0910d941dc34bdb481d61806f31cdcef)
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---
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filters/cache/lru.c | 32 ++++++++++++++++++++------------
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1 file changed, 20 insertions(+), 12 deletions(-)
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diff --git a/filters/cache/lru.c b/filters/cache/lru.c
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index 1c3c3e10..716b4984 100644
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--- a/filters/cache/lru.c
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+++ b/filters/cache/lru.c
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@@ -53,12 +53,14 @@
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/* LRU bitmaps. These bitmaps implement a simple, fast LRU structure.
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*
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- * bm[0] bm[1] blocks not in either bitmap
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- * ┌─────────┬──────────────────┬─────────────────────────────┐
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- * │ │ │ │
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- * └─────────┴──────────────────┴─────────────────────────────┘
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- * ↑ c1 bits set
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- * c0 bits set
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+ * bm[0]
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+ * ┌───────────────────────┐
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+ * │ X XX X XXX │ c0 bits set
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+ * └───────────────────────┘
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+ * bm[1]
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+ * ┌───────────────────────┐
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+ * │ X XX X X │ c1 bits set
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+ * └───────────────────────┘
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*
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* The LRU structure keeps track of the [approx] last N distinct
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* blocks which have been most recently accessed. It can answer in
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@@ -69,8 +71,7 @@
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*
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* When a new block is accessed, we set the corresponding bit in bm[0]
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* and increment c0 (c0 counts the number of bits set in bm[0]). If
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- * c0 == N/2 then we swap the two bitmaps, clear bm[0], and reset c0
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- * to 0.
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+ * c0 == N/2 then we move bm[1] <- bm[0], clear bm[0] and set c0 <- 0.
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*
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* To check if a block has been accessed within the previous N
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* distinct accesses, we simply have to check both bitmaps. If it is
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@@ -78,9 +79,11 @@
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* reclaimed.
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*
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* You'll note that in fact we only keep track of between N/2 and N
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- * recently accessed blocks. We could make the estimate more accurate
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- * by having more bitmaps, but as this is only a heuristic we choose
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- * to keep the implementation simple and memory usage low instead.
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+ * recently accessed blocks because the same block can appear in both
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+ * bitmaps. bm[1] is a last chance to hold on to blocks which are
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+ * soon to be reclaimed. We could make the estimate more accurate by
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+ * having more bitmaps, but as this is only a heuristic we choose to
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+ * keep the implementation simple and memory usage low instead.
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*/
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static struct bitmap bm[2];
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static unsigned c0 = 0, c1 = 0;
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@@ -129,7 +132,12 @@ lru_set_recently_accessed (uint64_t blknum)
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bitmap_set_blk (&bm[0], blknum, true);
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c0++;
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- /* If we've reached N/2 then we need to swap over the bitmaps. */
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+ /* If we've reached N/2 then we need to swap over the bitmaps. Note
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+ * the purpose of swapping here is to ensure that we do not have to
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+ * copy the dynamically allocated bm->bitmap field (the pointers are
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+ * swapped instead). The bm[0].bitmap field is immediately zeroed
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+ * after the swap.
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+ */
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if (c0 >= N/2) {
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struct bitmap tmp;
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--
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2.31.1
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