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[Qemu-block] [PATCH 5/6] qcow2: use a hash to look for entries in the L2
From: |
Alberto Garcia |
Subject: |
[Qemu-block] [PATCH 5/6] qcow2: use a hash to look for entries in the L2 cache |
Date: |
Thu, 30 Apr 2015 13:11:44 +0300 |
The current cache algorithm traverses the array starting always from
the beginning, so the average number of comparisons needed to perform
a lookup is proportional to the size of the array.
By using a hash of the offset as the starting point, lookups are
faster and independent from the array size.
The hash is computed using the cluster number of the table, multiplied
by 4 to make it perform better when there are collisions.
In my tests, using a cache with 2048 entries, this reduces the average
number of comparisons per lookup from 430 to 2.5.
Signed-off-by: Alberto Garcia <address@hidden>
---
block/qcow2-cache.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index e1bba20..c0e0278 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -237,6 +237,7 @@ static int qcow2_cache_do_get(BlockDriverState *bs,
Qcow2Cache *c,
BDRVQcowState *s = bs->opaque;
int i;
int ret;
+ int lookup_index;
uint64_t min_lru_counter = UINT64_MAX;
int min_lru_index = -1;
@@ -244,7 +245,8 @@ static int qcow2_cache_do_get(BlockDriverState *bs,
Qcow2Cache *c,
offset, read_from_disk);
/* Check if the table is already cached */
- for (i = 0; i < c->size; i++) {
+ i = lookup_index = (offset / c->table_size * 4) % c->size;
+ do {
const Qcow2CachedTable *t = &c->entries[i];
if (t->offset == offset) {
goto found;
@@ -253,7 +255,10 @@ static int qcow2_cache_do_get(BlockDriverState *bs,
Qcow2Cache *c,
min_lru_counter = t->lru_counter;
min_lru_index = i;
}
- }
+ if (++i == c->size) {
+ i = 0;
+ }
+ } while (i != lookup_index);
if (min_lru_index == -1) {
/* This can't happen in current synchronous code, but leave the check
--
2.1.4
- [Qemu-block] [PATCH 0/6] qcow2 L2/refcount cache improvements, Alberto Garcia, 2015/04/30
- [Qemu-block] [PATCH 5/6] qcow2: use a hash to look for entries in the L2 cache,
Alberto Garcia <=
- [Qemu-block] [PATCH 6/6] qcow2: style fixes in qcow2-cache.c, Alberto Garcia, 2015/04/30
- [Qemu-block] [PATCH 4/6] qcow2: remove qcow2_cache_find_entry_to_replace(), Alberto Garcia, 2015/04/30
- [Qemu-block] [PATCH 2/6] qcow2: simplify qcow2_cache_put() and qcow2_cache_entry_mark_dirty(), Alberto Garcia, 2015/04/30
- [Qemu-block] [PATCH 3/6] qcow2: use an LRU algorithm to replace entries from the L2 cache, Alberto Garcia, 2015/04/30
- [Qemu-block] [PATCH 1/6] qcow2: use one single memory block for the L2/refcount cache tables, Alberto Garcia, 2015/04/30