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authorAlberto Garcia <berto@igalia.com>2015-05-11 15:54:57 +0300
committerKevin Wolf <kwolf@redhat.com>2015-05-22 17:08:01 +0200
commit812e4082cae73e12fd425cace4fd3a715a7c1d32 (patch)
treed7405b5a8c87135628ea2d72a7ba93786969f96f /block/qcow2-cache.c
parentfdfbca82a0874916007ca76323cd35f2af8a2ef3 (diff)
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qcow2: use a hash to look for entries in the L2 cache
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 <berto@igalia.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Diffstat (limited to 'block/qcow2-cache.c')
-rw-r--r--block/qcow2-cache.c9
1 files changed, 7 insertions, 2 deletions
diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 2035cd8..121e6e9 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -248,6 +248,7 @@ 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;
@@ -255,7 +256,8 @@ 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 / s->cluster_size * 4) % c->size;
+ do {
const Qcow2CachedTable *t = &c->entries[i];
if (t->offset == offset) {
goto found;
@@ -264,7 +266,10 @@ 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