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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>
This commit is contained in:
Alberto Garcia 2015-05-11 15:54:57 +03:00 committed by Kevin Wolf
parent fdfbca82a0
commit 812e4082ca
1 changed files with 7 additions and 2 deletions
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9
block/qcow2-cache.c
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@ -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