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| author | Alberto Garcia <berto@igalia.com> | 2015-05-11 15:54:57 +0300 |
|---|---|---|
| committer | Kevin Wolf <kwolf@redhat.com> | 2015-05-22 17:08:01 +0200 |
| commit | 812e4082cae73e12fd425cace4fd3a715a7c1d32 (patch) | |
| tree | d7405b5a8c87135628ea2d72a7ba93786969f96f /block | |
| parent | fdfbca82a0874916007ca76323cd35f2af8a2ef3 (diff) | |
| download | qemu-812e4082cae73e12fd425cace4fd3a715a7c1d32.tar.gz qemu-812e4082cae73e12fd425cace4fd3a715a7c1d32.tar.bz2 qemu-812e4082cae73e12fd425cace4fd3a715a7c1d32.zip | |
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')
| -rw-r--r-- | block/qcow2-cache.c | 9 |
1 files changed, 7 insertions, 2 deletions
diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c index 2035cd8ab2..121e6e9227 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 |