Re: [Qemu-devel] [PATCH 3/7] qcow2: use an LRU algorithm to replace entries from the L2 cache

[Eric Blake <eblake@redhat.com>]

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On 05/06/2015 07:39 AM, Alberto Garcia wrote:
> The current algorithm to evict entries from the cache gives always
> preference to those in the lowest positions. As the size of the cache
> increases, the chances of the later elements of being removed decrease
> exponentially.
> 
> In a scenario with random I/O and lots of cache misses, entries in
> positions 8 and higher are rarely (if ever) evicted. This can be seen
> even with the default cache size, but with larger caches the problem
> becomes more obvious.
> 
> Using an LRU algorithm makes the chances of being removed from the
> cache independent from the position.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cache.c | 31 +++++++++++++++----------------
>  1 file changed, 15 insertions(+), 16 deletions(-)
> 

> @@ -318,12 +315,10 @@ static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c,
>  
>      /* Give the table some hits for the start so that it won't be replaced
>       * immediately. The number 32 is completely arbitrary. */
> -    c->entries[i].cache_hits = 32;
>      c->entries[i].offset = offset;

The comment is now dead.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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