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Re: [Qemu-devel] [PATCH 3/7] qcow2: use an LRU algorithm to replace entr

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 3/7] qcow2: use an LRU algorithm to replace entries from the L2 cache
Date: Thu, 07 May 2015 08:55:21 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0

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 <address@hidden>
> ---
>  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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