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Date: Thu, 5 Feb 2015 14:59:34 +0100
From: Alberto Garcia <berto@igalia.com>
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Subject: Re: [Qemu-devel] [PATCH] block: Give always priority to unused
 entries in the qcow2 L2 cache
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To: Max Reitz <mreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

On Thu, Feb 05, 2015 at 08:48:30AM -0500, Max Reitz wrote:

> >-        c->entries[i].cache_hits /= 2;
> >+        if (c->entries[i].cache_hits > 1) {
> >+            c->entries[i].cache_hits /= 2;
> >+        }
> >      }
> >      if (min_index == -1) {
> 
> Hm, I can't see where the code is actually giving priority to unused
> entries. qcow2_cache_find_entry_to_replace() is the only place which
> selects the entry to be used

Yes, and it looks for the one with the lowest cache hit count. That is
the only criteria:

        if (c->entries[i].cache_hits < min_count) {
            min_index = i;
            min_count = c->entries[i].cache_hits;
        }

If there are several with the same hit count then the first one is
chosen.

Since dividing the hit count by two everytime there's a cache miss can
make it go down to zero, an existing entry with cache_hits == 0 will
always be chosen before any empty one located afterwards in the array.

By never allowing the hit count to go down to zero, we make sure that
all unused entries are chosen first before a valid one is discarded.

Berto