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Re: [Qemu-devel] [PATCH] block: Give always priority to unused entries i

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH] block: Give always priority to unused entries in the qcow2 L2 cache
Date: Thu, 05 Feb 2015 09:03:20 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 2015-02-05 at 08:59, Alberto Garcia wrote:
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

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.

Oh, right. I was wondering because cache_hits is not reset to 0 in qcow2_cache_entry_flush(); but that function is not meant for emptying an entry but only making sure it's not dirty, and qcow2_cache_empty() indeed sets cache_hits to 0.


Reviewed-by: Max Reitz <address@hidden>

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