[Qemu-devel] [PATCH] block: Give always priority to unused entries in the qcow2 L2 cache

[Alberto Garcia <berto@igalia.com>]

The current algorithm to replace entries from the L2 cache gives
priority to newer hits by dividing the hit count of all existing
entries by two everytime there is a cache miss.

However, if there are several cache misses the hit count of the
existing entries can easily go down to 0. This will result in those
entries being replaced even when there are others that have never been
used.

This problem is more noticeable with larger disk images and cache
sizes, since the chances of having several misses before the cache is
full are higher.

If we make sure that the hit count can never go down to 0 again,
unused entries will always have priority.

Signed-off-by: Alberto Garcia <berto@igalia.com>
---
 block/qcow2-cache.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 904f6b1..b115549 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -253,7 +253,9 @@ static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
 
         /* Give newer hits priority */
         /* TODO Check how to optimize the replacement strategy */
-        c->entries[i].cache_hits /= 2;
+        if (c->entries[i].cache_hits > 1) {
+            c->entries[i].cache_hits /= 2;
+        }
     }
 
     if (min_index == -1) {
-- 
2.1.4