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