qcow2: use an LRU algorithm to replace entries from the L2 cache

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 <berto@igalia.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/qcow2-cache.c

@@ -28,10 +28,10 @@
 #include "trace.h"
 
 typedef struct Qcow2CachedTable {
-    int64_t offset;
-    bool    dirty;
-    int     cache_hits;
-    int     ref;
+    int64_t  offset;
+    bool     dirty;
+    uint64_t lru_counter;
+    int      ref;
 } Qcow2CachedTable;
 
 struct Qcow2Cache {
@@ -40,6 +40,7 @@ struct Qcow2Cache {
     int                     size;
     bool                    depends_on_flush;
     void                   *table_array;
+    uint64_t                lru_counter;
 };
 
 static inline void *qcow2_cache_get_table_addr(BlockDriverState *bs,
@@ -233,16 +234,18 @@ int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c)
     for (i = 0; i < c->size; i++) {
         assert(c->entries[i].ref == 0);
         c->entries[i].offset = 0;
-        c->entries[i].cache_hits = 0;
+        c->entries[i].lru_counter = 0;
     }
 
+    c->lru_counter = 0;
+
     return 0;
 }
 
 static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
 {
     int i;
-    int min_count = INT_MAX;
+    uint64_t min_lru_counter = UINT64_MAX;
     int min_index = -1;
 
 
@@ -251,15 +254,9 @@ static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
             continue;
         }
 
-        if (c->entries[i].cache_hits < min_count) {
+        if (c->entries[i].lru_counter < min_lru_counter) {
             min_index = i;
-            min_count = c->entries[i].cache_hits;
-        }
-
-        /* Give newer hits priority */
-        /* TODO Check how to optimize the replacement strategy */
-        if (c->entries[i].cache_hits > 1) {
-            c->entries[i].cache_hits /= 2;
+            min_lru_counter = c->entries[i].lru_counter;
         }
     }
 
@@ -316,14 +313,10 @@ 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;
 
     /* And return the right table */
 found:
-    c->entries[i].cache_hits++;
     c->entries[i].ref++;
     *table = qcow2_cache_get_table_addr(bs, c, i);
 
@@ -356,6 +349,10 @@ int qcow2_cache_put(BlockDriverState *bs, Qcow2Cache *c, void **table)
     c->entries[i].ref--;
     *table = NULL;
 
+    if (c->entries[i].ref == 0) {
+        c->entries[i].lru_counter = ++c->lru_counter;
+    }
+
     assert(c->entries[i].ref >= 0);
     return 0;
 }