[Qemu-devel] [PATCH 4/6] qcow2: remove qcow2_cache_find_entry_to_replace()

[Alberto Garcia]

A cache miss means that the whole array was traversed and the entry
we were looking for was not found, so there's no need to traverse it
again in order to select an entry to replace.

Signed-off-by: Alberto Garcia <address@hidden>
---
 block/qcow2-cache.c | 45 ++++++++++++++++-----------------------------
 1 file changed, 16 insertions(+), 29 deletions(-)

diff --git a/block/qcow2-cache.c b/block/qcow2-cache.c
index 477a209..e1bba20 100644
--- a/block/qcow2-cache.c
+++ b/block/qcow2-cache.c
@@ -231,51 +231,38 @@ int qcow2_cache_empty(BlockDriverState *bs, Qcow2Cache *c)
     return 0;
 }
 
-static int qcow2_cache_find_entry_to_replace(Qcow2Cache *c)
-{
-    int i;
-    uint64_t min_lru_counter = UINT64_MAX;
-    int min_index = -1;
-
-
-    for (i = 0; i < c->size; i++) {
-        if (c->entries[i].ref) {
-            continue;
-        }
-
-        if (c->entries[i].lru_counter < min_lru_counter) {
-            min_index = i;
-            min_lru_counter = c->entries[i].lru_counter;
-        }
-    }
-
-    if (min_index == -1) {
-        /* This can't happen in current synchronous code, but leave the check
-         * here as a reminder for whoever starts using AIO with the cache */
-        abort();
-    }
-    return min_index;
-}
-
 static int qcow2_cache_do_get(BlockDriverState *bs, Qcow2Cache *c,
     uint64_t offset, void **table, bool read_from_disk)
 {
     BDRVQcowState *s = bs->opaque;
     int i;
     int ret;
+    uint64_t min_lru_counter = UINT64_MAX;
+    int min_lru_index = -1;
 
     trace_qcow2_cache_get(qemu_coroutine_self(), c == s->l2_table_cache,
                           offset, read_from_disk);
 
     /* Check if the table is already cached */
     for (i = 0; i < c->size; i++) {
-        if (c->entries[i].offset == offset) {
+        const Qcow2CachedTable *t = &c->entries[i];
+        if (t->offset == offset) {
             goto found;
         }
+        if (t->ref == 0 && t->lru_counter < min_lru_counter) {
+            min_lru_counter = t->lru_counter;
+            min_lru_index = i;
+        }
+    }
+
+    if (min_lru_index == -1) {
+        /* This can't happen in current synchronous code, but leave the check
+         * here as a reminder for whoever starts using AIO with the cache */
+        abort();
     }
 
-    /* If not, write a table back and replace it */
-    i = qcow2_cache_find_entry_to_replace(c);
+    /* Cache miss: write a table back and replace it */
+    i = min_lru_index;
     trace_qcow2_cache_get_replace_entry(qemu_coroutine_self(),
                                         c == s->l2_table_cache, i);
     if (i < 0) {
-- 
2.1.4