Hashtable: Optimize replace large strings

Keep the old object in the database and just update its pointer
to the new string. Store the old ptr in the val to be freed.

Signed-off-by: Viktor Söderqvist <[email protected]>

src/db.c

@@ -314,8 +314,7 @@ static valkey *dbSetValue(serverDb *db, robj *key, robj *val, int overwrite, voi
     }
     serverAssertWithInfo(NULL, key, oldref != NULL);
     valkey *old = *oldref;
-
-    val->lru = old->lru;
+    valkey *new;
 
     if (overwrite) {
         /* VM_StringDMA may call dbUnshareStringValue which may free val, so we
@@ -331,16 +330,36 @@ static valkey *dbSetValue(serverDb *db, robj *key, robj *val, int overwrite, voi
         /* Because of VM_StringDMA, old may be changed, so we need get old again */
         old = *oldref;
     }
-    /* Replace the old value at its location in the key space. */
-    long long expire = objectGetExpire(old);
-    valkey *new = objectSetKeyAndExpire(val, key->ptr, expire);
-    *oldref = new;
-    /* Replace the old value at its location in the expire space. */
-    if (expire >= 0) {
-        int dict_index = getKVStoreIndexForKey(key->ptr);
-        void **expireref = kvstoreHashtableFindRef(db->expires, dict_index, key->ptr);
-        serverAssert(expireref != NULL);
-        *expireref = new;
+
+    if ((old->refcount == 1 && old->encoding != OBJ_ENCODING_EMBSTR) &&
+        (val->refcount == 1 && val->encoding != OBJ_ENCODING_EMBSTR)) {
+        /* Keep old object in the database. Just swap it's ptr, type and
+         * encoding with the content of val. */
+        int tmp_type = old->type;
+        int tmp_encoding = old->encoding;
+        void *tmp_ptr = old->ptr;
+        old->type = val->type;
+        old->encoding = val->encoding;
+        old->ptr = val->ptr;
+        val->type = tmp_type;
+        val->encoding = tmp_encoding;
+        val->ptr = tmp_ptr;
+        /* Return the old object as new. Set old to val to be freed below. */
+        new = old;
+        old = val;
+    } else {
+        /* Replace the old value at its location in the key space. */
+        val->lru = old->lru;
+        long long expire = objectGetExpire(old);
+        new = objectSetKeyAndExpire(val, key->ptr, expire);
+        *oldref = new;
+        /* Replace the old value at its location in the expire space. */
+        if (expire >= 0) {
+            int dict_index = getKVStoreIndexForKey(key->ptr);
+            void **expireref = kvstoreHashtableFindRef(db->expires, dict_index, key->ptr);
+            serverAssert(expireref != NULL);
+            *expireref = new;
+        }
     }
     /* For efficiency, let the I/O thread that allocated an object also deallocate it. */
     if (tryOffloadFreeObjToIOThreads(old) == C_OK) {