Do not update the LRU info on key lookup when we have a saving child. With...

Do not update the LRU info on key lookup when we have a saving child. With this trivial change the additional memory used while saving with a background child in presence of many read operations is zero.

src/db.c

@@ -11,7 +11,10 @@ robj *lookupKey(redisDb *db, robj *key) {
     if (de) {
         robj *val = dictGetEntryVal(de);
 
-        /* Update the access time for the aging algorithm. */
+        /* Update the access time for the aging algorithm.
+         * Don't do it if we have a saving child, as this will trigger
+         * a copy on write madness. */
+        if (server.bgsavechildpid == -1 && server.bgrewritechildpid == -1)
             val->lru = server.lruclock;
 
         if (server.vm_enabled) {