Refactor scanLaterList to fix latency issue (valkey-io#2514)

Author: asagege

src/defrag.c

@@ -377,43 +377,41 @@ static void defragLater(robj *obj) {
 static long scanLaterList(robj *ob, unsigned long *cursor, monotime endtime) {
     quicklist *ql = ob->ptr;
     quicklistNode *node;
-    long iterations = 0;
-    int bookmark_failed = 0;
     serverAssert(ob->type == OBJ_LIST && ob->encoding == OBJ_ENCODING_QUICKLIST);
 
+    /* Find starting node */
     if (*cursor == 0) {
-        /* if cursor is 0, we start new iteration */
         node = ql->head;
     } else {
         node = quicklistBookmarkFind(ql, "_AD");
         if (!node) {
-            /* if the bookmark was deleted, it means we reached the end. */
             *cursor = 0;
-            return 0;
+            return 0; /* either bookmark failed to create or deleted, skip this one and continue with other quicklists*/
         }
         node = node->next;
     }
 
-    (*cursor)++;
+    /* Process nodes until time expires or list ends */
     while (node) {
         activeDefragQuickListNode(ql, &node);
         server.stat_active_defrag_scanned++;
-        if (++iterations > 128 && !bookmark_failed) {
-            if (getMonotonicUs() > endtime) {
-                if (!quicklistBookmarkCreate(&ql, "_AD", node)) {
-                    bookmark_failed = 1;
-                } else {
-                    ob->ptr = ql; /* bookmark creation may have re-allocated the quicklist */
-                    return 1;
-                }
+
+        /* Check time limit after processing each node */
+        if (getMonotonicUs() > endtime) {
+            if (quicklistBookmarkCreate(&ql, "_AD", node)) {
+                ob->ptr = ql; /* bookmark creation may have re-allocated the quicklist */
+                (*cursor)++;
+                return 1;
             }
-            iterations = 0;
+            break; /* bookmark creation failed - skip this one and continue with other quicklists */
         }
         node = node->next;
     }
+
+    /* Completed processing all nodes */
     quicklistBookmarkDelete(ql, "_AD");
     *cursor = 0;
-    return bookmark_failed ? 1 : 0;
+    return 0;
 }
 
 static void scanLaterZsetCallback(void *privdata, void *element_ref) {

tests/unit/memefficiency.tcl

@@ -386,7 +386,7 @@ run_solo {defrag} {
             # number of total fields.  lists are progressively increasing sizes.
             set n 200000
 
-            perform_defrag_test $title populate {
+            perform_defrag_test $title latency 5 populate {
                 set rd [valkey_deferring_client]
                 $rd client reply off
                 set val [string repeat A 350]