From 7998a2a05f011fa2898be47ec44d24163988e526 Mon Sep 17 00:00:00 2001
From: Yuan Wang <yuan.wang@redis.com>
Date: Fri, 23 May 2025 19:21:29 +0800
Subject: [PATCH] To reduce memory churn during client list transfer (#14068)

When we transfer clients between IO thread and main thread, the creation
and destruction of list nodes also consume some CPU. In this commit, we
reuse list nodes to avoid this issue.
---
 src/iothread.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/src/iothread.c b/src/iothread.c
index cf28f6db2f..a170317911 100644
--- a/src/iothread.c
+++ b/src/iothread.c
@@ -51,17 +51,18 @@ void enqueuePendingClientsToMainThread(client *c, int unbind) {
     if (unbind) connUnbindEventLoop(c->conn);
     /* Just skip if it already is transferred. */
     if (c->io_thread_client_list_node) {
+        IOThread *t = &IOThreads[c->tid];
         /* If there are several clients to process, let the main thread handle them ASAP.
          * Since the client being added to the queue may still need to be processed by
          * the IO thread, we must call this before adding it to the queue to avoid
          * races with the main thread. */
-        sendPendingClientsToMainThreadIfNeeded(&IOThreads[c->tid], 1);
-        /* Remove the client from clients list of IO thread. */
-        listDelNode(IOThreads[c->tid].clients, c->io_thread_client_list_node);
-        c->io_thread_client_list_node = NULL;
+        sendPendingClientsToMainThreadIfNeeded(t, 1);
         /* Disable read and write to avoid race when main thread processes. */
         c->io_flags &= ~(CLIENT_IO_READ_ENABLED | CLIENT_IO_WRITE_ENABLED);
-        listAddNodeTail(IOThreads[c->tid].pending_clients_to_main_thread, c);
+        /* Remove the client from IO thread, add it to main thread's pending list. */
+        listUnlinkNode(t->clients, c->io_thread_client_list_node);
+        listLinkNodeTail(t->pending_clients_to_main_thread, c->io_thread_client_list_node);
+        c->io_thread_client_list_node = NULL;
     }
 }
 
@@ -544,7 +545,8 @@ int processClientsFromMainThread(IOThread *t) {
 
         /* Link client in IO thread clients list first. */
         serverAssert(c->io_thread_client_list_node == NULL);
-        listAddNodeTail(t->clients, c);
+        listUnlinkNode(t->processing_clients, ln);
+        listLinkNodeTail(t->clients, ln);
         c->io_thread_client_list_node = listLast(t->clients);
 
         /* The client now is in the IO thread, let's free deferred objects. */
@@ -575,7 +577,8 @@ int processClientsFromMainThread(IOThread *t) {
             }
         }
     }
-    listEmpty(t->processing_clients);
+    /* All clients must are processed. */
+    serverAssert(listLength(t->processing_clients) == 0);
     return processed;
 }