From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753675Ab2HDQeu (ORCPT ); Sat, 4 Aug 2012 12:34:50 -0400 Received: from mail-wg0-f44.google.com ([74.125.82.44]:58458 "EHLO mail-wg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751002Ab2HDQes (ORCPT ); Sat, 4 Aug 2012 12:34:48 -0400 Subject: Re: TCP Delayed ACK in FIN/ACK From: Eric Dumazet To: richard -rw- weinberger Cc: =?UTF-8?Q?S=C5=82awek?= Janecki , linux-kernel@vger.kernel.org, netdev@vger.kernel.org In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Date: Sat, 04 Aug 2012 18:34:43 +0200 Message-ID: <1344098083.9299.1503.camel@edumazet-glaptop> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2012-08-04 at 16:51 +0200, richard -rw- weinberger wrote: > On Sat, Aug 4, 2012 at 4:45 PM, Sławek Janecki wrote: > > I have a node.js client (10.177.62.7) requesting some data from http > > rest service from server (10.177.0.1). > > Client is simply using nodejs http.request() method (agent=false). > > Client is on Ubuntu 11.10 box. > > Why client sends FIN ACK after 475ms? Why so slow? He should send FIN > > ACK immediately. > > I have many situations like this. About 1% of whole traffic is request > > with delayed FIN ACK. > > Cpu idle on the client is about 99%, so nothing is draining CPU. > > How to debug this? What could it be? Is there any sysctl option I need to tune? > > I think this behaviour is the Delayed ACK feature of RFC1122 TCP stack. > > > > Link to tcpdump picture (done on a client machine) : > > http://i48.tinypic.com/35cpogx.png > > > > Can you tell why kernel delayed that FIN/ACK. > > In tcpflow data there is exacly one ACK per packet comming from server. > > Why kernel delayed client FIN/ACK. > > It could avoid sending ACK every 'data' packet. > > But it choose to delay FIN/ACK? > > Is this possible? Is this a bug? > > > > I've also posted question on stackexchange: > > http://stackoverflow.com/questions/11711218/client-sends-delayed-fin-ack-500ms-to-server > > > > Please help. > > CC'ing netdev. > I see nothing wrong in this tcpdump. You should strace the application instead. FIN/ACK is sent when client closes its socket (or calls shutdown()), and not in reply of FIN sent by the server. Kernel has no additional delay. I suspect your client is slow processing the server answer, then close() its socket _after_ data processing. Its possible tcp_send_fin() has to loop while allocating one skb under very high memory pressure, and it seems we have no counters for this case. But if it _was_ ever happening, you would have lot of messages in kernel log (dmesg) about alloc_skb_clone() failures.