Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756111AbXI0Oeo (ORCPT ); Thu, 27 Sep 2007 10:34:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757212AbXI0OeY (ORCPT ); Thu, 27 Sep 2007 10:34:24 -0400 Received: from smtp2.linux-foundation.org ([207.189.120.14]:49706 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756623AbXI0OeW (ORCPT ); Thu, 27 Sep 2007 10:34:22 -0400 Date: Thu, 27 Sep 2007 07:33:46 -0700 From: Stephen Hemminger To: "Majumder, Rajib" Cc: "'netdev@vger.kernel.org'" , "'linux-kernel@vger.kernel.org'" Subject: Re: TCP Spike Message-ID: <20070927073346.42d790dd@freepuppy.rosehill> In-Reply-To: References: Organization: Linux Foundation X-Mailer: Claws Mail 3.0.0 (GTK+ 2.10.14; x86_64-redhat-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1627 Lines: 38 On Thu, 27 Sep 2007 11:58:01 +0800 "Majumder, Rajib" wrote: > Hi, > > We have observed 40ms latency spikes in TCP connections in "burst" type of traffic. This affects regular TCP sockets. We observed this issue in kernels of 2.4.21 and kernel 2.6.5. Unfortunately, 2.6.5 is out of my short term memory at this point. I do remember that 2.6.5 used BIC for congestion control, and there were some math errors in the congestion control logic that caused it to be way to aggressive. > > Aparently, this seems to be fixed in 2.6.19. > > Can someone throw some light on this? My guess is that the addition of the SACK hinting might be the major win. The code takes 3 passes over the SACK list, so with large outstanding data that was a major bottleneck, not sure if it was 4ms worth though. > > Is this a congestion control/avoidance issue? What congestion control algorithm is used before 2.6.8? Default congestion control in early 2.6 was BIC, then after CUBIC stabilized it was made the default in 2.6.19. Another thing that may cause changes in latency is Appropriate Byte Counting (ABC). It was added in 2.6.14, but then turned off by default in 2.6.18. The problem is that ABC caused performance problems with some applications that sent messages as many small writes. -- Stephen Hemminger - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/