From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rick Jones Subject: Re: bizarre network timing problem Date: Tue, 06 Nov 2007 13:12:13 -0800 Message-ID: <4730D8AD.5060401@hp.com> References: <20071017205127.GA21334@codeblau.de> <47167BDE.4000103@redhat.com> <20071017220019.GA22765@codeblau.de> <471689BF.2040909@hp.com> <20071018094230.GA2978@codeblau.de> <4717964A.8080100@hp.com> <20071102221146.GA4354@codeblau.de> <472BA5BC.6000901@hp.com> <20071102223856.GA5121@codeblau.de> <472BAB90.8020306@hp.com> <20071102232332.GA5739@codeblau.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Cc: Chuck Ebbert , linux-kernel@vger.kernel.org, Netdev To: Felix von Leitner Return-path: Received: from palrel11.hp.com ([156.153.255.246]:35976 "EHLO palrel11.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754184AbXKFVMQ (ORCPT ); Tue, 6 Nov 2007 16:12:16 -0500 In-Reply-To: <20071102232332.GA5739@codeblau.de> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Felix von Leitner wrote: > Thus spake Rick Jones (rick.jones2@hp.com): > >>Past performance is no guarantee of current correctness :) And over an >>Ethernet, there will be a very different set of both timings and TCP >>segment sizes compared to loopback. > >>My guess is that you will find setting the lo mtu to 1500 a very >>interesting experiment. > > Setting the MTU on lo to 1500 eliminates the problem and gives me double > digit MB/sec throughput. I'm not in a position at the moment to test it as my IPoIB systems are offline, and not sure you are either, but I will note that with IPoIB bits circa OFED1.2 the default MTU for IPoIB goes up to 65520 bytes. If indeed the problem you were seeing was related to sub-mss sends and window probing and such, it might appear on IPoIB in addition to loopback. rick jones