From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965414AbXC0UlG (ORCPT ); Tue, 27 Mar 2007 16:41:06 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S965417AbXC0UlF (ORCPT ); Tue, 27 Mar 2007 16:41:05 -0400 Received: from ug-out-1314.google.com ([66.249.92.175]:27143 "EHLO ug-out-1314.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965416AbXC0UlD convert rfc822-to-8bit (ORCPT ); Tue, 27 Mar 2007 16:41:03 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=received:message-id:date:from:sender:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references:x-google-sender-auth; b=av07FTEvt04kHNk3n0Ynh7cSUXeiiwqZb7bWT8migcNmAZgU2inqsJIIDv1s+sxS9QafrXWQ2FTXTPO+JhFGklPUDojZ29jdSdGpEp7OWLK7bCeaFhSf9Yp7V+UX2Y+pd4p1L7ZiaciPOzU3H1d6Nk8SF/Xi5BF2bfzSQcKgtfo= Message-ID: <75b66ecd0703271341x34463c49w64bdac21a9741355@mail.gmail.com> Date: Tue, 27 Mar 2007 16:41:01 -0400 From: "Lee Revell" To: John Subject: Re: 2.6.20.3-rt8 - DMA suffers excessive delay periodically Cc: linux-kernel@vger.kernel.org, tglx@timesys.com, mingo@elte.hu, johnstul@us.ibm.com, akpm@osdl.org, linux-apps@vger.kernel.org In-Reply-To: <46094728.40300@free.fr> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8BIT Content-Disposition: inline References: <46094728.40300@free.fr> X-Google-Sender-Auth: 090166f26b4f1be4 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 3/27/07, John wrote: > I'm runnning 2.6.20.3 patched with -rt8 (and glibc 2.3.6). > http://people.redhat.com/mingo/realtime-preempt/older/patch-2.6.20-rt8 > > I've written a program to highlight a phenomenon I don't understand. > > This system includes a PCI board that provides data at ~38 Mbit/s. > I request data in lumps of 1316 bytes. > In other words, a new request should complete every 277 µs. > > However, if I time stamp each packet as soon as I receive it, I notice > that almost exactly every 2 seconds, the packet is received 30-100 > microseconds late. Sounds like periodic SMM traps. There's not much you can do about them other than to avoid such hardware if you have tight RT constraints. The RTAI people have some code to disable SMM but it does not work on all chipsets and may render some of your hardware inoperable. Is this a laptop? They are plagued with SMM problems... Lee