From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mark Hounschell Subject: Re: Latency Problems with 2.6.31/PREEMPT_RT on a 1.2GHz Celeron Date: Mon, 21 Dec 2009 05:05:53 -0500 Message-ID: <4B2F4881.1070203@cfl.rr.com> References: <4B2BB45A.9040909@mantatest.com> <4B2CBA28.1030405@cfl.rr.com> <200912211216.20588@zigzag.lvk.cs.msu.su> Reply-To: dmarkh@cfl.rr.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Vladimir Cotfas , linux-rt-users@vger.kernel.org, Scott Gilbertson To: "Nikita V. Youshchenko" Return-path: Received: from cdptpa-omtalb.mail.rr.com ([75.180.132.120]:65188 "EHLO cdptpa-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752860AbZLUKFz (ORCPT ); Mon, 21 Dec 2009 05:05:55 -0500 In-Reply-To: <200912211216.20588@zigzag.lvk.cs.msu.su> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On 12/21/2009 04:16 AM, Nikita V. Youshchenko wrote: >> On 12/18/2009 11:56 AM, Vladimir Cotfas wrote: >>> All, >>> >>> I have a a driver that must process an interrupt every 100 uS without >>> missing any. I am having latency issues and I am looking for some help >>> debugging my problem. >> >> You'll probably never get this working reliably with the RT kernel. >> Latencies are more deterministic with the RT kernel but slightly higher. >> You'll have better luck with a vanilla kernel running your app in run >> level 4. And then even better luck running on an SMP machine isolating >> your process and IRQ to a single processor. And stay away from the VGA >> console. You have better luck with faster hardware too. > > We have seen <20us avg latencies on RT kernel on slower powerpc-based SoC. > > The only problem is that sometimes we got single-shot latency >150 us - > that happened when sched tick was running for >100 us with interrupts > disabled. > Averages are one thing. Guaranteeing he doesn't miss _any_ and still has time to do whatever he needs to do before the next one, is different. Local timer interrupts alone are enough to kill ya. Mark