From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Gleixner Subject: Re: rt-tests-0.82 available on github Date: Wed, 21 Sep 2011 23:24:30 +0200 (CEST) Message-ID: References: <20110921155455.61ab4745@redhat.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: RT , LKML , Carsten Emde , =?ISO-8859-15?Q?Uwe_Kleine-K=F6nig?= , John Kacur To: Clark Williams Return-path: Received: from www.linutronix.de ([62.245.132.108]:51206 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750777Ab1IUVYd (ORCPT ); Wed, 21 Sep 2011 17:24:33 -0400 In-Reply-To: <20110921155455.61ab4745@redhat.com> Sender: linux-rt-users-owner@vger.kernel.org List-ID: On Wed, 21 Sep 2011, Clark Williams wrote: > While kernel.org is sorting out the security stuff, the rt-tests code > may be pulled from: > > git://github.com/clrkwllms/rt-tests.git > > Note that we're now at version 0.82. Presently I only have the git > archive available (no tarballs). > > While investigating latency spikes in the 3.0.x-rt kernels, Thomas > spotted a case where an Intel quad-core Xeon was going into deep > sleep states and were all fighting to come out of sleep at the same > time (and consequently causing a big latency spike in cyclictest). > > While trying to figure out how to prevent deep cstates I remembered a > conversation I had with Arjan at the last Plumbers conference in > Boston. He mentioned the /dev/cpu_dma_latency interface to the power > managment code and that if you opened it and wrote a zero to it, you > effectively put the system into "idle=poll" mode until you closed the > file descriptor (see: Documentation/power/pm_qos_interface.txt). > > I've added a set_latency_target() function to cyclictest that by > default opens /dev/cpu_dma_latency and writes a zero to it, then holds > the file descriptor open for the duration of the cyclictest run. This > made a *huge* difference on some Intel Xeon's. Without this option, when > I was running cyclictest with the -b option, I saw latencies over > 300us. When I added it, while tracing I never saw a latency over 30us. > Turning of -b, I never saw it go over 10us. I am doing further testing > now with other x86_64 systems. > > Of course this is very architecture specific, so YMMV, but I think it's > a valid mechanism to be used when measuring latency and I believe a > technique that many latency-sensitive applications might use to good > effect. This is not a big surprise as the kernel got more agressive going into deep C-states since 2.6.33-rt especially with "intel_idle" on modern (Nehalem+) cpus. Thanks, tglx