linux-kernel - Re: rt-tests-0.82 available on github

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Date:	Wed, 21 Sep 2011 23:24:30 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Clark Williams <williams@...hat.com>
cc:	RT <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, Carsten Emde <ce@...g.ch>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>, John Kacur <jkacur@...hat.com>
Subject: Re: rt-tests-0.82 available on github

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
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