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Date: Tue, 20 Nov 2007 09:57:04 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Pavel Machek <pavel@ucw.cz>
Cc: kernel list <linux-kernel@vger.kernel.org>, tglx@linutronix.de
Subject: Re: nohz and strange sleep latencies
Message-ID: <20071120085704.GA19950@elte.hu>
References: <20071119203152.GA1772@elf.ucw.cz> <20071119205555.GA28696@elte.hu> <20071119211142.GA1857@elf.ucw.cz>
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* Pavel Machek <pavel@ucw.cz> wrote:

> > > 0.00user 0.00system 0.08 (0m0.081s) elapsed 3.71%CPU
> > > 0.00user 0.00system 0.01 (0m0.013s) elapsed 23.33%CPU
> > > 0.00user 0.00system 0.01 (0m0.019s) elapsed 15.92%CPU
> > > 
> > > nohz=off helps a lot. while true; do time sleep 0.0; done does not 
> > > have unexpected latencies.
> > 
> > does hpet=disable on the boot line also help? (without nohz=off)
> 
> hpet-disable helps.. a bit. 200msec latencies are gone. (What is used 
> for wakeups in this case?)
> 
> pavel@amd:~$ while true; do time sleep 0.01; done
> 0.00user 0.00system 0.01 (0m0.013s) elapsed 22.96%CPU
> 0.00user 0.00system 0.01 (0m0.018s) elapsed 11.05%CPU
> 0.00user 0.00system 0.01 (0m0.013s) elapsed 15.54%CPU

could you run this while doing the sleep test:

  http://people.redhat.com/mingo/cfs-scheduler/tools/cfs-debug-info.sh

and send us the output? (Enabling CONFIG_TIMER_STATS, CONFIG_SCHED_DEBUG 
and CONFIG_SCHEDSTATS would maximize the amount of information.)

	Ingo