linux-kernel - Re: nohz and strange sleep latencies

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Date:	Tue, 20 Nov 2007 09:57:04 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Pavel Machek <pavel@....cz>
Cc:	kernel list <linux-kernel@...r.kernel.org>, tglx@...utronix.de
Subject: Re: nohz and strange sleep latencies


* Pavel Machek <pavel@....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
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