linux-kernel - Re: Scheduler regression: Too frequent timer interrupts(?)

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Message-Id: <1240001585.27840.24.camel@laptop>
Date:	Fri, 17 Apr 2009 22:53:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: Scheduler regression: Too frequent timer interrupts(?)

On Fri, 2009-04-17 at 16:34 -0400, Christoph Lameter wrote:
> On Fri, 17 Apr 2009, Peter Zijlstra wrote:
> 
> > > often must occur in parallel on multiple cores. Processing is delayed if
> > > any of the cores encounters a delay due to OS noise.
> >
> > So you have hard deadlines in the order of us? Are these SCHED_FIFO
> > tasks or SCHED_OTHER?
> 
> SCHED_FIFO has the effect of removing all the involuntary context switches
> but it does not effect the other interrutions.

OK, that's good to know.

> > Your Xeon is a core2 class machine and should have relatively stable
> > TSC, however its also a dual socket, which I think defeats the
> > stable-ness.
> 
> > What clocksource do you have?
> >
> > cat /sys/devices/system/clocksource/clocksource0/current_clocksource
> 
> cat /sys/devices/system/clocksource/clocksource0/current_clocksource
> tsc

Ah, good. I could measure a significant difference on my testbox between
tsc and acpi_pm.

> > Also, looking over the rest of the scheduler tick code, I can't really
> > see what would be so expensive.
> 
> The expensiveness may be fine if we can limit the number of occurences.
> Maybe the histograms for those releases give more insight.

Yeah, curious to see what .22 looks like -- readprofile/oprofile runs of
the kernel for those might be interesting as well.

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