linux-kernel - Re: Large increase in context switch rate

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Message-Id: <200807241126.48364.nickpiggin@yahoo.com.au>
Date:	Thu, 24 Jul 2008 11:26:48 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"Alex Nixon (Intern)" <Alex.Nixon@...citrix.com>
Cc:	"Andi Kleen" <andi@...stfloor.org>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	"Ian Campbell" <Ian.Campbell@...citrix.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: Large increase in context switch rate

On Wednesday 23 July 2008 19:34, Alex Nixon (Intern) wrote:
> > -----Original Message-----
> > From: Andi Kleen [mailto:andi@...stfloor.org]
> > Sent: 17 July 2008 22:43
> > To: Jeremy Fitzhardinge
> > Cc: Alex Nixon (Intern); Peter Zijlstra; Ingo Molnar; Linux
> > Kernel Mailing List; Ian Campbell
> > Subject: Re: Large increase in context switch rate
> >
> > Jeremy Fitzhardinge <jeremy@...p.org> writes:
> > > OK, but that still doesn't account for the relatively large increase
> > > from 2.6.18 -> 2.6.26.  You're using HZ=100 in both cases,
> >
> > I presume.
> >
> > > The other variable is NOHZ and highres timers.  You could
> >
> > try turning
> >
> > > those off in 2.6.26.  Also, CONFIG_PREEMPT could well make a
> > > difference.  2.6.18-xen doesn't support CONFIG_PREEMPT at all, but
> > > pvops(-xen) does.
> >
> > If it's that easily reproducible you could just bisect it?
> >
> > -Andi
>
> I've bisected down to commit ba52de123d454b57369f291348266d86f4b35070 -
> [PATCH] inode-diet.  Before that kernbench consistently reports about
> 35k context switches (total), and after that commit about 53k.  The
> benchmarks are being run on a tmpfs.  I've verified the results on a
> different machine, albeit with an almost identical setup (the same
> kernels and debian distro, kernbench version, and benchmarking a build
> of the same source).
>
> Seems to be a mystery why that patch is (seemingly) the culprit...anyone
> have any ideas?  Maybe there's some other variable I'm not keeping
> constant?

Weird. It could possibly be triggering some different userspace behaviour
if blocksize reporting has changed anywhere. strace might help there.

Interesting if you could post the top results of profile=schedule for
a kernel with and without the patch.

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