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

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Message-ID: <0E902970173AF84089673FA54B7FE78A32906F@lonpexch01.citrite.net>
Date:	Fri, 18 Jul 2008 18:00:22 +0100
From:	"Alex Nixon (Intern)" <Alex.Nixon@...citrix.com>
To:	"Andi Kleen" <andi@...stfloor.org>,
	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"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>
Subject: RE: Large increase in context switch rate



> -----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 the majority of the increase down to some time between
2.6.18 and 2.6.19 - kernbench results are:

Kernel version    Elapsed    User    System    Context Switches
V2.6.18             232      427       28          32713
V2.6.19	        232      429       28          52812

NOHZ and highres timers are disabled wherever available, HZ=100, and
CONFIG_PREEMPT_VOLUNTARY=y

Will continue further on Monday.

- Alex
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