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

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Date:	Thu, 17 Jul 2008 09:11:11 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Alex Nixon (Intern)" <Alex.Nixon@...citrix.com>
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

(Don't top-post.)

Alex Nixon (Intern) wrote:
> I'm talking about total number of context switches - kernbench gets it
> from
>
> time -f "%c" make -j 4
>
> Dividing through it gives me a rate of around 250/sec (vs Peters 3000),
> but I've set CONFIG_HZ=100 (vs Peters 1000), so they don't wildly
> conflict.
>
> Well spotted :-)
>   

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.

    J
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