linux-kernel - Re: Dynticks Causing High Context Switch Rate in ksoftirqd

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Message-ID: <20071126213847.4768b1f2@laptopd505.fenrus.org>
Date:	Mon, 26 Nov 2007 21:38:47 -0800
From:	Arjan van de Ven <arjan@...radead.org>
To:	Robert Hancock <hancockr@...w.ca>
Cc:	bdupree@...hfinesse.com, linux-kernel@...r.kernel.org
Subject: Re: Dynticks Causing High Context Switch Rate in ksoftirqd

On Mon, 26 Nov 2007 22:36:17 -0600
Robert Hancock <hancockr@...w.ca> wrote:

> bdupree@...hfinesse.com wrote:
> > Question: Why is ksoftirqd eating about 5 to 10 percent of my CPU
> > on an idle system? The problem occurs if I config the kernel with
> > tickless support (i.e. CONFIG_TICK_ONESHOT=y).  (Thanks to
> > "oprofile" for putting me onto this.)
> > 
> > I have noted this same problem on kernel versions: 2.6.23.1,
> > 2.6.23.8 and 2.6.23.9
> > 
> > **************************************************************************
> > *** Output from "vmstat -n 1 10" -- Note very high context switch
> > rate *** *** This is on a idle
> > machine!                                         ***
> > **************************************************************************
> > 
> > procs -----------memory---------- ---swap-- -----io---- --system--
> > ----cpu----
> >  r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs
> > us sy id wa
> >  0  0      0 1925556   4768 116104    0    0   124     2    6
> > 7538  1  2 96  1
> >  0  0      0 1925556   4768 116104    0    0     0     0    2
> > 147329  0  1 99  0
> 
> What did oprofile show? It should be able to narrow down what 
> function(s) are responsible for the CPU usage..
> 

or better, what does powertop version 1.9 show?
that tends to show tickless wakeup artifacts quite nicely


-- 
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