linux-kernel - Re: Costly Context Switches

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Date:	Mon, 13 Dec 2010 15:25:54 +0100
From:	Andres Freund <andres@...razel.de>
To:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Cc:	Ralf Hildebrandt <Ralf.Hildebrandt@...rite.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Costly Context Switches

On Monday 13 December 2010 14:51:04 Arnaldo Carvalho de Melo wrote:
> Em Sun, Dec 12, 2010 at 08:07:07PM +0100, Andres Freund escreveu:
> > On Sunday 12 December 2010 16:11:12 Ralf Hildebrandt wrote:
> > > I recently made a parallel installation of dovecot-2.0 on my mailbox
> > > server, which is running dovecot-1.2 without any problems whatsoever.
> > > 
> > > Using dovecot-2.0 on the same hardware, same kernel, with the same
> > > users and same mailboxes and usage behaviour results in an immense
> > > increase in the load numbers.
> > > 
> > > Switching back to 1.2 results in a immediate decrease of the load back
> > > to "normal" numbers.
> > > 
> > > This is mainly due to a 10-20 fold increase of the number of context
> > > switches. The same problem has been reported independently by Cor
> > > Bosman of XS4All, on different hardware (64bit instead of 32bit,
> > > real hardware instead of virtual hardware).
> > > 
> > > So, now the kernel related question: How can I find out WHY the
> > > context switches are happening? Are there any "in kernel" statistics
> > > I could look at?
> > 
> > "strace" or "perf trace syscall-counts" would be a good start.
> 
> Better to record just "cs" (Context Switches) events and also to collect
> callchains when those events take place:
Hm. Its also a good starting point but it may be harder to see the differences 
between dovecot-2.0 and dovecot-1.2 that way because its harder to see if its 
the usage being different causing the problem (i.e. calling in a different 
order, trashing caches) or if its the amount of syscalls that changed. But I 
agree that both are very usefull analyze problems like that.

Andres
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