Re: Costly Context Switches

[Arnaldo Carvalho de Melo]

Em Mon, Dec 13, 2010 at 03:25:54PM +0100, Andres Freund escreveu:
> 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 

Well, for that he can try 'perf diff', i.e.:

1. run perf record on dovecot-1.2
2. run perf record on dovecot-2.0
3. perf diff

> 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.

Well, I tried to answer his question:

"How can I find out WHY the context switches are happening?"

:-)

- Arnaldo
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