linux-kernel - Re: SMP performance degradation with sysbench

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Message-ID: <45F67796.4040508@yahoo.com.au>
Date:	Tue, 13 Mar 2007 21:06:14 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrea Arcangeli <andrea@...e.de>
CC:	Anton Blanchard <anton@...ba.org>, Rik van Riel <riel@...hat.com>,
	Lorenzo Allegrucci <l_allegrucci@...oo.it>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: SMP performance degradation with sysbench

Andrea Arcangeli wrote:
> On Tue, Mar 13, 2007 at 04:11:02PM +1100, Nick Piggin wrote:
> 
>>Hi Anton,
>>
>>Very cool. Yeah I had come to the conclusion that it wasn't a kernel
>>issue, and basically was afraid to look into userspace ;)
> 
> 
> btw, regardless of what glibc is doing, still the cpu shouldn't go
> idle IMHO. Even if we're overscheduling and trashing over the mmap_sem
> with threads (no idea if other OS schedules the task away when they
> find the other cpu in the mmap critical section), or if we've
> overscheduling with futex locking, the cpu usage should remain 100%
> system time in the worst case. The only explanation for going idle
> legitimately could be on HT cpus where HT may hurt more than help but
> on real multicore it shouldn't happen.
> 

Well ignoring the HT issue, I was seeing lots of idle time simply
because userspace could not keep up enough load to the scheduler.
There simply were fewer runnable tasks than CPU cores.

But it wasn't a case of all CPUs going idle, just most of them ;)

-- 
SUSE Labs, Novell Inc.
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