linux-kernel - Re: SMP performance degradation with sysbench

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Date:	Tue, 13 Mar 2007 21:37:54 +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 09:06:14PM +1100, Nick Piggin wrote:
> 
>>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.
> 
> 
> When you said idle I thought idle and not waiting for I/O. Waiting for
> I/O would be hardly a kernel issue ;). If they're not waiting for I/O
> and they're not scheduling in userland with nanosleep/pause, the cpu
> shouldn't go idle. Even if they're calling sched_yield in a loop the
> cpu should account for zero idle time as far as I can tell.

Well it wasn't iowait time. From Anton's analysis, I would probably
say it was time waiting for either the glibc malloc mutex or MySQL
heap mutex.

-- 
SUSE Labs, Novell Inc.
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