linux-kernel - Re: Performance regression of specjbb2005/aim7 with 2.6.29-rc1

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Message-Id: <1231839464.14655.12.camel@marge.simson.net>
Date:	Tue, 13 Jan 2009 10:37:44 +0100
From:	Mike Galbraith <efault@....de>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, ming.m.lin@...el.com,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: Performance regression of specjbb2005/aim7 with 2.6.29-rc1

On Tue, 2009-01-13 at 16:57 +0800, Zhang, Yanmin wrote:
> Comparing with 2.6.28's results, specjbb2005 has about 7% regression with 2.6.29-rc1
> on my a couple of x86_64 machines. aim7 has about 1.7% regression.
> 
> Ming did a quick bisect with aim7 and located below patch.
> 
> commit 0a582440ff546e2c6610d1acec325e91b4efd313
> Author: Mike Galbraith <efault@....de>
> Date:   Fri Jan 2 12:16:42 2009 +0100
> 
>     sched: fix sched_slice()
>     
>     Impact: fix bad-interactivity buglet
>     
>     Fix sched_slice() to emit a sane result whether a task is currently
>     enqueued or not.
>     
>     Signed-off-by: Mike Galbraith <efault@....de>
>     Tested-by: Jayson King <dev@...sonking.com>
>     Signed-off-by: Ingo Molnar <mingo@...e.hu>
> 
> 
> After we revert the patch, aim7 regression disappeared. specjbb2005 regression becomes
> less than 1.5% on 8-core stokley and disappears on 16-core tigerton. I don't know what
> causes the last 1.5% regression.
> 
> As tbench has about 5% improvement and oltp(mysql+sysbench) has 5% improvement, we also tested
> to make sure such improvement isn't related to above patch. volanoMark's improvement is also not
> related to the patch. So it seems safe to revert it.

No, it's not safe to just revert.  You can replace it with something
else, but as long as sched_slice() is called for unqueued tasks, it must
emit sane slices, otherwise you can experience a latency-hit-from-hell.

See thread: problem with "sched: revert back to per-rq vruntime"?

	-Mike

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