linux-kernel - tbench regression with 2.6.32-rc1

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Message-Id: <1255081889.25078.42.camel@ymzhang>
Date:	Fri, 09 Oct 2009 17:51:29 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: tbench regression with 2.6.32-rc1

Comparing with 2.6.31's results, tebench has some regression with
2.6.32-rc1.
COmmandline to start tbench:
#./tbench_srv &
#./tbench -t 600 CPU_NUM*2 127.0.0.1  #Use real cpu num to replace CPU_NUM
So start 2 client processes per cpu.

1) On 4*4 core tigerton: 30%;
2) On 2*4 core stoakley: 15%;
3) On 2*8 core Nehalem: 6%.

As there are couple of patches which try to turn on/off some sched domain
flags such like SD_BALANCE_WAKE, I used some walkaround to bisect it.
On tigerton, below patch is captured. 
commit 59abf02644c45f1591e1374ee7bb45dc757fcb88
Author: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Date:   Wed Sep 16 08:28:30 2009 +0200

    sched: Add SD_PREFER_LOCAL

The patch reverting is not clean, so I did some testing by turning on/off
some domain flags and sched_feaures manually.

1) On tigerton: if SD_PREFER_LOCAL=0 (disable it), the regression becomes about 2%.
2) On stoakley: if SD_PREFER_LOCAL=0 (disable it), the regression becomes about 4%.
3) On Nehalem: Above method couldn't improve result. I'm still checking it.

I also tried to turn on/off FAIR_SLEEPERS and GENTLE_FAIR_SLEEPERS. It seems they
has limited impact on tbench. I need double check these 2 flags.

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