linux-kernel - Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets

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Date:	Tue, 25 Sep 2012 14:23:13 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Nikolay Ulyanitsky <lystor@...il.com>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Mon, Sep 24, 2012 at 07:44:17PM +0200, Peter Zijlstra wrote:
> On Mon, 2012-09-24 at 18:54 +0200, Peter Zijlstra wrote:
> > But let me try and come up with the list thing, I think we've
> > actually got that someplace as well. 
> 
> OK, I'm sure the below can be written better, but my brain is gone for
> the day...
> 

It crashes on boot due to the fact that you created a function-scope variable
called sd_llc in select_idle_sibling() and shadowed the actual sd_llc you
were interested in. Result: dereferenced uninitialised pointer and kaboom.
Trivial to fix so it boots at least.

This is a silly test for a scheduler patch but as "sched: Avoid SMT siblings
in select_idle_sibling() if possible" regressed 2% back in 3.2, it seemed
reasonable to retest with it.

KERNBENCH
                               3.6.0                 3.6.0                 3.6.0
                         rc6-vanilla    rc6-mikebuddy-v1r1  rc6-idlesibling-v1r1
User    min         352.47 (  0.00%)      351.77 (  0.20%)      352.30 (  0.05%)
User    mean        353.10 (  0.00%)      352.78 (  0.09%)      352.77 (  0.09%)
User    stddev        0.41 (  0.00%)        0.56 (-36.13%)        0.35 ( 15.16%)
User    max         353.55 (  0.00%)      353.43 (  0.03%)      353.31 (  0.07%)
System  min          34.86 (  0.00%)       34.83 (  0.09%)       35.37 ( -1.46%)
System  mean         35.35 (  0.00%)       35.29 (  0.16%)       35.63 ( -0.80%)
System  stddev        0.41 (  0.00%)        0.40 (  0.10%)        0.15 ( 62.26%)
System  max          35.94 (  0.00%)       36.05 ( -0.31%)       35.81 (  0.36%)
Elapsed min         110.18 (  0.00%)      109.65 (  0.48%)      110.04 (  0.13%)
Elapsed mean        110.21 (  0.00%)      109.75 (  0.42%)      110.15 (  0.06%)
Elapsed stddev        0.03 (  0.00%)        0.07 (-167.83%)        0.09 (-207.56%)
Elapsed max         110.26 (  0.00%)      109.86 (  0.36%)      110.26 (  0.00%)
CPU     min         352.00 (  0.00%)      353.00 ( -0.28%)      352.00 (  0.00%)
CPU     mean        352.00 (  0.00%)      353.00 ( -0.28%)      352.00 (  0.00%)
CPU     stddev        0.00 (  0.00%)        0.00 (  0.00%)        0.00 (  0.00%)
CPU     max         352.00 (  0.00%)      353.00 ( -0.28%)      352.00 (  0.00%)

mikebuddy-v1r1 is Mike's patch that just got reverted. idlesibling is
Peters patch. "Elapsed mean" time is the main value of interest. Mike's
patch gains 0.42% which is less than the 2% lost but at least the gain is
outside the noise. idlesibling make very little difference. "System mean"
is also interesting because even though idlesibling shows a "regression", it
also shows that the variation between runs is reduced. That might indicate
that fewer cache misses are being incurred in the select_idle_sibling()
code although that is a bit of a leap of faith.

The machine is in use at the moment but I'll queue up a test this evening to
gather a profile to confirm time is even being spent in select_idle_sibling()
Just because 2% was lost in select_idle_sibling() back in 3.2 does not
mean squat now.

-- 
Mel Gorman
SUSE Labs
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