linux-kernel - Re: REGRESSION: Performance regressions from switching anon

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Date:	Wed, 15 Jun 2011 14:12:16 -0700
From:	Tim Chen <tim.c.chen@...ux.intel.com>
To:	Andi Kleen <ak@...ux.intel.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Shaohua Li <shaohua.li@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Russell King <rmk@....linux.org.uk>,
	Paul Mundt <lethal@...ux-sh.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	"Luck, Tony" <tony.luck@...el.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Namhyung Kim <namhyung@...il.com>,
	"Shi, Alex" <alex.shi@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: REGRESSION: Performance regressions from switching
 anon_vma->lock to mutex

On Wed, 2011-06-15 at 13:57 -0700, Andi Kleen wrote:
> >       7.44%        exim  [kernel.kallsyms]              [k] format_decode
> >                    |
> >                    --- format_decod
> 
> 
> This is a glibc issue. exim calls libdb and libdb asks sysconf for the 
> number of CPUs to tune
> its locking, and glibc reads /proc/stat.  And /proc/stat is incredible slow.
> 
> I would blame glibc, but in this case it's really the kernel to blame 
> for not providing proper
> interface.
> 
> This was my motivation for the sysconf() syscall I submitted some time ago.
> https://lkml.org/lkml/2011/5/13/455
> 
> Anyways a quick workaround is to use this LD_PRELOAD: 
> http://halobates.de/smallsrc/sysconf.c
> But it's not 100% equivalent.
> 

Thanks to Andi for providing the info.  We've used this workaround in
our testing so it will not mask true kernel scaling bottlenecks.

Tim

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