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

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Message-ID: <4DFA6442.9000103@linux.intel.com>
Date:	Thu, 16 Jun 2011 13:14:58 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Shaohua Li <shaohua.li@...el.com>,
	Andrew Morton <akpm@...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


> /proc/stat may be slow, but it's not slower than doing real work -
> unless you call it millions of times.


I haven't analyzed it in detail, but I suspect it's some cache line 
bounce, which
can slow things down quite a lot.  Also the total number of invocations
is quite high (hundreds of messages per core * 32 cores)

Ok even with cache line bouncing it's suspicious.

> And you didn't actually look at glibc sources, did you?

I did, but I gave up fully following that code path because it's so 
convoluted :-/

Ok if you want I can implement caching in the LD_PRELOAD and see
if it changes things.

> There is very clearly no caching going on. And since exim doesn't even
> execve, it just forks, it's very clear that it could cache things just
> ONCE, so your argument that caching wouldn't be possible at that level
> is also bogus.

So you mean caching it at startup time? Otherwise the parent would
need to do sysconf() at least , which it doesn't do (the exim source doesn't
really know anything about libdb internals)

That would add /proc/stat overhead to every program execution. Is that 
what you
are proposing?

-Andi

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