linux-kernel - Re: [PATCH v2] proc: speedup /proc/stat handling

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Message-Id: <20120124172732.19b3d9f4.akpm@linux-foundation.org>
Date:	Tue, 24 Jan 2012 17:27:32 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Glauber Costa <glommer@...allels.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Paul Tuner <pjt@...gle.com>
Subject: Re: [PATCH v2] proc:  speedup /proc/stat handling

On Wed, 25 Jan 2012 01:22:25 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Le mardi 24 janvier 2012 __ 16:12 -0800, Andrew Morton a __crit :
> 
> > Did you measure the improvement from this patch?
> 
> Unfortunately I can not reboot the server where I noticed this
> performance problem.
> 
> On the smaller one, performance improvement is about 20%, because the
> second run of show_stat() can use data present in cpu cache.
> 
> On big machines, I guess the 128 bytes per possible cpu reservation can
> avoid the second run. (since a typical cpuXXX line is smaller than 128
> bytes)

I had a fiddle on an 8-way x86_64 machine.  I'm unable to demonstrate
any improvement for either of

time (for i in $(seq 1000); do; cat /proc/self/stat > /dev/null; done)
time (for i in $(seq 1000); do; cat /proc/1/stat > /dev/null; done)

oh well.
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