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

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Message-Id: <20120124161221.032325d1.akpm@linux-foundation.org>
Date:	Tue, 24 Jan 2012 16:12:21 -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 01:01:23 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> On a typical 16 cpus machine, "cat /proc/stat" gives more than 4096
> bytes, and is slow :

It is pretty bad:

akpm:/usr/src/25> time (for i in $(seq 1000); do; cat /proc/self/stat > /dev/null; done)     
(; for i in $(seq 1000); do; cat /proc/self/stat > /dev/null; done; )  0.05s user 0.94s system 29% cpu 3.367 total

(29% CPU?)

> # strace -T -o /tmp/STRACE cat /proc/stat | wc -c
> 5826
> # grep "cpu " /tmp/STRACE
> read(0, "cpu  1949310 19 2144714 12117253"..., 32768) = 5826 <0.001504>
> 

Did you measure the improvement from this patch?

> Thats partly because show_stat() must be called twice since initial
> buffer size is too small (4096 bytes for less than 32 possible cpus)
> 
> Fix this by :
> 
> 1) Taking into account nr_irqs in the initial buffer sizing.
> 
> 2) Using ksize() to allow better filling of initial buffer.

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