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

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Message-Id: <20120125091818.45e00b3c.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Wed, 25 Jan 2012 09:18:18 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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 :
> 
> # 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>
> 
> 
> 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.
> 
> 
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> Cc: Glauber Costa <glommer@...allels.com>
> Cc: Russell King - ARM Linux <linux@....linux.org.uk>
> Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Paul Tuner <pjt@...gle.com>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Ingo Molnar <mingo@...e.hu>


Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

BTW, what is the reason of this change ?

> -	unsigned size = 4096 * (1 + num_possible_cpus() / 32);
> +	unsigned size = 1024 + 128 * num_possible_cpus();

I think size of buffer is affected by the number of online cpus.
(Maybe 128 is enough but please add comment why 128 ?)

And
> size += 2 * nr_irqs;

If this assumption fails, the issue comes again...


-Kame

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