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Date: Thu, 26 Jan 2012 16:43:42 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        Glauber Costa <glommer@parallels.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org,
        Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Paul Tuner <pjt@google.com>
Subject: Re: [PATCH v2] proc:  speedup /proc/stat handling
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On Thu, 26 Jan 2012 18:55:20 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Wed, 25 Jan 2012 17:04:16 -0800
> Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > On Wed, 25 Jan 2012 06:29:32 +0100
> > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > 
> > > Le mardi 24 janvier 2012 __ 17:27 -0800, Andrew Morton a __crit :
> > > 
> > > > 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.
> > > 
> > > What size is /proc/stat ?
> > 
> > About 40mm, but it depends on the font size.
> > 
> > > wc -c /proc/stat
> > > 
> > > If under 4096, there is no problem with existing code.
> > 
> > akpm2:/home/akpm> wc -c /proc/stat 
> > 2800 /proc/stat
> > 
> > > I had the problem on a 16-way machine.
> > 
> > OK..
> 
> 
> I wrote following patch just for my fun, which makes /proc/stat twice fast.
> But I'm not sure whether this kind of dirty && special printk is worth to do or not..
> because I can't see /proc/stat cost at shell-scripting.

It is rather a lot of not-very-general infrastructure.

>
> ...
>
> @@ -131,8 +143,8 @@ static int show_stat(struct seq_file *p, void *v)
>  	seq_printf(p, "intr %llu", (unsigned long long)sum);
>  
>  	/* sum again ? it could be updated? */
> -	for_each_irq_nr(j)
> -		seq_printf(p, " %u", kstat_irqs(j));
> +	j = 0;
> +	seq_printnum_batch(p, " %u", &j, get_next_kstat_irq);

I expect most of these numbers are zero.  I wonder if we would get
useful speedups from

	for_each_irq_nr(j) {
		/* Apologetic comment goes here */
		if (kstat_irqs(j))
			seq_printf(p, " %u", kstat_irqs(j));
		else
			seq_puts(p, " 0");
	}