linux-kernel - Re: [PATCH] proc: much faster /proc/vmstat

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Message-ID: <20160807013510.GM2356@ZenIV.linux.org.uk>
Date:	Sun, 7 Aug 2016 02:35:13 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc: much faster /proc/vmstat

On Sat, Aug 06, 2016 at 03:54:56PM +0300, Alexey Dobriyan wrote:

[sprintf sucks, let's convert numbers manually]

> --- a/mm/vmstat.c
> +++ b/mm/vmstat.c
> @@ -1592,7 +1592,10 @@ static int vmstat_show(struct seq_file *m, void *arg)
>  {
>  	unsigned long *l = arg;
>  	unsigned long off = l - (unsigned long *)m->private;
> -	seq_printf(m, "%s %lu\n", vmstat_text[off], *l);
> +
> +	seq_puts(m, vmstat_text[off]);
> +	seq_put_decimal_ull(m, ' ', *l);
> +	seq_putc(m, '\n');
>  	return 0;
>  }

If that manages to be a hotspot, we really should
	* educate the wankers responsible for the userland code in question,
until they repent and cease committing such abominations.
	* look into fixing vsnprintf().  

Seriously, what the hell is vsnprintf() doing that takes so much time?  It's
not as if it was a complex format anyway.  WTF is going on there?  Where is
it spending that much time?