From: Alexey Dobriyan <adobriyan@gmail.com>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] proc: much faster /proc/vmstat
Date: Sun, 7 Aug 2016 11:42:50 +0300 [thread overview]
Message-ID: <20160807084249.GA1871@p183.telecom.by> (raw)
In-Reply-To: <20160807013510.GM2356@ZenIV.linux.org.uk>
On Sun, Aug 07, 2016 at 02:35:13AM +0100, Al Viro wrote:
> 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.
I'll get right on that.
> * 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?
1. format_decode() is busy looking for format specifier: 2 branches per character
(not in this case, but in others)
2. approximately million branches while parsing format mini language
and everywhere
3. just look at what string() does
/proc/vmstat is good case because most of its content are strings
But the patch will still be faster.
prev parent reply other threads:[~2016-08-07 8:42 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-06 12:54 [PATCH] proc: much faster /proc/vmstat Alexey Dobriyan
2016-08-07 1:35 ` Al Viro
2016-08-07 8:42 ` Alexey Dobriyan [this message]
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