[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <1327915555.2288.18.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date: Mon, 30 Jan 2012 10:25:55 +0100
From: Eric Dumazet <eric.dumazet@...il.com>
To: Jörg-Volker Peetz <jvpeetz@....de>
Cc: linux-kernel@...r.kernel.org,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Glauber Costa <glommer@...allels.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Ingo Molnar <mingo@...e.hu>,
Russell King - ARM Linux <linux@....linux.org.uk>,
Paul Tuner <pjt@...gle.com>
Subject: Re: [PATCH v2] proc: speedup /proc/stat handling
Le lundi 30 janvier 2012 à 09:06 +0100, Jörg-Volker Peetz a écrit :
> Eric Dumazet wrote, on 01/25/12 01:26:
> > Le mercredi 25 janvier 2012 à 09:18 +0900, KAMEZAWA Hiroyuki a écrit :
> >
> >> 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 ?)
> >>
> >
> > There is no change, as 4096/32 is 128 bytes per cpu.
> >
>
> Wrong math, only num_possible_cpus() is divided by 32. Thus,
>
> - unsigned size = 4096 * (1 + num_possible_cpus() / 32);
> + unsigned size = 4096 + 128 * num_possible_cpus();
>
> <snip>
It is good math, once you take the time to think a bit about it.
The original question was about the 128 * num_possible_cpus()
4096/32 is 128 as I said.
The 4096 -> 1024 is just taking into account fact that once you do the
correct computations, you dont need initial 4096 value, and 1024 is more
than enough.
Example on a dual core machine :
# dmesg|grep nr_irq
[ 0.000000] nr_irqs_gsi: 40
[ 0.000000] NR_IRQS:2304 nr_irqs:712 16
size = 1024 + 2*128 + 2*712 = 2704 bytes (rounded to 4096 by kmalloc())
# wc -c /proc/stat
1767 /proc/stat
Problem with original math was that for a machine with 16 cpus or a
machine with 1 cpu, we ended with the same 4096 value. That was a real
problem.
If we instead use "unsigned size = 4096 + 128 * num_possible_cpus();" as
you suggest, we would always allocate 2 pages of memory, this is not
needed at all for typical 1/2/4 way machines.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists