linux-kernel - Re: [tip:sched/core] sched: cpuacct: Use bigger percpu counter batch values for stats counters

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Message-Id: <20090821152159.12c8529d.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Fri, 21 Aug 2009 15:21:59 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, balbir@...ux.vnet.ibm.com,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Ingo Molnar <mingo@...e.hu>, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, schwidefsky@...ibm.com,
	balajirrao@...il.com, dhaval@...ux.vnet.ibm.com,
	tglx@...utronix.de, akpm@...ux-foundation.org
Subject: Re: [tip:sched/core] sched: cpuacct: Use bigger percpu counter
 batch values for stats counters

On Thu, 20 Aug 2009 16:24:51 +1000
Anton Blanchard <anton@...ba.org> wrote:

>  
> Hi,
> 
> > Could you share contex-switch-test program ?
> > I'd like to play with it to find out what I can do against percpu counter.
> 
> Sure:
> 
> http://ozlabs.org/~anton/junkcode/context_switch.c
> 
> Very simple, just run it once per core:
> 
> for i in `seq 0 31`
> do
> 	taskset -c $i ./context_switch &
> done
> 
> Then look at the context switch rates in vmstat.
> 

Ok, I reproduced it on ia64 box (ia64 supports VIRT_CPU_ACCOUNTING)

On 8cpu
-- config=on--
procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
11  0      0 7185152  66048 284416    0    0     0     0 5918 12354  0 100  0  0  0
--config=off--
11  0      0 7194496  66304 283392    0    0     0    20 8023 1424022 32 68  0  0  0

Wow, yes. 100x regression.

I'll dig this more. 

IIUC, If CONFIG_VIRT_CPU_ACCOUNTING=on, process's stime/utime records "nanosecond".
Then, batch=32 is too small. How about once per msec ?

Thanks,
-Kame

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