linux-kernel - Re: inaccurate cpu idle time from top command

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Date:	Mon, 27 Jul 2009 09:58:41 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Fred Fan <fanyefeng@...il.com>
Cc:	cocala <syy.wxd@...il.com>,
	linux-arm-kernel@...ts.arm.linux.org.uk, rick.jones2@...com,
	gallatin@...i.com, akpm@...ux-foundation.org, dada1@...mosbay.com,
	brice@...i.com, paulus@...ba.org, benh@...nel.crashing.org,
	mingo@...e.hu, gregkh@...e.de, linux-kernel@...r.kernel.org
Subject: Re: inaccurate cpu idle time from top command

On Sun, 26 Jul 2009 11:34:02 +0800
Fred Fan <fanyefeng@...il.com> wrote:

> Hi cocala:
>     I have met same issue. And we found the total utilization
> count(added by idle's utilization) is not 100% in detail processes
> information fields.
>    best Regards
> Fred
> 
> 2009/7/24 cocala <syy.wxd@...il.com>:
> > I saw one commit "3209ada8285a9fa6ab8f7a731d54031ec884c745 - sched: account
> > system time properly"
> > This commit is to take into account timer IRQ interrupting the idle task
> > servicing a hard or soft irq.
> >
> > If a softirq is started in a tick, all the time in the tick will be counted
> > as soft irq time even most time in the tick is idle.
> > In our use case, one jiffy is 10ms and the 1G net driver will generate many
> > software irq, so the "top" command will show high softare irq time and cpu
> > idle time approaches to zero.
> > If I change the jiffy to 1ms, the "top" command shows much higher cpu idle
> > time. I think this algorithm is also not fair for some cases.
> >
> > Is there any better way to count the softare irq time?
> >
> > Thanks
> > cocala

There is a solution to this problem but not all architectures have
implemented it. Currently only s390, powerpc and ia64 have defined
CONFIG_VIRT_CPU_ACCOUNTING which allows the architecture backend to do
its own, precise math. The backdraw of the improved accounting is
i) some complicated code, and ii) a slight performance penalty. So far
nobody wanted to implement it for x86.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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