linux-kernel - Re: [RFC][PATCH v2 4/7] taskstats: Add per task steal time accounting

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Date:	Mon, 15 Nov 2010 18:59:23 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>,
	Shailabh Nagar <nagar1234@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Venkatesh Pallipadi <venki@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>, Oleg Nesterov <oleg@...hat.com>,
	John stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	"jeremy.fitzhardinge" <jeremy.fitzhardinge@...rix.com>
Subject: Re: [RFC][PATCH v2 4/7] taskstats: Add per task steal time
 accounting

On Mon, 15 Nov 2010 18:50:41 +0100
Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Mon, 2010-11-15 at 18:42 +0100, Martin Schwidefsky wrote:
> > The steal time of a task tells us how much more progress a task could have
> > done if the hypervisor would not steal cpu. Now you could argue that the
> > steal time for a cpu is good enough for that purpose but steal time is not
> > necessarily uniform over all tasks. And we already do calculate this number,
> > we just do not store it right now. 
> 
> If you make the scheduler take steal time into account like Jeremy
> proposed then you schedule on serviced time and the steal time gain is
> proportional to the existing service distribution.
> 
> Still, then you know, then what are you going to do about it? Are you
> going to avoid the hypervisor from scheduling when that one task is
> running?
> 
> What good is knowing something you cannot do anything about.

Steal time per task is at least good for performance problem analysis.
Sometimes knowing what is not the cause of a performance problem can help you
tremendously. If a task is slow and has no steal time, well then the hypervisor
is likely not the culprit. On the other hand if you do see lots of steal time
for a task while the rest of the system doesn't cause any steal time can tell
you something as well. That task might hit a specific function which causes
hypervisor overhead. The usefulness depends on the situation, it is another
data point which may or may not help you.

-- 
blue skies,
   Martin.

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

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