linux-kernel - Re: [RFC][PATCH v2 5/7] taskstats: Improve cumulative CPU time accounting

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Message-ID: <20101116095456.7389bebc@mschwide.boeblingen.de.ibm.com>
Date:	Tue, 16 Nov 2010 09:54:56 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	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>, 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
Subject: Re: [RFC][PATCH v2 5/7] taskstats: Improve cumulative CPU time
 accounting

On Mon, 15 Nov 2010 19:10:06 +0100
Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Mon, 2010-11-15 at 19:00 +0100, Martin Schwidefsky wrote:
> > On Mon, 15 Nov 2010 18:51:57 +0100
> > Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> > 
> > > On Mon, 2010-11-15 at 18:49 +0100, Martin Schwidefsky wrote:
> > > > > To what purpose? 
> > > >  
> > > > Is that a trick question? Why do we have tools like "top"? Or process
> > > > accounting? The point is that the quality of the numbers we get right
> > > > now is rather bad, the overhead of scanning /proc is horrendous and
> > > > the 10ms granularity is rather coarse. 
> > > 
> > > But you're not just replacing top, you're adding all kinds of new
> > > accounting crap all over the place.
> > 
> > We DO replace top. Patch #7 of 7.
> 
> You _also_ replace top, but its not by far the only thing you do. If you
> simply replaced top you wouldn't need to add tons of extra accounting,
> would you?

There are basically two things we want to accomplish:
1) Make top faster by replacing the /proc based data gathering with taskstats.
   To really make it go fast with taskstats we filter with last_depart to
   read only tasks that have changed since the last snapshot.
2) Make top more precise. That is where all of the extra accounting comes
   into play.

-- 
blue skies,
   Martin.

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

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