linux-kernel - Re: [PATCH v2 3/6] KVM-GST: KVM Steal time accounting

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Date:	Tue, 01 Feb 2011 13:57:04 -0200
From:	Glauber Costa <glommer@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	aliguori@...ibm.com, Rik van Riel <riel@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 3/6] KVM-GST: KVM Steal time accounting

On Sun, 2011-01-30 at 16:04 +0200, Avi Kivity wrote:
> On 01/28/2011 09:52 PM, Glauber Costa wrote:
> > This patch accounts steal time time in kernel/sched.
> > I kept it from last proposal, because I still see advantages
> > in it: Doing it here will give us easier access from scheduler
> > variables such as the cpu rq. The next patch shows an example of
> > usage for it.
> >
> > Since functions like account_idle_time() can be called from
> > multiple places, not only account_process_tick(), steal time
> > grabbing is repeated in each account function separatedely.
> >
> 
> I accept that steal time is worthwhile, but do you have some way to 
> demonstrate that the implementation actually works and is beneficial?
> 
> Perhaps run two cpu-bound compute processes on one vcpu, overcommit that 
> vcpu, and see what happens to the processing rate with and without steal 
> time accounting.  I'd expect a fairer response with steal time accounting.

Avi,

There are two things here:
One of them, which is solely the accounting of steal time, (patches 1 to
4) has absolutely nothing to do with what you said. Its sole purpose is
to provide the user with information about "why is my process slow if I
am using 100 % of my cpu?")

The last patch is the only one that actually tries to rebalance cpus
according to steal time information. For that, I do have some
experiments I did here to see if it is working, will try to provide more
precise data in the next submission.


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