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

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Date:	Wed, 02 Feb 2011 09:57:49 -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 Wed, 2011-02-02 at 12:51 +0200, Avi Kivity wrote:
> On 02/02/2011 12:11 PM, Avi Kivity wrote:
> > On 02/01/2011 05:57 PM, Glauber Costa wrote:
> >> 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?")
> >
> > Right.  Like irq and softirq time, we need to report this to the user, 
> > as it's potentially much higher.
> 
> Of course, it's not enough to just account for this time, you also have 
> to expose it somewhere, and update tools like top(1) to display it.
Yes, what I meant is that just the accounting will just expose it to the
tools, won't affect the scheduler in any sense.


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