linux-kernel - Re: Réf. : Re: [PATCH 0/4] Virtual Machine Time Accounting

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Message-ID: <5d6222a80708211046xe248cdcw5193a0dc8f4ff0a8@mail.gmail.com>
Date:	Tue, 21 Aug 2007 14:46:02 -0300
From:	"Glauber de Oliveira Costa" <glommer@...il.com>
To:	"Laurent Vivier" <Laurent.Vivier@...l.net>
Cc:	"Jeremy Fitzhardinge" <jeremy@...p.org>,
	"John Stoffel" <john@...ffel.org>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: Réf. : Re: [PATCH 0/4] Virtual Machine Time Accounting

On 8/21/07, Laurent Vivier <Laurent.Vivier@...l.net> wrote:
> Glauber de Oliveira Costa wrote:
> >> by doing this at kernel level, we can:
> >> - measure exactly the guest time,
> >> - move this part of system time to user time (as you think it should be
> >> user time),
> >> - have consistency between system, user and guest time,
> >> - report values in /proc/state and /proc/<pid>/state, at system wide level
> >>
> >> I'm not sure we can measure the guest time at the qemu user level.
> >>
> >> Perhaps Rusty can say what he thinks about this ?
> >>
> > Even if we cannot _now_, isn't that an easier, and safer change? (and
> > I don't think we lose anything by design).
>
> Could you explain ? How should I do this ?
> I'm _sure_ it is not easier to do that at qemu level.
>
> I don't like to patch kernel (it is the last thing I do to solve a problem: I
> know there is always at least one guy to not agree the patch :-P ) but in this
> case I think this is the best way to do that.
>
> I think the virtualization notion should be introduced at the kernel level, at
> least in the kernel statistics: it is generic, it can be used by other
> virtualization tools. As I said, until know CPUs have got only two states
> reflected in statistics by "user time" and "system time". Since recently, they
> have introduced a third state, the virtual CPU, that, in my opinion, should be
> also reflected in the CPU statistics as the "guest time".

After a second thought on this, you seem to be right.

> >
> > Although I don't know KVM to a that deep level, I think it should be
> > possible to keep the virtual cpus in different process (or threads),
> > and take the accounting time from there. Perfectly possible to know
> > the time we spent running (user time), and the time the hypervisor
> > spent doing things on our behalf (system time).
>
> But we have always user time accounted as system time. CPU stats are wrong if we
> do not modify the kernel. Can you live with wrong statistics ? (yes, I think,
> you can, but perhaps someone else not)
>
No, I can't. Just because I suggested an alternate way (even if it was
wrong), it does not mean I want things to be done wrongly.

-- 
Glauber de Oliveira Costa.
"Free as in Freedom"
http://glommer.net

"The less confident you are, the more serious you have to act."
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