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Date: Tue, 21 Aug 2007 09:45:53 +0200
From: Laurent Vivier <Laurent.Vivier@bull.net>
Organization: Bull S.A.S.
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To: Glauber de Oliveira Costa <glommer@gmail.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>, John Stoffel <john@stoffel.org>,
       kvm-devel <kvm-devel@lists.sourceforge.net>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>,
       virtualization <virtualization@lists.linux-foundation.org>
Subject: Re: =?ISO-8859-1?Q?R=E9f=2E_=3A_Re=3A_=5BPATCH_0/4=5D_?=
 =?ISO-8859-1?Q?Virtual_Machine_Time_Accounting?=
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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 b=
e
>> user time),
>> - have consistency between system, user and guest time,
>> - report values in /proc/state and /proc/<pid>/state, at system wide l=
evel
>>
>> 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 proble=
m: 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 leve=
l, 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, shou=
ld be
also reflected in the CPU statistics as the "guest time".

>=20
> 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 wron=
g if we
do not modify the kernel. Can you live with wrong statistics ? (yes, I th=
ink,
you can, but perhaps someone else not)

Laurent
--=20
------------- Laurent.Vivier@bull.net  --------------
          "Software is hard" - Donald Knuth


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