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Message-ID: <46C9A7AB.1030003@bull.net>
Date: Mon, 20 Aug 2007 16:39:39 +0200
From: Laurent Vivier <Laurent.Vivier@bull.net>
Organization: Bull S.A.S.
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To: John Stoffel <john@stoffel.org>
Cc: Ingo Molnar <mingo@elte.hu>, kvm-devel <kvm-devel@lists.sourceforge.net>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       virtualization <virtualization@lists.linux-foundation.org>
Subject: Re: [PATCH 0/4] Virtual Machine Time Accounting
References: <46C99371.7070705@bull.net> <18121.39541.790665.686081@stoffel.org>
In-Reply-To: <18121.39541.790665.686081@stoffel.org>
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Hi John,

John Stoffel wrote:
>=20
> Laurent> The aim of these four patches is to introduce Virtual Machine
> Laurent> time accounting.
>=20
> So what does this buy us?  What increased functionality? =20
>=20

functionnalities:

- allow to measure time spent by a CPU in a virtual CPU.
- allow to display in /proc/state this value by CPU
- allow to display in /proc/<pid>/state this value by process
- allow KVM to use these 3 previous functionnalities

> Laurent> [PATCH 1/4] as recent CPUs introduce a third running state,
> Laurent> after "user" and "system", we need a new field, "guest", in
> Laurent> cpustat to store the time used by the CPU to run virtual
> Laurent> CPU. Modify /proc/stat to display this new field.
>=20
> What happens when you have multiple guests, is this state cummulative? =
=20

Yes, like system time and user time are.

> I've glanced over these patches and the #ifdefs are ugly..., esp since

#ifdefs are ugly, I agree... I can remove them.
But "#ifdef" allows to save space and time (and it is not a beauty show).=


> the overhead seems to exist even if you don't run any Guest CPUs.  Or
> whatever you're doing here.

Can you explain ?

> Please justify this patch more in terms of what it provides and why
> it's a good thing to have. =20

Well... it provides the time spent by CPUs in virtual CPUs.
It is usefull only if you run Virtual Machine on your host (like KVM, lgu=
est,
kqemu...) to know how many time your CPUs dedicate to virtual machine exe=
cution.
It's a good thing to have like user time and system time are good things =
to
have... do you use 'top' or 'time' sometime ?


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


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