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Message-ID: <20070820191208.GB3714@elte.hu>
Date: Mon, 20 Aug 2007 21:12:08 +0200
From: Ingo Molnar <mingo@...e.hu>
To: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Christian Borntraeger <borntraeger@...ibm.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org,
Jan Glauber <jang@...ux.vnet.ibm.com>,
heiko.carstens@...ibm.com, Paul Mackerras <paulus@...ba.org>
Subject: Re: [accounting regression since rc1] scheduler updates
* Martin Schwidefsky <schwidefsky@...ibm.com> wrote:
> > If a virtual CPU is idle then i think the "real = steal, virtual =
> > 0" way of thinking about idle looks a bit unnatural to me - wouldnt
> > it be better to think in terms of "steal = 0, virtual = real" ?
> > Basically a virtual CPU can idle at "perfect speed", without the
> > host "stealing" any cycles from it. And with that way of thinking,
> > if s390 passed in the real-idle-time value to the new callbacks
> > below it would all fall into place. Hm?
>
> How you think about an idle cpu depends on your viewpoint. The source
> for the virtual cpu time on s390 is the cpu timer. This timer is
> stopped when a virtual cpu looses the physical cpu, so it seems
> natural to me to think that real=steal, virtual=0 because the cpu
> timer is stopped while the cpu is idle. The other way of thinking
> about it is as valid though.
my thinking is this: the structure of "idle time" only matters if it can
be observed from "within" a virtual machine - via timers. Are on s390
any of the typical app-visible timers (timer_list, etc.) driven by the
virtual tick? [which slows down if a virtual CPU is scheduled away by
the host/monitor/hypervisor?] Or is the virtual tick only affecting
scheduling/cpu-accounting statistics in essence?
Ingo
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