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Re: [Xen-devel] [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform

To: "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Rik van Riel <riel@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform changes
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Fri, 31 Aug 2007 10:23:00 +0100
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Mark Langsdorf <mark.langsdorf@xxxxxxx>
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On 31/8/07 03:42, "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:

> d) I guess final power saving of cpufreq (either approach) is not obvious,
> since average CPU utilization should be higher than native which is the
> goal of virtualization. C-state may be more interesting.


I would love to see some C-state support in Xen, both for normal idle-loop
execution and, as further work, deeper sleeps for hot-unplugged CPUs (which
can be under control of management/performance tools in dom0).

In the now prevalent multi-core environments, I'll be surprised if it's not
better to deep-sleep whole cores rather than run them all at continually
varying half speeds. And, simultaneously with making C-states a viable
power-saving model, I think multi-core makes it harder to decide what the
'right' per-cpu cpu frequency changes should be.

 -- Keir

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