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

To: "Keir Fraser" <keir@xxxxxxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform changes
From: "Langsdorf, Mark" <mark.langsdorf@xxxxxxx>
Date: Thu, 30 Aug 2007 09:45:50 -0500
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Thread-topic: [Xen-devel] [PATCH] 1/2: cpufreq/PowerNow! in Xen: Time and platform changes
> > a) Current approach is simple to let Dom0 conduct frequency
> > change. That should be OK in the start, but at the same time we
> > should also consider the on-demand governor within Xen itself.
> > Xen can always get first-hand data about domain status, while
> > dom0 (either user-level or in-kernel) can't achieve in time. Fine-
> > grained frequency change is more likely to be achieved within
> > Xen directly.
> Personally I'm a fan of doing it in dom0 userspace, although 
> doing it within Xen can also be argued for. Doing it in dom0
> kernel doesn't seem very attractive apart from the obvious
> pragmatic advantage that all the code is
> already in the Linux kernel. :-)

The advantage to doing it in the dom0 kernel is that the
distributions have just switched from doing it in userspace,
and thus have all their tools set up to do it in the kernel.

To me, it makes more sense to simplify the user interface,
so that a native mode machine and a virtual machine uses the
same tools.  The end user shouldn't need to learn cpuspeed
when running power management on a virtual machine host if
the same computer uses ondemand when running a native mode

> If we're doing it in the Linux kernel, I don't see much point 
> in hacking up the defunct powernow (or equivalent Intel) code.
> Why not fix the generic acpi-cpufreq.c? That is supposed to
> work on any modern CPU.  I'm not sure the 2.6.18 version is
> new enough, but I'd rather see a backported and fixed
> version of that file, rather than bother to maintain modified 
> versions of obsolete source files.

powernow-k8 and the Intel SpeedStep equivalents are being
maintained in preference to acpi-cpufreq.  I don't think 
the code is obsolete or defunct.

-Mark Langsdorf
Operating System Research Center

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