RE: [Xen-users] xen power management

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> tbrown@xxxxxxxxxxxxx
> Sent: 20 February 2006 23:20
> To: Xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] xen power management
> 
> On Mon, 20 Feb 2006, sriram govindan wrote:
> 
> > hi guys,
> >
> >  I am new to xen and would like to know the power 
> management options 
> > provided to the virtual machines like,
> >
> >  ?? Are the VMs allowed to manipulate processor frequency 
> or aloowed 
> > to do some kind of voltage scaling. ??
> 
> [ I'm no XEN expert, but I think the answer is self evident. ]
> 
> no. Think about it. None of the VMs have any concept of how 
> busy the CPU is (since they can't peak at the other VMs). If 
> this is permitted, I'm pretty sure it would be considered a bug.
> 
> XEN (the hypervisor) could conceivably use this capacity... 
> as _it_ knows how fast guests voluntarily give up the CPU.

Exactly, only the hypervisor knows the entire picture.

The way I imagine it working (having discussed with several other Xen
developers), would be that Dom0 runs a daemon of some sort that monitors
the entire systems CPU usage (using a call into Xen Hypervisor).
Depending on the results of this call, adjustment to CPU speed can be
done. 

The hardest part would be to determine what CPU load is suitable for
reduction in speed and at what level do we increase the CPU speed - and
plumbing it into the existing cpufreq architecture so that it works
smoothly with existing kernels and drivers. 

Another factor would be to actually factor in the settings being told by
the individual kernels - say you run 5 guests, and one is using 100%
cpu, but it's also talking to the Power Management system to say "set me
to the slowest CPU-speed", perhaps because the task at hand is a
low-priority one, and it's not affecting user interaction. For
unmodified (fully virtualized) guests, we could monitor the MSR's used
for power management, for example. This would of course be a more
complex solution.

--
Mats


> 
> -Tom
> 


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