RE: [Xen-users] XEN hardware compatibility list

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

> -----Original Message-----
> From: Mark Williamson [mailto:mark.williamson@xxxxxxxxxxxx] 
> Sent: 27 July 2005 13:50
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Cc: Petersson, Mats; koxman
> Subject: Re: [Xen-users] XEN hardware compatibility list
> 
> > Having some understanding of the inner workings of the CPU's 
> > Speed/Power handling, I can see how it would be necessary 
> for the Xen 
> > portion to be the manager of frequency switching, as Dom0 or DomU 
> > doesn't know what's going on in any of the other domains per se. I 
> > guess it would technically be possible to have a monitor process in 
> > Dom0 that manages the speed of the CPU, but it's probably 
> better to do this in Xen itself.
> 
> Yup.  Another thing to consider might be providing Xen 
> interfaces for collecting stats and setting the CPU scaling, 
> then allow a userspace daemon (e.g. something like cpufreqd) 
> to deal with policy.

Yes, if there's an interface to get the information from Xen into (for
exampl) Dom0, then we'd be able to use that for the userspace daemon,
and then use a kernel module for Dom0 (or some other Dom, but it makes
sense that it's a privilige domain handling this). 

> 
> It's a bit laptop-centric at the moment but I guess this'll 
> be necessary for servers, as their chips also get frequency 
> scaling (I think there are at least some Intel chips that do 
> this now).  Of course, if you're running Xen you should be 
> getting high server utilisation... ;-)

Yes, I'm pretty sure that there's the Cool'n'Quiet option on AMD
Opterons too (but I'm a bit removed from the products side, so I
wouldn't know exactly). However, if the server, as you suggest, is
mostly in the 50-90% utilisation range, you'd probably not get that much
out of it, as server workloads tend to be very transient. One of the
problems with PowerNow/Cool'n'Quiet (and probably in Intels solution
too, but I wouldn't know for sure, as I haven't worked with that
technology) is that it's somewhat costly to change the frequency, so you
need many milliseconds of the same level performance before you decide
to change the frequency. Do it too often, and you loose more performance
due to the speed switch than you gain from increasing the speed.
However, if you have a server sitting doing some work that is based on,
for instance, the working day of your employees, then it would make
sense to lower the CPU-speed overnight. A multinational mail-server that
receives mail spread over the whole 24 hour period probably don't get
much gain from this, as it would probably be roughly at the same
processing level all throughout the 24 hour period [and if it's only
running 10% performance for 90% of the time, you'd be shuffling some
other load onto that machine, right?]

--
Mats

[snip]


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