[Fedora-xen] CPU management

[Dustin Henning]

That being noted, in Xen, I believe that a CPU is randomly selected and pinned to a VCPU for the entire session of the VM (this may only apply to HVMs), whereas, according to someone, virsh uses any available CPU.  I assign my CPUs to my VMs for that reason.  However, here is some information on scheduling processor usage as well:
http://wiki.xensource.com/xenwiki/Scheduling?highlight=%28scheduling%29
It's not especially detailed, but it may be what you are looking for, and while virsh may have a similar method, I am unfamiliar with virsh.
Dustin

-----Original Message-----
From: fedora-xen-bounces at redhat.com [mailto:fedora-xen-bounces at redhat.com] On Behalf Of John Summerfield
Sent: Friday, January 18, 2008 08:52
To: fedora-xen at redhat.com
Subject: Re: [Fedora-xen] CPU management

Richard W.M. Jones wrote:
>> I would like to be able to give, for example, 33% for dom1 and dom2
>> and 33% for dom0, but  I know that we can only work with VCPUs.
> 
> It doesn't work like this.  Guests don't normally consume CPU 
> continuously (well, unless you're running something like numerical 
> simulations, which I really wouldn't recommend for virtualization).
> 
> First of all, if you do nothing at all then guests' virtual CPUs are 
> scheduled to run on physical CPUs at random.  When a guest starts to do 
> something, a physical CPU is picked and the guest runs on that for some 
> short timeslice.  You can easily have more virtual CPUs than physical 
> CPUs.  If your guests are mostly idle, the this is fine.

Something that has occurred to me, and which troubled someone else, is 
that if the host is busy running Firefox, OOo and other desktop software 
as well as guest operating systems, those guests probably are not going 
to perform well.

I think Xen will perform better in this sort of environment than KVM.

The virtualisation software I'm most familiar with (from a theoretic 
POV) is IBM's VM which runs on its mainframes. It's a hypervisor only, 
and all users have their own virtual computer, and it's those virtual 
computers that do the real work.

Tuning VM to work best is an intricate job.

VM is now about 40 years old.


-- 

Cheers
John

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