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Re: [Xen-devel] Re: Xen cpu Scheduling

To: ncmike@xxxxxxxxxx
Subject: Re: [Xen-devel] Re: Xen cpu Scheduling
From: pak333@xxxxxxxxxxx
Date: Wed, 21 Mar 2007 05:48:41 +0000
Cc: "Mike D. Day" <ncmike@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Thanks for the explanation.
I still have questions about how cpu utilization is reported by xm top/xentop. If someone can answer the question as to how the cpu% is calculated, when a VM with 2 vcpus, is only using 1 pcpu at any given point of time.
Also, a vcpu is given the full pcpu. What if it doesn't use the full pcpu. How is it scheduled?
-------------- Original message --------------
From: "Mike D. Day" <ncmike@xxxxxxxxxx>

> On 20/03/07 01:37 +0000, pak333@xxxxxxxxxxx wrote:
> >
> > 1. Suppose I give a VM 2 vcpus. When this VM gets scheduled will it
> > always be given two pcpus or can it be given 1vcpu also sometimes.
> > For example if we have mutiple VMs running, and if a cpu gets freed up
> > will my 2vcpu VM be scheduled on this single free cpu. If a single
> > cpu cna be given how is the cpu % calculated by xentop
> It depends on how busy the CPUs are on the machine, and the ratio of
> When the VM has two VCPUs, each VCPU (when runnable) will be put on
> the run queue of a physical CPU. This VM will run concurrently on AT
> MOST 2 PCPUs, sometimes only 1 PCPU. If on the platf orm VCPU == PCPU
> in aggregate then the VM with 2 VCPUs will frequently run concurrently
> on two PCPUs.
> >
> > 2. If the 2vcpu is scheduled on two cpus, and one of its threads goes
> > to sleep can that single thread be context swtiched out. or will it be
> > kept spinning.
> VM's get scheduled as VCPUs, not threads. Having two VCPU's for a VM
> is sort of like a process having two threads. But there are a lot of
> differences as well so don't take the analogy too far.
> When a VCPU for a VM is not running it is not executing any code. The
> VCPU can be runnable, running, or sleeping. It only executes code when
> it is running.
> However, remember that *kernel threads* are scheduled by the guest kernel,
> while the guest kernel is scheduled as one or more VCPUs by *xen.* The
> guest kernel is allowed to put its own thre ads to sleep and awaken
> other threads, but when Xen decides that the time slice is over for a
> VCPU, then that VCPU gets rescheduled at the back of the q, or put to
> sleep.
> I don't know very much about xentop.
> Mike
> --
> Mike D. Day
> Cell: 919 412-3900
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