[Xen-devel] Re: Xen cpu Scheduling

["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
VCPU to PCPU.

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 platform 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 threads 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
IBM LTC
Cell: 919 412-3900
Sametime: ncmike@xxxxxxxxxx AIM: ncmikeday  Yahoo: ultra.runner
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