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Re: [Xen-users] how many VCPUs per physical CPU?

On Mon, 2009-07-06 at 17:22 +0300, Peter Peltonen wrote:
> Hi and thanks for your reply,
> On Mon, Jul 6, 2009 at 4:47 PM, Siddharth Wagh<siddharth.wagh@xxxxxxxxx> 
> wrote:
> > The Xen document specifies that upto 32 VCPUs may be allocated to a single
> > domU. But it is recommended that one allocates the same number of VCPUs or
> > less than the number of logical CPUs on your machine, for optimum
> > performance. Since, you have a quad-core processor, the maximum number of
> > VCPUs you could allot to each domU w/o compromising on performance is 4.
> But what is the benefit of having more VCPUs in a logical CPU?

None that I know of, and probably that's why the manual recommends
limiting the VCPUs to the number of host CPUs.

> In what kind of cirumstances does a domU perform better with a 4 VCPUs
> than with 1 VCPU?

If you have more than one thread eating CPU, the threads can span across
multiple physical CPUs, resulting in better performance. If you have
only one VCPU, all the work will be done by a single host CPU.

> I don't undrestand the difference, as in the end there is just one
> logical CPU doing all the work.

As far as I know, different VCPUs can be mapped to different host CPUs
by the xen scheduler.


Radu R.

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