Re: [Xen-users] Looking for clarity about physical cores/cpus and VCPUs

["Todd Deshane" <deshantm@xxxxxxxxx>]

Hi,

Some answers inline.

On Jan 23, 2008 3:49 PM, Kayvan Sylvan <kayvan@xxxxxxxxx> wrote:

Hi everyone,

 

I'm looking for a clear explanation of how the CPU scheduling works and the correspondence between physical CPUs and VCPUS.

 

The Xen manuals state that there is "up to 32 virtual CPUs per guest virtual machine".

 

So, If I have a server with 64 physical CPUs, and I boot into Xen, with dom0_max_vcpu set to 1, and I have 2 guest domains, like this:

 

Dom0 (1 vcpu)

Guest1 (15 vcpu)

Guest2 (16 vcpu)

 

What is the actual correspondence between the virtual CPUs and physical CPUs?

Xen will schedule based on its scheduling policies. So there would be not direct correspondence. You can override this behavior with xm vcpu-pin and other xm vcpu* commands. You can even change scheduling policies, priorities etc.
 

 

If there is up to 32 vcpus per guest, can I exceed the number of physical CPUs? For example:

 

I am pretty sure you can do this, but I don't think it makes much sense to do this, since it would simply increase scheduling overhead.
 

Dom0 (4 vcpu)

Guest1 (32 vcpu)

Guest2 (32 vcpu)

Guest3 (32 vcpu)

 

 If I do that, how does Xen operate?

 

I believe Xen can do the scheduling of more vcpus than physical CPUs, but I can't think of a good motivation to do so.

Best Regards,
Todd

Thank you very much for your answers.

 

---Kayvan

 

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