Xen users - Jan 2008 - Looking for clarity about physical cores/cpus and VCPUs

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?

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

Dom0 (4 vcpu)
Guest1 (32 vcpu)
Guest2 (32 vcpu)
Guest3 (32 vcpu)

 If I do that, how does Xen operate?

Thank you very much for your answers.

---Kayvan


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Hi,

Some answers inline.


On Jan 23, 2008 3:49 PM, Kayvan Sylvan <kayvan@psiww.com> 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
>



>
> _______________________________________________
> Xen-users mailing list
> Xen-users@lists.xensource.com
> http://lists.xensource.com/xen-users
>

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> 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?
That depends on whether you restrict some VCPUs to only run on some PCPUs.

There''s not normally a fixed mapping between VCPUs and PCPUs and so
there''s
not a requirement that each VCPU ends up running on a separate PCPU (although 
I''d expect something close to that to happen in the case you describe, 
especially if the VCPUs were occupied with something computer intensive).
> If there is up to 32 vcpus per guest, can I exceed the number of physical
> CPUs? For example:
>
> Dom0 (4 vcpu)
> Guest1 (32 vcpu)
> Guest2 (32 vcpu)
> Guest3 (32 vcpu)
>
>  If I do that, how does Xen operate?
VCPUs are to domains as threads are to a process under Unix.  VCPUs are the 
units that get scheduled.  If you have more VCPUs in a domain than there are 
PCPUs then that''s less efficient because those VCPUs will never be able
to
run in parallel.  But the scheduler can cope with it and it might be useful 
for some kind of testing, or if you migrate an SMP domain to a smaller host 
temporarily.

Cheers,
Mark
> Thank you very much for your answers.
>
> ---Kayvan


-- 
Push Me Pull You - Distributed SCM tool (http://www.cl.cam.ac.uk/~maw48/pmpu/)

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