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Re: [Xen-users] [xs-devel] Odd number of VCPU and performance impact?

On 16/01/15 21:35, mad Engineer wrote:
> Hi all,
>         Is it good practice to always create vm with even number of VCPU?
> What could be the impact if we create vms with odd number of CPU on
> NUMA or SMP systems.Is there any recommendation
> Thanks

There should not be any performance impact as a result of having an odd
number of vcpus.

I cannot speak to the internals of an OS scheduler pe say, but there is
real hardware with odd numbers of cpus (AMD Phenom X3 series).

For HVM guests, the VM actually boots with ACPI tables claiming either
15[1] or 128 cpus.  The processors between VCPUs-at-startup and this are
marked as offline in the tables, and the difference between
VCPUs-at-startup and VCPUs-max may be hot-added using the standard APCI
mechanism.  HVMLoader chooses one of the two ACPI tables to place in
memory based upon VCPUs-max.

An OS capable of hot-adding cpus must be capable of dealing with the
maximum number of cpus appearing at some later point, and will set up
its resources accordingly.


[1] Windows 2000 would BSOD if it ever found 16 processor objects in the
ACPI tables.

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