Re: [Xen-devel] [PATCH v3 00/47] xen: add core scheduling support

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.09.2019 10:52, Juergen Gross wrote:
> This is achieved by switching the scheduler to no longer see vcpus as
> the primary object to schedule, but "schedule units". Each schedule
> unit consists of as many vcpus as each core has threads on the current
> system. The vcpu->unit relation is fixed.

There's another aspect here that, while perhaps obvious, I didn't
realize so far: Iirc right now schedulers try to place vCPU-s on
different cores, as long as there aren't more runnable vCPU-s than
there are cores. This is to improve overall throughput, since
vCPU-s on sibling hyperthreads would compete for execution
resources. With a fixed relation this is going to be impossible.
Otoh I can of course see how, once we have proper virtual
topology, this allows better scheduling decisions inside the
guest, in particular if - under the right circumstances - it is
actually wanted to run two entities on sibling threads.

Jan

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