Re: [PATCH] qemu-kvm: response to SIGUSR1 to start/stop a VCPU (v2)

Avi Kivity
2010-12-02T09:08:31Z

On 12/01/2010 09:07 PM, Peter Zijlstra wrote:
> >
> > The pause loop exiting& directed yield patches I am working on
> > preserve inter-vcpu fairness by round robining among the vcpus
> > inside one KVM guest.
>
> I don't necessarily think that's enough.
>
> Suppose you've got 4 vcpus, one is holding a lock and 3 are spinning.
> They'll end up all three donating some time to the 4th.
>
> The only way to make that fair again is if due to future contention the
> 4th cpu donates an equal amount of time back to the resp. cpus it got
> time from. Guest lock patterns and host scheduling don't provide this
> guarantee.
>

That is fine. Fairness is pointless if it there's no work to be done
for the other three threads. Just like if you have four processes, one
of which if processing, the others sleeping, cpu is not divided equally.

--
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