Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

Avi Kivity
2010-12-13T11:57:37Z

On 12/11/2010 03:57 PM, Balbir Singh wrote:
> * Avi Kivity<avi@redhat.com> [2010-12-11 09:31:24]:
>
> > On 12/10/2010 07:03 AM, Balbir Singh wrote:
> > >>
> > >> Scheduler people, please flame me with anything I may have done
> > >> wrong, so I can do it right for a next version :)
> > >>
> > >
> > >This is a good problem statement, there are other things to consider
> > >as well
> > >
> > >1. If a hard limit feature is enabled underneath, donating the
> > >timeslice would probably not make too much sense in that case
> >
> > What's the alternative?
> >
> > Consider a two vcpu guest with a 50% hard cap. Suppose the workload
> > involves ping-ponging within the guest. If the scheduler decides to
> > schedule the vcpus without any overlap, then the throughput will be
> > dictated by the time slice. If we allow donation, throughput is
> > limited by context switch latency.
> >
>
> If the vpcu holding the lock runs more and capped, the timeslice
> transfer is a heuristic that will not help.

Why not? as long as we shift the cap as well.

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