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

[Avi Kivity <avi@redhat.com>]

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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