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

[Balbir Singh]

* Avi Kivity <avi@xxxxxxxxxx> [2010-12-13 13:57:37]:

> On 12/11/2010 03:57 PM, Balbir Singh wrote:
> >* Avi Kivity<avi@xxxxxxxxxx>  [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.
>

Shifting the cap would break it, no? Anyway, that is something for us
to keep track of as we add additional heuristics, not a show stopper. 

-- 
	Three Cheers,
	Balbir
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