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

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Message-ID: <20101213123944.GA14178@balbir.in.ibm.com>
Date:	Mon, 13 Dec 2010 18:09:44 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Rik van Riel <riel@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

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

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

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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