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

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Message-ID: <4D0614CC.5060900@redhat.com>
Date:	Mon, 13 Dec 2010 14:42:52 +0200
From:	Avi Kivity <avi@...hat.com>
To:	balbir@...ux.vnet.ibm.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

On 12/13/2010 02:39 PM, Balbir Singh wrote:
> * 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?

The total cap for the guest would remain.

> Anyway, that is something for us
> to keep track of as we add additional heuristics, not a show stopper.

Sure, as long as we see a way to fix it eventually.

-- 
error compiling committee.c: too many arguments to function

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