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

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Message-ID: <20101211135727.GU3158@balbir.in.ibm.com>
Date:	Sat, 11 Dec 2010 19:27:27 +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> [2010-12-11 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. 

-- 
	Three Cheers,
	Balbir
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