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

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Message-ID: <20101214092535.GD14178@balbir.in.ibm.com>
Date:	Tue, 14 Dec 2010 14:55:35 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Avi Kivity <avi@...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>,
	Bharata B Rao <bharata.rao@...ibm.com>
Subject: Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

* Rik van Riel <riel@...hat.com> [2010-12-13 12:02:51]:

> On 12/11/2010 08:57 AM, Balbir Singh wrote:
> 
> >If the vpcu holding the lock runs more and capped, the timeslice
> >transfer is a heuristic that will not help.
> 
> That indicates you really need the cap to be per guest, and
> not per VCPU.
>

Yes, I personally think so too, but I suspect there needs to be a
larger agreement on the semantics. The VCPU semantics in terms of
power apply to each VCPU as opposed to the entire system (per guest).
 
> Having one VCPU spin on a lock (and achieve nothing), because
> the other one cannot give up the lock due to hitting its CPU
> cap could lead to showstoppingly bad performance.

Yes, that seems right!

-- 
	Three Cheers,
	Balbir
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