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Date: Thu, 27 Sep 2012 16:56:58 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Avi Kivity <avi@redhat.com>
CC: Jiannan Ouyang <ouyang@cs.pitt.edu>, Peter Zijlstra <peterz@infradead.org>,
        Rik van Riel <riel@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Ingo Molnar <mingo@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
        Srikar <srikar@linux.vnet.ibm.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
        KVM <kvm@vger.kernel.org>, chegu vinod <chegu_vinod@hp.com>,
        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Gleb Natapov <gleb@redhat.com>
Subject: Re: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE
 handler
References: <20120921115942.27611.67488.sendpatchset@codeblue> <20120921120000.27611.71321.sendpatchset@codeblue> <505C654B.2050106@redhat.com> <505CA2EB.7050403@linux.vnet.ibm.com> <50607F1F.2040704@redhat.com> <5060851E.1030404@redhat.com> <506166B4.4010207@linux.vnet.ibm.com> <5061713D.5060406@redhat.com> <CAJocwcf+8u84_yDC-PK0Yni93YSTWzYvr69nq6b3pNv1MwVJzQ@mail.gmail.com> <50641356.5070008@redhat.com>
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On 09/27/2012 02:20 PM, Avi Kivity wrote:
> On 09/25/2012 04:43 PM, Jiannan Ouyang wrote:
>> I've actually implemented this preempted_bitmap idea.
>
> Interesting, please share the code if you can.
>
>> However, I'm doing this to expose this information to the guest, so the
>> guest is able to know if the lock holder is preempted or not before
>> spining. Right now, I'm doing experiment to show that this idea works.
>>
>> I'm wondering what do you guys think of the relationship between the
>> pv_ticketlock approach and PLE handler approach. Are we going to adopt
>> PLE instead of the pv ticketlock, and why?
>
> Right now we're searching for the best solution.  The tradeoffs are more
> or less:
>
> PLE:
> - works for unmodified / non-Linux guests
> - works for all types of spins (e.g. smp_call_function*())
> - utilizes an existing hardware interface (PAUSE instruction) so likely
> more robust compared to a software interface
>
> PV:
> - has more information, so it can perform better

Should we also consider that we always have an edge here for non-PLE
machine?

>
> Given these tradeoffs, if we can get PLE to work for moderate amounts of
> overcommit then I'll prefer it (even if it slightly underperforms PV).
> If we are unable to make it work well, then we'll have to add PV.
>
Avi,
Thanks for this summary.. It is of great help to proceed in right
direction..