Message-ID: <4FA7E06E.20304@linux.vnet.ibm.com>
Date: Mon, 07 May 2012 20:17:10 +0530
From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Organization: IBM
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To: Avi Kivity <avi@redhat.com>
CC: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        "H. Peter Anvin" <hpa@zytor.com>,
        Marcelo Tosatti <mtosatti@redhat.com>, X86 <x86@kernel.org>,
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Subject: Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks
References: <20120502100610.13206.40.sendpatchset@codeblue.in.ibm.com> <20120507082928.GI16608@gmail.com> <4FA7888F.80505@redhat.com> <4FA7AAD8.6050003@linux.vnet.ibm.com> <4FA7BABA.4040700@redhat.com> <4FA7CC05.50808@linux.vnet.ibm.com> <4FA7CCA2.4030408@redhat.com> <4FA7D06B.60005@linux.vnet.ibm.com> <20120507134611.GB5533@linux.vnet.ibm.com> <4FA7D2E5.1020607@redhat.com> <4FA7D3F7.9080005@linux.vnet.ibm.com> <4FA7D50D.1020209@redhat.com>
In-Reply-To: <4FA7D50D.1020209@redhat.com>
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On 05/07/2012 07:28 PM, Avi Kivity wrote:
> On 05/07/2012 04:53 PM, Raghavendra K T wrote:
>>> Is the improvement so low, because PLE is interfering with the patch, or
>>> because PLE already does a good job?
>>>
>>
>>
>> It is because PLE already does a good job (of not burning cpu). The
>> 1-3% improvement is because, patchset knows atleast who is next to hold
>> lock, which is lacking in PLE.
>>
>
> Not good.  Solving a problem in software that is already solved by
> hardware?  It's okay if there are no costs involved, but here we're
> introducing a new ABI that we'll have to maintain for a long time.
>

Hmm agree that being a step ahead of mighty hardware (and just an
improvement of 1-3%) is no good for long term (where PLE is future).

Having said that, it is hard for me to resist saying :
  bottleneck is somewhere else on PLE m/c and IMHO answer would be
combination of paravirt-spinlock + pv-flush-tb.

But I need to come up with good number to argue in favour of the claim.

PS: Nikunj had experimented that pv-flush tlb + paravirt-spinlock is a 
win on PLE where only one of them alone could not prove the benefit.

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