linux-kernel - Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

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Message-ID: <4FA7E06E.20304@linux.vnet.ibm.com>
Date:	Mon, 07 May 2012 20:17:10 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Attilio Rao <attilio.rao@...rix.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	linux-doc@...r.kernel.org, KVM <kvm@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC V8 0/17] Paravirtualized ticket spinlocks

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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