linux-kernel - Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

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Message-ID: <4F743247.5080407@redhat.com>
Date:	Thu, 29 Mar 2012 11:58:31 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	Alan Meadows <alan.meadows@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KVM <kvm@...r.kernel.org>, Andi Kleen <andi@...stfloor.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Virtualization <virtualization@...ts.linux-foundation.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Stephan Diestelhorst <stephan.diestelhorst@....com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Attilio Rao <attilio.rao@...rix.com>
Subject: Re: [PATCH RFC V6 0/11] Paravirtualized ticketlocks

On 03/28/2012 08:21 PM, Raghavendra K T wrote:
>
>>
>>
>>         Looks like a good baseline on which to build the KVM
>>         implementation.  We
>>         might need some handshake to prevent interference on the host
>>         side with
>>         the PLE code.
>>
>
> I think I still missed some point in Avi's comment. I agree that PLE
> may be interfering with above patches (resulting in less performance
> advantages). but we have not seen performance degradation with the
> patches in earlier benchmarks. [ theoretically since patch has very
> slight advantage over PLE that atleast it knows who should run next ].

The advantage grows with the vcpu counts and overcommit ratio.  If you
have N vcpus and M:1 overcommit, PLE has to guess from N/M queued vcpus
while your patch knows who to wake up.

>
> So TODO in my list on this is:
> 1. More analysis of performance on PLE mc.
> 2. Seeing how to implement handshake to increase performance (if PLE +
> patch combination have slight negative effect).

I can think of two options:
- from the PLE handler, don't wake up a vcpu that is sleeping because it
is waiting for a kick
- look at other sources of pause loops (I guess smp_call_function() is
the significant source) and adjust them to use the same mechanism, and
ask the host to disable PLE exiting.

This can be done incrementally later.

>
> Sorry that, I could not do more analysis on PLE (as promised last time)
> because of machine availability.
>
> I 'll do some work on this and comeback. But in the meantime, I do not
> see it as blocking for next merge window.

-- 
error compiling committee.c: too many arguments to function

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