Message-ID: <1366663714.8337.12.camel@laptop>
Subject: Re: Preemptable Ticket Spinlock
From: Peter Zijlstra <peterz@infradead.org>
To: Rik van Riel <riel@redhat.com>
Cc: Jiannan Ouyang <ouyang@cs.pitt.edu>, LKML <linux-kernel@vger.kernel.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        Avi Kivity <avi.kivity@gmail.com>, Gleb Natapov <gleb@redhat.com>,
        Ingo Molnar <mingo@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>,
        Srikar <srikar@linux.vnet.ibm.com>, "H. Peter Anvin" <hpa@zytor.com>,
        "Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
        KVM <kvm@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
        Chegu Vinod <chegu_vinod@hp.com>,
        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Andrew Jones <drjones@redhat.com>, Karen Noel <knoel@redhat.com>
Date: Mon, 22 Apr 2013 22:48:34 +0200
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On Mon, 2013-04-22 at 22:44 +0200, Peter Zijlstra wrote:
> On Mon, 2013-04-22 at 16:32 -0400, Rik van Riel wrote:
> > 
> > IIRC one of the reasons was that the performance improvement wasn't
> > as obvious.  Rescheduling VCPUs takes a fair amount of time, quite
> > probably more than the typical hold time of a spinlock.
> 
> IIRC it would spin for a while before blocking..
> 
> /me goes re-read some of that thread...
> 
> Ah, its because PLE is curing most of it.. !PLE it had huge gains but
> apparently nobody cares about !PLE hardware anymore :-)

Hmm.. it looked like under light overcommit the paravirt ticket lock
still had some gain (~10%) and of course it brings the fairness thing
which is always good.

I can only imagine the mess unfair + vcpu preemption can bring to guest
tasks.

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