linux-kernel - Re: Preemptable Ticket Spinlock

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Date:	Tue, 23 Apr 2013 08:57:44 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Avi Kivity <avi.kivity@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Chegu Vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	Karen Noel <knoel@...hat.com>
Subject: Re: Preemptable Ticket Spinlock

On Mon, Apr 22, 2013 at 07:08:06PM -0400, Rik van Riel wrote:
> On 04/22/2013 04:55 PM, Peter Zijlstra wrote:
> >On Mon, 2013-04-22 at 16:46 -0400, Jiannan Ouyang wrote:
> 
> >>- pv-preemptable-lock has much less performance variance compare to
> >>pv_lock, because it adapts to preemption within  VM,
> >>   other than using rescheduling that increase VM interference
> >
> >I would say it has a _much_ worse worst case (and thus worse variance)
> >than the paravirt ticket implementation from Jeremy. While full
> >paravirt ticket lock results in vcpu scheduling it does maintain
> >fairness.
> >
> >If you drop strict fairness you can end up in unbounded starvation
> >cases and those are very ugly indeed.
> 
> If needed, Jiannan's scheme could easily be bounded to prevent
> infinite starvation. For example, we could allow only the first
> 8 CPUs in line to jump the queue.
> 
> However, given the way that virtual CPUs get scheduled in and
> out all the time, I suspect starvation is not a worry, and we
> will not need the additional complexity to deal with it.
> 
FWIW RHEL6 uses unfair spinlock when it runs as a guest. We never got
reports about problems due to this on any scale.

> You may want to play around with virtualization a bit, to get
> a feel for how things work in virt land.
> 
> -- 
> All rights reversed

--
			Gleb.
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