linux-kernel - Re: Preemptable Ticket Spinlock

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Message-ID: <5175C2D6.9020202@redhat.com>
Date:	Mon, 22 Apr 2013 19:08:06 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	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>,
	Gleb Natapov <gleb@...hat.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 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.

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

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