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Date:	Mon, 7 Jul 2008 15:46:51 -0400
From:	Rik van Riel <>
To:	Jeremy Fitzhardinge <>
Cc:	Peter Zijlstra <>,
	Christoph Lameter <>,
	Petr Tesarik <>, Ingo Molnar <>,, Nick Piggin <>
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin
 with irqs disable

On Wed, 25 Jun 2008 19:51:12 -0700
Jeremy Fitzhardinge <> wrote:

> Thomas Friebel presented results at the Xen Summit this week showing 
> that ticket locks are an absolute disaster for scalability in a virtual 
> environment, for a similar reason.  It's a bit irritating if the lock 
> holder vcpu gets preempted by the hypervisor, but its much worse when 
> they release the lock: unless the vcpu scheduler gives a cpu to the vcpu 
> with the next ticket, it can waste up to N timeslices spinning.
> I'm experimenting with adding pvops hook to allow you to put in new 
> spinlock implementations on the fly.

Alternatively, the guest could tell the host which vcpus
are next in line for a ticket spinlock, or a vcpu that gets
scheduled but is not supposed to grab the lock yet can give
some CPU time to the vcpu that should get the lock next.

I believe the IBM PPC64 people have done some work to implement
just that.

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