linux-kernel - Re: [PATCH RFC 0/4] Paravirtual spinlocks

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Date:	Tue, 8 Jul 2008 11:01:55 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	virtualization@...ts.linux-foundation.org,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Jens Axboe <axboe@...nel.dk>,
	Xen devel <xen-devel@...ts.xensource.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Lameter <clameter@...ux-foundation.org>,
	Petr Tesarik <ptesarik@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Friebel <thomas.friebel@....com>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH RFC 0/4] Paravirtual spinlocks

On Tuesday 08 July 2008 10:37:54 Jeremy Fitzhardinge wrote:
> Rusty Russell wrote:
> > On Tuesday 08 July 2008 05:07:49 Jeremy Fitzhardinge wrote:
> >> At the most recent Xen Summit, Thomas Friebel presented a paper
> >> ("Preventing Guests from Spinning Around",
> >> http://xen.org/files/xensummitboston08/LHP.pdf) investigating the
> >> interactions between spinlocks and virtual machines.  Specifically, he
> >> looked at what happens when a lock-holding VCPU gets involuntarily
> >> preempted.
> >
> > I find it interesting that gang scheduling the guest was not suggested as
> > an obvious solution.
>
> It's an obvious answer, but not an obvious solution.  You trade off
> wasting time spinning vs wasting time waiting for N vcpus to be free for
> scheduling.

Perhaps, but with huge numbers of cores (as The Future seems to promise) and 
significant overcommit not sure how bad this would be.

> Or something; seems much more complex, particularly if you
> can do a small guest tweak to solve the problem.

But AFAICT it's one of a related set of problems where all VCPUs are required 
for a task.  Hackbench comes to mind.  There's going to be a lot of 
ping-ponging and you'll approach gang scheduling to get decent performance.

> > A little disappointing that you can't patch your version inline.
>
> Spinlock code isn't inlined currently, so I hadn't considered it.  The
> fast path code for both lock and unlock is nearly small enough to
> consider it, but it seems a bit fiddly.

Yeah, OK.

Thanks,
Rusty.
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