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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin with irqs disable
Date: Mon, 7 Jul 2008 21:52:59 +1000
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Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Christoph Lameter <clameter@sgi.com>, Petr Tesarik <ptesarik@suse.cz>,
       Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
References: <Pine.LNX.4.64.0805061220570.30604@schroedinger.engr.sgi.com> <48630420.1090102@goop.org> <200807072150.39571.nickpiggin@yahoo.com.au>
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On Monday 07 July 2008 21:50, Nick Piggin wrote:
> On Thursday 26 June 2008 12:51, 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 didn't realise it is good practice to run multiple "virtual CPUs"
> of the same guest on a single physical CPU on the host...
>
> > I'm experimenting with adding pvops hook to allow you to put in new
> > spinlock implementations on the fly.  If nothing else, it will be useful
> > for experimenting with different algorithms.  But it definitely seems
> > like the old unfair lock algorithm played much better with a virtual
> > environment, because the next cpu to get the lock is the next one the
> > scheduler gives time, rather than dictating an order - and the scheduler
> > should mitigate the unfairness that ticket locks were designed to solve.
>
> ... if it is good practice, then, virtualizing spinlocks I guess is
> reasonable. If not, then "don't do that". Considering that probably
> many bare metal systems will run pv kernels, every little cost adds
> up.

Although, you wouldn't need to oversubscribe physical CPUs to hit
suboptimal behaviour.

Basically, I just ask for performance improvement to be measured
with some "realistic" configuration, then it should be easier to
justify.
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