linux-kernel - Re: [PATCH] raise tsc clocksource rating

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Date:	Tue, 30 Oct 2007 08:19:19 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Ian Pratt <Ian.Pratt@...cam.ac.uk>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Zachary Amsden <zach@...are.com>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	rusty@...tcorp.com.au, Avi Kivity <avi@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Glauber de Oliveira Costa <glauber@....localdomain>,
	Dan Hecht <dhecht@...are.com>,
	Garrett Smith <garrett@...are.com>
Subject: Re: [PATCH] raise tsc clocksource rating


* Ian Pratt <Ian.Pratt@...cam.ac.uk> wrote:

> > > Sigh, I don't really want to have this fight again.
> > 
> > i dont remember us having discussed this before, ever. If there's any
> > "fight" about monotonicity and SMP then it would be a pretty onesided
> > affair, with you being beaten up seriously ;-)
> 
> Actually, it is possible, even for NUMA systems with CPUs running off 
> completely different oscillators, and in the presence of CPU frequency 
> changes, power management, and even in the presence of thermal 
> throttling (though the latter introduces temporary inaccuracies it 
> doesn't affect monotonicity or rate).
> 
> Take a look at the Xen code to see how each physical CPU is 
> independently calibrated on an ongoing basis, how movement of VCPUs 
> between physical CPUs is tracked, and how shared variables are used to 
> ensure montonicity if a guest requires it.

I think that's wishful thinking. Check out:

    http://people.redhat.com/mingo/time-warp-test/time-warp-test.c

change TEST_TSC to 0, run it on an SMP guest (on a reasonably fast 
machine) and let me know whether you can make SMP guests not come up 
with monotonicity violations in the CLOCK tests.

	Ingo
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