linux-kernel - Re: [RFC patch 0/4] TSC calibration improvements

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Message-ID: <20080904163644.GA28340@elte.hu>
Date:	Thu, 4 Sep 2008 18:36:44 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Alok Kataria <akataria@...are.com>,
	Arjan van de Veen <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RFC patch 0/4] TSC calibration improvements


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Thu, 4 Sep 2008, Ingo Molnar wrote:
> > 
> > ah - perhaps a dynamic statistical approach with an estimation of 
> > worst-case calibration error (~= standard deviation) and a quality 
> > threshold to reach? That could dramatically increase the number of 
> > samples while also making it much faster in practice. Nifty!
> 
> Oh, no, I'm _much_ more nifty than that!
> 
> Instead of being very clever, I have a very _stupid_ algorithm, one 
> that has very hardcoded expectations of exactly what it will see. And 
> if it doesn't see exactly that, it just fails early.
>
> I'd post the patch, but I really need to actually _test_ it first, and 
> I haven't rebooted yet.

ok, will wait with patience :)

i've been using adaptive calibration with great success in user-space, 
to run benchmarks on multiple boxes with a variable number of lmbench 
iterations. Once the observable statistical properties of the series of 
measurements looks valid it stops the test iterations and emits a 
result. This both makes things faster (more predictable hw/kernel 
executes certain lmbench tests much faster) and it makes the results 
more reliable (less noise, better cross-hardware, cross-kernel and 
cross-test comparisons). I've got a quality threshold (and a max 
iterations threshold) hardcoded as well.

	Ingo
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