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

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Date:	Thu, 4 Sep 2008 18:00:37 +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:
> > 
> > i've added them to tip/x86/tsc and merged it into tip/master - if 
> > there's test success we can merge it into x86/urgent as well and push it 
> > into v2.6.27. Any objections to that merge route?
> 
> I don't think it's quite that urgent, and wonder what the downside is 
> of just changing the timeout to 10ms. On 32-bit x86, it was 30ms (I 
> think) before the merge, so it sounds like 50ms was a bit excessive 
> even before the whole "loop five times"/
>
> So _short_ term, I'd really prefer (a) looping just three times and 
> (b) looping with a smaller timeout.

ok - sounds very good to me! It's quite late in the cycle so the more 
commits we can delay to v2.6.28 the better IMHO.

> Long-term, I actually think even 10ms is actually a total waste. I'll 
> post my trial "quick calibration" code that is more likely to fail 
> under virtualization or SMM (or, indeed, perhaps even on things like 
> TMTA CPU's that can have longer latencies due to translation), but 
> that is really fast and knows very intimately when it succeeds. I just 
> need to do slightly more testing.

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!

	Ingo
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