linux-kernel - RE: [PATCH] x86: Export tsc related information in sysfs

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Date:	Mon, 24 May 2010 16:16:26 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	john stultz <johnstul@...ibm.com>, "H. Peter Anvin" <hpa@...or.com>
Cc:	Brian Bloniarz <bmb@...enacr.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: RE: [PATCH] x86: Export tsc related information in sysfs

> From: john stultz [mailto:johnstul@...ibm.com]
> Also, you don't really need extra accuracy, you just need it to be the
> same from boot to boot. NTP keeps the correction factor persistent from
> boot to boot via the drift file. The boot argument is just trying to
> save the time (possibly hours depending on ntp config) after a reboot
> for NTP to correct for the new error introduced by calibration.

I was assuming that extra accuracy would decrease the ntp
convergence time by about the same factor (5-6 bits of extra
accuracy would decrease ntp convergence time by 32-64x).
Is that an incorrect assumption?

> From: H. Peter Anvin [mailto:hpa@...or.com]
> A longer sample would make sense if the goal is to freeze it into a
> kernel command line variable, but the real question is how many people
> would actually do that (and how many people would then suffer problems
> because they upgraded their CPU/mobo and got massive failures on post-
> boot.)

Not sure why upgraded mobo's would fail due to a longer sample?

As more and more systems become dependent on clocksource==tsc
and more and more people assume nanosecond-class measurements
are relatively accurate, I'd expect the accuracy of tsc_khz
to become more important.  While desktop users might bristle
at an extra second of boot delay, I'll bet many server
farm administrators would gladly pay that upfront cost
if they know an option exists.
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