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

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Date:	Mon, 24 May 2010 16:19:44 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	john stultz <johnstul@...ibm.com>,
	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

On 05/24/2010 04:16 PM, Dan Magenheimer wrote:
>> 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?
> 

Yes.

>> 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?

Not due to a longer sample, but a frozen 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.

Not really.  The delta measurements aren't the issue here, but rather
walltime convergence.

	-hpa
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