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

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Date:	Mon, 24 May 2010 13:39:26 -0700
From:	john stultz <johnstul@...ibm.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Dan Magenheimer <dan.magenheimer@...cle.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 Mon, 2010-05-24 at 13:20 -0700, H. Peter Anvin wrote:
> On 05/24/2010 11:51 AM, john stultz wrote:
> > 
> > Hmmm. That could be an option for newer cpus that I wouldn't oppose.
> > 
> > While Peter is correct that the stamped value is probably not very
> > accurate, atleast it would be constant from boot to boot, and NTP's
> > calculated drift value would be correct. 
> > 
> > We'd need a check to make sure its not way off, since NTP will give up
> > if its outside 500ppm.  So as long as its close to the calibrated value,
> > we probably could use it.
> > 
> 
> Is that still the case?  I thought newer versions of NTP could deal with
> large values.  Inaccuracies of way more than 500 ppm are everyday.

That's scary. 

Yea, in the kernel the ntp freq correction tops out at 500ppm. Almost
all the systems I see tend to fall in the +/- 200ppm range (if there's
not something terribly wrong with the hardware).

So maybe things aren't so bad out there? Or is that wishful thinking?

thanks
-john


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