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

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Date:	Wed, 26 May 2010 16:22:58 -0400
From:	Brian Bloniarz <bmb@...enacr.com>
To:	john stultz <johnstul@...ibm.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Dan Magenheimer <dan.magenheimer@...cle.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 05/26/2010 03:49 PM, john stultz wrote:
> On Wed, 2010-05-26 at 14:44 -0400, Brian Bloniarz wrote:
>> On 05/26/2010 12:25 PM, john stultz wrote:
>>> Brian: is this something the NTPd folks actually want? Has anyone
>>> checked with them before we hand down the solution from high upon on
>>> lkml mountain?
>>
>> I haven't checked, it's been a while since I dealt with
>> this problem. The NTP maintainers definitely complain about the
>> quick TSC calibration code like it's a bug:
>> (e.g. http://www.mail-archive.com/questions@lists.ntp.org/msg02079.html).
>> Anyway I'll reach out before I spend any time investing in
>> a solution that they don't want (and you don't like :).
>>
>>> Personally I think NTPd should be a little more savvy about how far it
>>> trusts the drift file when it starts up. Since I believe its
>>> fast-startup mode can quickly estimate the drift well within 100ppm,
>>> which is about the maximum variance I've seen from the calibration code.
>>
>> The workaround we went with was to remove the drift file on 
>> every reboot. But in our experience, even with iburst, converging takes
>> a long time. I don't have hard numbers since it's been a long time since
>> I investigated the problem, but we defined failure as >1ms offset syncing
>> to a server in our LAN, and a cold NTP boot takes 10-20 hours to get
>> there.
> 
> Ok. If its been awhile, you may find recent kernels (2.6.31+) are much
> faster to converge due to adjustments made to the SHIFT_PLL constant.
> This was done explicitly to address issues similar to what you describe
> above.

My tests were pre-2.6.31, this is really good to know. I'll take
another look on recent kernels.
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