linux-kernel - Re: [RFC][PATCH] Adjust SHIFT_PLL to improve NTP convergence.

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Message-ID: <4A02C2A7.4033.ED06D54@Ulrich.Windl.rkdvmks1.ngate.uni-regensburg.de>
Date:	Thu, 07 May 2009 11:14:45 +0200
From:	"Ulrich Windl" <ulrich.windl@...uni-regensburg.de>
To:	"George Spelvin" <linux@...izon.com>
CC:	linux-kernel@...r.kernel.org, linux@...izon.com,
	tglx@...utronix.de, ulrich.windl@...uni-regensburg.de,
	williams@...hat.com, zippel@...ux-m68k.org
Subject: Re: [RFC][PATCH] Adjust SHIFT_PLL to improve NTP convergence.

On 6 May 2009 at 10:46, George Spelvin wrote:

[...]
> One thing I just noticed, although I'm sure it has happened in the past,
> is that there's a frequency jump each boot due to TSC recalibration.
> 
> 		Machine 1	Machine 2
> Old CPU MHz	2500.170	2500.138
> Old NTP ppm	-24.63 +/-0.01	-30.27 +/-0.02
> 
> New CPU MHz	2500.176	2500.193
> New NTP ppm	-22.26 +/-0.01	-8.20 +/-0.015 
> 
> "True" CPU MHz	2500.2316	2500.2136
> 
> I should look and see if there's an easy way to tighten that tolerance.
> The current algorithm is fine for jiffies purposes, but a few seconds'
> worth of background calibration would produce a more stable estimate.

Hi,

IMHO, the "value" of calibration during boot is dubious (unless you just reboot a 
"hot" machine): Usually the machine is cold on reboot, and the oscillators will 
drift a lot initially. Depending on your environment, the temperatures inside your 
box may rise about 10°C or more within a few minutes.

Maybe an experimental feature "recalibrate anytime (and as long as you'd like)" 
would be interesting...

Regards,
Ulrich

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