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

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Date:	6 May 2009 10:46:45 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	johnstul@...ibm.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.

> As always, feedback or testing (especially on non-x86 arches) would be
> greatly appreciated.

Applying it to a couple of PPS-synchronized machines (where I had already
dropped the poll interval into the basement for better convergence, have
to try reverting that) seems to be working.

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.
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