[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)
unruh-spam at physics.ubc.ca
Sat Jan 26 22:02:52 UTC 2008
"David L. Mills" <mills at udel.edu> writes:
>There were several different architecture computers considered in the
>1995 and 1998 studies, incluing SPARC, Alpha, Intel and several lab
>instruments. All oscillators conformed to a simple model: white phase
>noise (slope -1) below the intercept, random-walk frequency noise (slope
>+0.5) above the intercept. This is equivalent to your model.
Unfortunately this does not seem to be the way that actual computers
behave, or should \i say, my computers. The problem is that there are
frequency shifts, which occur over very short time scales, and last hours.
These are probably because the copmpters do lots of work during theday and
get hot, and then when the students go home, or have a weekend, they get
cold. Ie, non-linear non-gaussian processes. I certainly understand the
white noise/1/f model, just worry about its applicability to actual
computers used by real people.
>Additional data are in the nanokernel documentation. The only
>differences are in the (x, y) intercept. You don't need das Buch to
>justify this model; there is evidence all over the place. Clocks of all
>kinds from cold rocks to Cesium oscillators all show very similar
>chacteristics, whether modelled in the time domain or frequency domain.
>It's easy to make your own Allan characteristic. Just let the computer
>clock free-run for a couple of weeks and record the offset relative to a
>known and stable standard, preferable at the smallest poll interval you
>can. The PPS from a GPS receiver is an ideal source, but you have to
>jerry-rig a means to capture each transition.
Yes, I did that. I adapted a parallel interrupt routine to capture the
times and make them available as a /dev. It is what shows the behavour of
the clock. on one of my real systems.
>Richard B. Gilbert wrote:
>> Unruh wrote:
>>> "David L. Mills" <mills at udel.edu> writes:
>>>> 1. I have explained in very gory detail in many places how the time
>>>> constant is chosen for the best accuracy using typical computer
>>>> oscillators and network paths. See the briefings on the NTP project
>>>> page and especially the discussion about the Allan intercept. If you
>>>> want the
>>> The Allan intercept is predicated on a very specific model of the
>>> noise in
>>> a clock ( as I recall basically random gaussian noise at high
>>> and 1/f noise at low). It is not at all clear that real computers comply
>>> with that.
>>>> best accuracy over the long term, you had better respect that. Proof
>>>> positive is in my 1995 SIGCOMM paper, later IEEE Transactions on
>>>> Networking paper and das Buch. I abvsolutely relish scientific
>>>> critique, but see the briefings and read the papers first.
>>>> 2. To reduce the convergence time, reduce the time constant, but only
>>>> at the expense of long term accuracy. An extended treatise on that is
>>>> in das Buch, especially Chaptera 4, 6 and 12. I would be delighted to
>>>> hear critique of the material, but read the chapters first.
>>> While you may know what in the world Das Buch is (Hitlers Mein Kampf?)
>>> I do
>>> not. Nor do I know where to get it.
>> Computer Network Time Synchronization: The Network Time Protocol by
>> David L. Mills (Hardcover - Mar 24, 2006)
>> Available from Amazon.com. You may be able to find a copy at a
>> University Book store. Be prepared for "Sticker Shock". It ain't
>> cheap! Publishing in small quantities is EXPENSIVE!!! It's different
>> when you can amortize your setup costs over 50,000 copies!
>> "Das Buch" is unlikely to become a best seller!
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