[ntp:questions] NTP vs chrony comparison (Was: oscillations in ntp clock synchronization)

David L. Mills mills at udel.edu
Sat Jan 26 18:51:39 UTC 2008


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.

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.

Compute the RMS frequency differences, decimate and repeat. Don't take 
the following seriously, I lifted it without considering context, but 
that's the general idea. Be very careful about missing data, etc., as 
that creates spectral lines that mess up the plot.

p = w; r = diff(x); q = y; i = 1; d = 1;
while (length(q) >= 10)
     u = diff(p) / d;
     x2(i) = sqrt(mean(u .* u) / 2);
     u = diff(r) / d;
     x1(i) = sqrt(mean(u .* u) / 2);
     u = diff(q);
     y1(i) = sqrt(mean(u .* u) / 2);
     p = p(1:2:length(p));
     r = r(1:2:length(r));
     q = q(1:2:length(q));
     m1(i) = d; i = i + 1; d = d * 2;
loglog(m1, x2 * 1e6, m1, x1 * 1e6, m1, y1 * 1e6, m1, (x1 + y1) * 1e6)
axis([1 1e5 1e-4 100]);
xlabel('Time Interval (s)');
ylabel('Allan Deviation (PPM)');
print -dtiff allan


Richard B. Gilbert wrote:
> Unruh wrote:
>> "David L. Mills" <mills at udel.edu> writes:
>>> David,
>>> 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 
>> frequencies,
>> 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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