[ntp:questions] Allan deviation survey
David L. Mills
mills at udel.edu
Sat Sep 11 00:05:12 UTC 2010
Running a precision time server on a busy public machine with a widely
varying load is not a good idea and I have no interest in that. Running
experiments on a dedicated, but very busy, time server such as
rackety.udel.edu is much more interesting. As for load-induced
temperature variations, even on a busy NTPserver, the CPU is loaded to
about five percent and the load is constant.
As for your concern about diurnal variations for any reason, that's what
the clock discipline algorithm is for and has nothing to do with Allan
As for the question about the graph, it's from my book. However, there
are examples in the Precision Time Synchronization briefing slides on
the NTP project page at www.eecis.udel.edu/~mills/ntp.html. be advised,
most of those briefings are from the 1990s.
>On 2010-09-10, David L. Mills <mills at udel.edu> wrote:
>>This is a multi-part message in MIME format.
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>>I've done this many times with several machines in several places and=20
>>reported the results in Chapter 12 and 6 in both the first and second=20
>>editions of my book, as well as my 1995 paper in ACM Trans. Networking.=20
>>Judah Levine of NIST has done the same thing and reported in IEEE=20
>>Transactions. He pointed out valuable precautions when making these=20
>>measurements. You need to disconnect all time disciplines and let the=20
>>computer clock free-wheel. You need to continue the measurements for at=20
>>least a week, ten times longer than the largest lag in the plot. You=20
>>need to display on log-log coordinates and look for straight lines=20
>>intersecting at what I have called the Allan intercept. I have Matlab=20
>>programs here that do that and produce graphs like the attached.
>What was the load on those computers? Were they running just this time
>measurement software or were they being used for real work by normal people?
>>From what I have seen from machines that are in use, they heat up
>during the day when people use them and cool off at night when they are
>idle. The amplitude of this component depends on how much work is being
>done. This does not fit the model of random phase noise/random walk
>frequency noise. It has a strong periodic component with period of a
>day. I see this in most of my systems. Such a periodic noise is not part
>of the noise model on which the Allan intercept is based.
>Also this assumes a very particular model of how the measurements are
>made, of how the time corrections are made and of what is desired from
>>Most of the prior work was done 15 years ago. I strongly suspect the=20
>>Allan intercept has moved to lower time values due to the fact that=20
>>modern processors are faster and the interrupt latency is smaller. The=20
>>current NTP distribution includes a NTP simulator that can be excited=20
>>with white phase noise and random-walk frequency noise that very nicely=20
>>models the real noise sources.
>>For those that might want to repeat the experiments, see the attached=20
>>figure. Trace 1 is from an old Sun SPARC IPC; trace 2 is from a Digital=20
>>Alpha. Traces 3 and 4 were generated using artificial noise sources with=20
>>parameters chosen to closely match the measured characteristics. Phase=20
>>noise is generated from an exponential distribution, while frequency=20
>>nose is generated from the integral of a Gaussian distribution, in other=20
>>words a random walk. Trace 4 is the interesting one. It shows the=20
>>projected performance with precision of one nanosecond. The fastest=20
>>machines I have found have a precision of about 500 ns. Note, precision=20
>>is the time taken to read the kernel clock and is not the resolution.
>You graph of course did not make it through to newsnet.
> Have you archived the figure somewhere?
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