[ntp:questions] Allan deviation survey

Miroslav Lichvar mlichvar at redhat.com
Fri Sep 10 13:04:18 UTC 2010


I'm trying to find out how a typical computer clock oscillator
performs in normal conditions without temperature stabilization or a
stable CPU load and how far it is from the ideal case which includes
only a random-walk frequency noise.

A very useful statistics is the Allan deviation. It can be used to
compare performance of oscillators, to make a guess of the optimal
polling interval, whether enabling ntpd daemon loop to use FLL will
help, how much better chrony will be than ntpd, etc.

If you have a PPS device and would be willing to run the machine
unsynchronized for a day, I'd like to ask you to measure the Allan
deviation and send it to me.

I wrote a small ncurses program that can be used with LinuxPPS to
capture the PPS samples and create an Allan deviation plot. An
overview is displayed and continuously updated while samples are
collected. Data which can be used to make an accurate graph (e.g. in
gnuplot) are written to the file specified by -p option when the
program is ended or when the 'w' key is pressed.

Available at:

Obligatory screenshot :-)
           Allan deviation plot (span 11:09:55, skew +0.0)
     │  + +
1e-06├      +     
     │       +++
     │          ++ 
     │            +++ 
     │               ++  
1e-07├                 +++
     │                    ++ 
     │                      +++
     │                         ++                ++++++
     │                           +++         ++++      
1e-08├                              +++++++++         
   1e+00       1e+01	   1e+02       1e+03	   1e+04       1e+05
w:Write   q:Quit   r:Reset   1:Skew 0.0   2:Skew +1.0   3:Skew -0.5

To make a good plot:
1. disable everything that could make system clock adjustments
2. start ./ppsallan -p adev.plot /sys/devices/virtual/pps/pps0/assert
   (change the sys file as appropriate)
3. let it collect the PPS samples for at least one day
4. hit q and send me the adev.plot file


Miroslav Lichvar

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