How to measure synchronisation performance?

Question

I'm looking for a tool for Debian Linux that can measure the synchronisation performance between two desktops.

I'm using RADClock instead of the NTP client to synchronise my desktops.

Any recommendations?

Answer 1

One of the best tools you can use to answer your question is to use an Endace DAG card. That would give you a hardware-based and reliable "external time reference" to assess the performance of the two computers. It is quite pricey though and perhaps not what you are after.

The RADclock running on the desktops should give you an upper bound on its error. /var/log/radclock.log should regularly print messages similar to this:

i=12345: Clock Error Bound (cur,avg,std) 0.015686 0.011903 0.005100 [ms]

The first value is the "instantaneous" measurement, the second a sliding window average and the last the corresponding standard deviation. The second value is a good estimate and in the example above, the upper bound on the error is about 12 microseconds. All the RADclock can tell you is that its error is smaller than this, which is a realistic comment without a better "external reference".

Now you can try to use ntpdate or write your own ntpdate script to obtain the RADclock time from each desktop computers. The RADclock serves time, so you can cross check the synchronisation, but it is likely the RTT between the two machines and the asymmetry will be too large for you to conclude anything.

Have a look at this paper if you are interested in some gory details:

A Methodology for Clock Benchmarking