Folkert van Heusden wrote: > I would like to compare 2 NTP implementations. What would be the best > way?
The biggest problem is finding out the time on the machines without using NTP. One approach is to use a simulator, but that assumes that the simulator correctly represents clock imperfections and changes in in the environment. It is also fairly easy to output a quite accurate indication of the time that the machine thinks it has, provided that you have a local (non-USB) parallel port. However, the problem there is that they will not output at the same time, which means that you cannot use very simple hardware to measure the difference, but will need hardware that accurately log both the actual time of the report and the time the reporter thought it had. The clock for this probably doesn't have to be too accurate, providing that you monitor your source of true time frequently, but it does have to have good precision and predictable latency. You cannot output at the same time because of indeterminate interrupt latency and because modern systems interpolate between clock interrupts and correct the time by adjusting the interpolation, not by aligning the clock interrupts onto the exact 100ms. The other requirement, that has been noted in the recent chrony discussion, is that you must run the test with real workloads on the machines. > I was thinking of configuring 7 upstream servers on these 2 physical > servers and then on a third pc (which is also synced against these 7) > check the difference? You need to have physically distinct servers, so that clock variations are not correlated. They ought to be in different environments. Alternatively, the server needs to directly read a high stability hardware clock and simulate perturbations. > _______________________________________________ questions mailing list questions@lists.ntp.org https://lists.ntp.org/mailman/listinfo/questions
