[ntp:questions] comparing 2 NTP implementations
david at ex.djwhome.demon.co.uk.invalid
Sun Jan 27 09:18:32 UTC 2008
Folkert van Heusden wrote:
> I would like to compare 2 NTP implementations. What would be the best
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
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
> 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.
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