How accurate can I expect the time to be from a stratum 0 NTP server on the same subnet on ethernet?

Question

I have an application that depends on gpsd and ntpd to accurately set the system time on a linux machine.

gpsd is fed NMEA + PPS

The application is punping ~25MB per second over the network and I think the loading on the system is causing jitter in the time somehow. (loaded PCI express bus causing irregular interrupt latency)

I have another machine that is not loaded at all that I could setup to read the GPS and act as an NTP server for the loaded machine. (the loaded machine would be getting startum 1 ???)

How accurate can I expect the time to be from a stratum 0 NTP server on the same subnet on Ethernet?

I hope this is not too off topic, I am sure sometime someone else will be happy the answer is documented here. ;-)

Answer 1

Best info I could find on NTP accuracy, seems to point at 1-2 ms in a LAN setting.

Answer 2

NTP is usually considered good for small single-digit ms in this sort of situation.

After it has been running for a few days, there shouldn't really be much jitter in any of the the actual clocks, because the ntpd implements a heap of very long time-constant filtering.

However, you don't really say how you're measuring the time, and whatever mechanism you're using might be just as jittery as (if not more than) the underlying synchronisation.

If you do have a busy network and network cards with really deep buffering, then that might not be helping things, as the jitter between packet arrival and interrupt service will be larger. The fancier your Ethernet switching is the worse it is for timing too - old fashioned hubs are better than switches in this regard.