This is the 13th article that I’ve written lately on NTP and PTP timing with Linux. I set out to answer a couple questions for myself and ended up spending two months swimming in an ocean of nanosecond-scale measurements.
When I started, I saw a lot of misinformation about NTP and PTP online. Things like:
Conventional wisdom said that NTP was good for millisecond-scale timing accuracy. I expected that to be rather pessimistic, and expected to see low microsecond to high nanosecond-range syncing with Chrony, at least under controlled circumstances.In a lab environment, it’s possible to get single-digit nanosecond time skew out of Chrony. With a less-contrived setup, 500 ns is probably a better goal. In any case “milliseconds” is grossly underselling what’s possible.
Conventional wisdom also said that PTP was better than NTP when you really cared about time, but that it was more difficult to use and made more requirements on hardware.You know, conventional wisdom is actually right sometimes. PTP is somewhat more difficult to set up and really wants to have hardware support from every switch and every NIC, but once you have that it’s pretty solid.
Along the way I tested NTP and PTP “in the wild” on my network, built a few new GPS-backed NTP (and PTP) servers, collected a list of all known NICs with timing features,Specifically GNSS modules or PPS inputs.
built a testing environment for measuring time-syncing accuracy to within a few nanoseconds, tested the impact of various Chrony polling settings, tested 14 different NICs for time accuracy, and tested how much added latency PTP-aware switches add.
I ran into problems with PTP on Mellanox/nVidia ConnectX-4 and Intel X710 NICs.Weird stuff. The X710 doesn’t seem to like PTP v2.1, and it doesn’t like it when you ask it to timestamp packets too frequently.
I fought with Raspberry Pis. I tested NICs until my head hurt. I fought with statistics.
This little project that I’d expected to last most of a week has now dragged on for two months. It’s finally time to summarize what I’ve learned and celebrate The End Of Time.