STARGRASP NTP client

As of SVN revision 4427 of the stage 2 software, the STARGRASP controller has included experimental support for synchronising with an NTP server to provide timing for various events. The client can be started via the ntp command.

Startup Performance

When first started, the NTP client first sets the system time from the first NTP transaction in a single step. Since it cannot make any assumptions about the system's clock drift, it initially starts with a drift correction of zero and attempts to figure out the drift from first principles over successive updates. The result is an initial overshoot that's typically on the order of a couple of milliseconds (the vertical scale below is in usec):

After a couple of minutes, the NTP client typically settles to an NTP offset in the region of 100-200 usec, barring perturbations from the NTP server.

Stargrasp Client: Observed Stability

Show time offset when connected to NTP server via 100Mbit and Gigabit, long-term stability, etc.

Linux NTP Daemon: Observed Stability

The Linux NTP client can be quite unstable at times. The following is the reported NTP time offset (in milliseconds) from a cluster of identical Linux systems synced with an NTP server on the local network:

There are both sudden large offsets observed (tens of milliseconds) and smaller, longer-term offsets compared to the server's clock that are believed to be real (audited by means of a simple program that returns the server's time directly and compares it against each system's clock without heed of the round-trip time on the LAN, which is typically sub-1ms). These departures are as yet unexplained.