Contents
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
Show how unstable linux NTP can be?
Attachments
- stargrasp_ntp_startup.png (47 kB) - STARGRASP NTP client startup, added by crae on Wed Oct 10 11:18:09 2012.
- linux_ntp_stability.png (86 kB) - Example Linux NTP performance, added by crae on Thu Oct 11 14:58:35 2012.
- ntp_controller_100mbit.png (135 kB) - NTP time offset of 16 STARGRASP controllers synched to linux NTP server via 100Mbit link, added by crae on Wed Oct 17 15:40:02 2012.
- ntp_controller_gigabit.png (46 kB) - NTP time offset of 16 STARGRASP controllers synched to linux NTP server via 1Gigabit link, added by crae on Wed Oct 17 15:40:22 2012.
- ntp_linux_server_bounce.png (29 kB) - Sudden changes in linux NTP server's reported time offset, added by crae on Wed Oct 17 16:19:57 2012.
- ntp_controller_bounce.png (56 kB) - Ringing in reported time offset of 16 stargrasp controllers after abrupt NTP server time offset changes, added by crae on Wed Oct 17 16:20:54 2012.
