From what I've seen on the NTP groups, others' sites, and my own systems,
refclocks must be polled frequently (maxpoll 4).
You will get a more jittery trace but much lower offset and jitter.
The current stable NTP GPS NMEA driver has added user mode PPS support for
systems where a kernel PPS driver is unsupported, and how it does so may be
worth a look, to get similar results from your PTP packets.
Help is available by subscribing to [email protected].
You may also want to see whether you can use what the Linux PTP project offers.
On 2013-09-29 06:11, Anders Wallin wrote:
Thanks for all replies,
I can try changing maxpoll to a larger value and see if the trace is
smoother.
The refclock driver is a userspace C-program (daemon) that essentially does:
while(1) {
gettimeofday(&tv,NULL) // system time, for NTP receiveTimeStamp
get_wr_time(&wr_tv); // WR time, for NTP clockTimeStamp
// write tv and wr_tv to shared memory where NTP expects to see them
sleep(8);
}
This may be the cause of a constant negative offset I see, since one
time-stamp is always read before the other. Perhaps this could be improved
by reading system time both before and after get_wr_time() and reporting
the average of the two readings as receiveTimeStamp? Or measure the offset
and put it as a "time1" offset-value in ntp.conf
If the driver was written as a kernel module, would that run with higher
priority and less variable delay?
I use the same piece of code to log how well system time tracks WR-time.
Here I sometimes see sudden spikes of 100s of microseconds. Could this be
caused by the OS context switching in the middle of my program between the
two timestamp-reading functions? Again, would this improve if the
time-logger was written as a kernel module, or is there some other way of
coding it that avoids context switches and keeps the two time-stamp reading
functions "atomic"?
Standard Ubuntu nowadays has a pre-packaged "lowlatency" kernel which I
think is RT-Preempt with some modifications. But I assume both the
refclock-driver and the logger would need a re-write to take advantage of
the RT-kernel. Does anyone have experienced with that?
thanks,
Anders
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