I've been hearing about PTP in a few places. Does anyone here have experience with it to know if it would provide better performance in a situation like this?
On Sun, Sep 29, 2013 at 5:11 AM, Anders Wallin <[email protected]>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 > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
