[time-nuts] refclock -> NTP server settings/tuning?

Eric Williams wd6cmu at gmail.com
Sun Sep 29 11:07:47 EDT 2013


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
<anders.e.e.wallin at gmail.com>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 -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>


More information about the time-nuts mailing list