NTP Performance

[ASSI]

Richard Laager via devel writes:
> On 11/21/19 12:55 AM, ASSI via devel wrote:
>>> I could try fiddling around with the polling interval. Next steps might
>>> be to try raising the polling the interval to 4 and/or lowering it to 1.
>
>> It is generally inadvisable to use too short polling intervals unless
>> rthe clock you are disciplining is exceptionally unstable.
>
> Do you have a recommended polling interval for the GPS PPS source? I've
> looked at some ADEV charts (in general, nothing generated here)
> comparing quartz and GPS, and it seems like maybe I want ~30-60 seconds?

I use 4 at the moment, but mainly to have the statistics for all
machines on the same basis.  The best measurements I've seen for
contemporary hardware are from the RADclock papers and they all suggest
that the Allan intercept is between 300…3000s.  So once you've removed
all the bumps from other influences, a polling interval of 10 (1024s)
should be OK.

> I'm currently running with the values removed (i.e. using the defaults
> of minpoll 6 maxpoll 10). It's still at 64. I'm avoiding touching the
> server until it's been at least 24 hours of stability.

In general, a short polling interval will keep the clock close to
the PPS pulse, but chases the frequency around a lot to do so.  A
longer polling interval will keep the frequency more stable while
allowing the PPS to be less well aligned over short periods of time.

> I tried changing from gpsd SHM to the PPS driver, but then my offset
> (which was with SHM too if I added it back) from ntpq -p was -100, which
> I believe is milliseconds. That made me wonder if it was catching the
> wrong edge of the PPS signal, as the pulse width on this GPS is 100 ms.

Install pps-tools and look at the PPS sequence with ppstest or ppswatch
and it should become pretty clear what the correct edge is.  You said
you use a "motherboard serial port".  If that's RS232 it probably needs
clear edge, otherwise if it's TTL most likely assert edge.  Depending on
what the serial controller actually is it may need to poll one of the
two edges (which can actually be more precise than an interrupt on some
hardware).  Don't forget to put the serial into low-latency mode.

> At this point, I'm focusing on proving ntp2.wiktel.com is back to normal
> GPS operation and seeing how the polling interval change behaves. Then I
> might try to switch from SHM to PPS again.

You might want to say what you changed and when…


Achim.
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