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

[Chris Albertson]

I guess a LOT would depend on your driver and the interrupt latency in the
system.

The pulse per second system can work much better then you are getting even
when the PPS has far less accuracy then WR.  I think because must of the
work for PPS happens inside a low latency handler that only runs few
machine instructions.   Are you working mostly in user space?  Are the
interrupts maskable?


On Fri, Sep 27, 2013 at 9:14 AM, Anders Wallin
<anders.e.e.wallin at gmail.com>wrote:

> Hi all,
>
> If I have an NTP server with a refclock that is assumed very accurate (~few
> nanoseconds/day drift at most, with << 1 ns jitter), how well should NTP be
> able to keep the system clock "on time"?
>
> I'm using a White Rabbit (WR) [1] SPEC PCI-card that receives accurate time
> over WR, and I wrote a shared-memory refclock driver [2] for it. I added it
> to ntp.conf with
> server 127.127.28.0 minpoll 4 maxpoll 4
>
> and I get the following graph. The error is system time compared to
> WR-time, where we can assume that WR-time is very accurate.
> http://www.anderswallin.net/wp-content/uploads/2013/09/ntp_graph.png
>
> Are the minpoll and maxpoll parameters the only way to tune how NTP locks
> onto a refclock? To me the trace (blue in the graph) looks quite noisy and
> I would guess it could be improved by "PID-tuning" inside NTP?
> As a comparison the red trace shows another computer using standard NTP
> without a refclock and the trace is much smoother.
> If it matters I measured the free running system clock on this computer to
> have an error of about 40 ppm [3].
>
>
> thanks,
> Anders
>
> [1] http://en.wikipedia.org/wiki/White_Rabbit
> [2] https://github.com/aewallin/ptp2ntpd
> [3]
>
> http://www.anderswallin.net/wp-content/uploads/2013/07/freerunning_vs_ntp_2013jul26.png
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-- 

Chris Albertson
Redondo Beach, California