[time-nuts] Recommendations for NTP server

Geoff Powell geoff at g8kbz.demon.co.uk
Sun Apr 23 17:42:58 EDT 2006

In article <444BED98.7060401 at febo.com>, John Ackermann N8UR
<jra at febo.com> writes
>FreeBSD is definitely the best tuned OS for NTP timekeeping, but Linux
>can do OK.  The biggest problem is that there's no kernel support for
>PPS signals in the 2.6 series of kernels.  There is a patch available
>for a few 2.6 versions (including 2.6.15) called "PPS-kit-alpha" that
>implements at least the ability to pass the PPS signal through the
>serial port to NTP, but doesn't implement the kernel time discipline.

I suspected that, but couldn't be sure - net sources are ambiguous on
the point, even the NTP documentation.

Is PPS kernel discipline compiled into the default FreeBSD kernel?

>I am not running 2.6.15 because it has some problems with the linux-gpib
>drivers that I depend on for data logging, but instead using a shared
>memory driver called "shm_linux_clock" that runs as a separate program
>and monitors a serial port for PPS and passes that through to NTP.  It
>works very well and holds within 10s of microseconds, most of the time.

Does this require a kernel recompile, is it a module, or does it run in

Initially, I'd like to stick with Debian, since I have a (somewhat)
better understanding of it - OpenBSD would be a whole new ballgame. I'm
a newbie in either case.

>I plot my NTP servers' performance at
>http://www.febo.com/time-freq/ntp/stats so you can see the results I'm
>getting with both Linux and FreeBSD if you'd like.  At the moment the
>data is a mess because I've been changing configurations and rebooting,
>etc., but if you look past the noise, you can see the steady state
>performance where all five stratum 1 servers are tracking within 100us
>of each other.  If you subtract the WWVB clock, which has some problems
>when it loses lock, the GPS and Cs clocks track each other within 10 or
>20 us when everything is stable.

Yes, I noticed that all the machines vary more-or-less together, which
suggests a common-mode effect - is databox playing up? 

I know that radio clocks (I'd be using MSF Rugby, or DCF Mainflingen)
are poor in comparison, and probably worse than your WWVB results,
because the receivers I'd be using would have a lot less intelligence
than yours.

Geoff Powell

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