[time-nuts] Recommendations for NTP server

[John Ackermann N8UR]

Geoff Powell said the following on 04/23/2006 05:00 PM:

> So my question is - should I continue with Debian Stable, or would
> OpenBSD be better for sub-microsecond accuracy? Indeed, is sub-
> microsecond offset achievable with this hardware? GPSDOs and Rb or Cs
> standards are not yet practical politics.

Hi Geoff --

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 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.

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.

John