[time-nuts] Typical NTP performance? Monitoring multiple NTPservers?

[David J Taylor]

Hi all, I have two NTP-related questions:

1.
We are setting up a White-Rabbit[1] network for time-distribution. We
'seed' the WR-network with 10MHz and PPS signals form atomic clocks. This
means on each computer in the network there's a very accurate PTP-server
running on the WR-card, as well as the normal system time on the computer.
For fun I logged both the system-time (kept on time using NTP) as well as
the PTP time and plotted the error:
http://www.anderswallin.net/wp-content/uploads/2013/07/NTP_time_2013jul26.png
I was wondering if this plot is typical for a (good?) NTP-disciplined
computer clock?
Without NTP the free-running clock shows >40 ppm error:
http://www.anderswallin.net/wp-content/uploads/2013/07/freerunning_vs_ntp_2013jul26.png

The -16ms offset in this graph is probably due to my naive program where we
first call a simple function that asks the NTP time, and then over a serial
link ask for the PTP time. In reality the two time-stamps might be better
synchronized - I don't know. The slow variation I see should be real
however and completely due to drift in the NTP time, since the WR-time is
much more accurate.

2.
Is it possible to run several NTP-clients on one machine? That means I'd
have multiple "system-times" each synchronized to its own NTP server.
If this is possible I'd like to monitor several NTP-servers at once and log
their time-stamps against our WR-time which is known to be good.

thanks,

Anders
==============================================

Anders,

(1) Is this a Windows system?  It has some scheduling which can be at 60 ms 
intervals.  My own PCs here show much less offset than 16 ms, you can see 
the LAN-synced (with some optimisation) Windows PCs here:

  http://www.satsignal.eu/mrtg/performance_ntp.php#windows

Click on a graph for the longer-term measurements.

Using a simple PPS signal you can get much better performance, of course, 
from FreeBSD, Linux and Windows:

  http://www.satsignal.eu/mrtg/performance_ntp.php
  http://www.satsignal.eu/mrtg/performance_ntp.php#windows-stratum-1

(2) No, but you can have multiple servers listed in the NTP configuration 
file, each with the "noselect" option, so that you can use  ntpq -p  to see 
what the offset NTP reports for those servers.

I hope that helps.

Cheers,
David
-- 
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Web: http://www.satsignal.eu
Email: david-taylor at blueyonder.co.uk