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

[Anders Wallin]

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
[1] http://en.wikipedia.org/wiki/The_White_Rabbit_Project