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

[Magnus Danielson]

Hej Anders,

On 26/07/13 09:21, Anders Wallin wrote:
> 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?

There is folks here that have done tons of plots. You can do better I 
think, but it depends on network, load and configuration.

> 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.

Rather, you set up multiple peers to your NTPD and you then monitor them 
as well as the system-time, and by combining source_n-NTPD and NTPD-WR 
differences you get your full set of source_n-WR set of differences.

Looks like you have good reasons to tweak that NTPD setup, it should be 
able to perform better than what you have logged.

Cheers,
Magnus