[ntp:questions] Bug 2341 - ntpd fails to keep up with clock drift at poll>7

Martin Burnicki martin.burnicki at meinberg.de
Thu Nov 28 08:34:28 UTC 2013


David Taylor wrote:
> On 28/11/2013 07:46, xiaoniao112233 at gmail.com wrote:
> []
>>    My time source come from GPS software getting GPS time running in
>> Windows Server 2008 R2;
>> Windows Server 2008 R2 --NTP sync--Linux Cent OS 6.2--w32TIME
>> Sync--Windows XP/7
>> NTP Version NTP 4.2.4 P8
>
> If that is /just/ GPS time, it's likely no better than an Internet
> server.

xiaoniao112233 wrote:
"My time source come from GPS software getting GPS time running in 
Windows Server 2008 R2"

In my opinion this sounds like some other piece of software running on 
the Windows machine, which gets the time from some GPS receiver and 
disciplines the system time.

In such case ntpd only had to make the disciplined system time available 
on the network, e.g. using the local clock driver with stratum-0.

We use a similar approach with the driver software for our GPS PCI cards 
under Windows.

However, the question is how good and smoothly the other software 
synchronized the system time. If it does this only by setting the 
Windows time in periodic intervals, maybe only with limited resolution 
or accuracy, this could result in time steps in Windows, which could 
cause the Linux client also to step the time if the steps back and forth 
exceed 12 ms.

This would explain the reason for the OP's initial questions, how the 
128 ms could be changed in ntpd.

If my assumptions are true then this is a very bad overall timekeeping 
configuration.

> You should be using a PPS (pulse per second) feed to your
> primary NTP server to get best performance.  As others have said, if you
> could (a) use the Linux box as your primary NTP server and (b) feed it
> with PPS, you would have a better configuration.
>
> You can even use the low-cost Raspberry Pi as a Linux NTP server, in a
> variety of configurations, as I describe here:
>
>    http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
>
> Perhaps you could use the Raspberry Pi as your low-cost, primary NTP
> server?

Basically you're right, but it the OP relies on some other Windows 
software getting the time from his GPS device then eventually he can't 
switch to Linux. We'd need to know more details from the OP, but his 
replies are very short and not very precise.

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany



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