[ntp:questions] NTP Cheat Sheet

Heiko Gerstung heiko.gerstung at meinberg.de
Fri May 9 09:51:21 UTC 2008


David Woolley schrieb:
> Heiko Gerstung wrote:
>> For me maxpoll is an important parameter as well because ntpd switches 
>> to larger polling intervals pretty fast and for a lot of our customers 
>> it is important that ntpd recognizes as fast as possible if a GPS 
>> signal is lost or the GPS receiver/IRIG time code reader/AM radio 
>> looses sync 
> 
> But ntpd should only switch to a long poll interval if it believes that 
> it will not accumulate a significant error if it drifts for many times 
> the poll interval, so, if the poll interval is long the time available 
> before you need to switch to an alternative source is also long.
> 
> I think what you are actually saying is that you believe there is a bug 
> in ntpd that causes it to increase the loop time constant prematurely.
> 
> The other possibility is that you are are saying that your reference 
> clocks will give out wrong times for a significant period before they 
> declare themselves down, and you want the accumuated error wiped out as 
> soon as possible after the clock discovers its is giving bad times. Even 
> then, a long loop time constant will require bad input for a long time 
> to cause real problems.

No, I apologize if I did not make this clear. I said that our customers want 
ntpd to show that something is wrong with the internal time source, probably for 
management reasons. Additionally they often deploy more than one refclock and if 
one of them looses sync, they want the second unit to take over.

I strongly assume that ntpd is correctly increasing the polling interval.

Of course our refclocks will not give out wrong time for a significant period 
:-) ...

> However, it looks to me as though the code only ever uses the minpoll 
> value for reference clocks unless the clock driver itself actually 
> overrides the NTP_FIXPOLL flag, or the poll rate, directly.  I'm not 
> sure I've not missed something, though, and I don't have a system with a 
> reference clock to check.

I have a system with a refclock and it honours the minpoll / maxpoll settings 
just like other association types do.

>> for some other reason. They want the unit to switch as soon as 
>> possible to a fallback source which will take significantly longer 
>> with a polling interval of 64s when compared to 16s ... the same 
>> applies when the refclock recovers after becoming unsynchronized.
> 
> There is a better case for this, always assuming that maxpoll is 
> relevant for reference clocks.

What do you mean with "better case"?

Best Regards,
  Heiko




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