[ntp:questions] Change poll interval at runtime?

unruh unruh at invalid.ca
Sun Feb 26 23:20:41 UTC 2012


On 2012-02-26, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
> On 2/26/2012 12:55 AM, Ron Frazier (NTP) wrote:
>> On 2/25/2012 5:05 PM, A C wrote:
>>> On 2/25/2012 13:09, Richard B. Gilbert wrote:
>>>> On 2/25/2012 1:20 AM, A C wrote:
>>>>> On 2/24/2012 21:26, A C wrote:
>>>>>> Is it possible to change the polling interval of one or more
>>>>>> associated
>>>>>> servers at runtime? It seems like I should be able to run:
>>>>>>
>>>>>> ntpq -c "writevar &associd hpoll=N" or is it ppoll?
>>>>>>
>>>>>
>>>>> Actually, I should have been more specific and say change the minimum
>>>>> polling interval. In other words, be able to adjust the conf file's
>>>>> minpoll flag at runtime instead of restarting.
>>>>
>>>> What problem are you trying to solve?
>>>>
>>>> NTPD does a pretty good job of adjusting itself most of the time.
>>>>
>>>> Short poll intervals are useful when correcting large errors.
>>>> Long poll intervals allow NTPD to make small corrections very
>>>> accurately.
>>>
>>> The idea was to bump up the minimum poll interval after ntpd has been
>>> running for a day or so to something more kind to the remote servers
>>> because the refclock is holding the remote servers clamped to 64
>>> seconds. If I set minpoll in the config file, then ntpd's start up
>>> takes a long time because of a long poll interval. If I don't set the
>>> minpoll, then ntpd doesn't do "a pretty good job" because it clamps
>>> the polling interval.
>>>
>>
>> I've noticed the same thing. You could try what I'm doing, although I'm
>> still testing for the best configuration.
>>
>> # GPS Lines
>> server 127.127.20.5 prefer minpoll 3 maxpoll 6 mode 72
>> fudge 127.127.20.5 time2 0.3100 refid GPS1
>>
>> # Internet server lines
>> # NIST New York
>> server nist1-ny.ustiming.org minpoll 8 maxpoll 13
>>
>> # other internet server lines similar
>>
>> Sincerely,
>>
>> Ron
>>
>>
> NTPD adjusts the poll interval dynamically.  Just because MINPOLL=4 does 
> not mean that the poll interval is "stuck" there.  Give NTPD a
> rock solid 1 second per second source it will ramp up the poll interval
> to 1024 seconds.  Those "rock solid" ticks can frequently be found
> 1:00AM to 5:00AM local time.  The net quiets down and NTPD takes
> shameless advantage.
>
> If you really want good time and can afford a GPS *timing* receiver 
> that's the way to do it.  The last I heard, you could get a timing 
> receiver for $100 -- $300.

Try $35. The Sure gps is a "timing receiver" in that it has a PPS pulse
with a 20ns or so rise time, and probably comparable accuracy. 
If you want a receiver which a) does a location survey and then uses
that location to derive the time even from a single sattelite, b) tells
you what the sawtooth correction is
(which may be what you think of as a timing receiver) then that is in
the range you mention. Mind you if you are feeding a computer, you
cannot actually get that ns level time accuracy into your system, since
interrupts take 1-10us to be processed.


>
> ANY GPS receiver knows what time it is but the navigation receivers
> give priority to location.  Timing receivers give priority to delivering
> the correct time.
>
Unfortunately most gps may know what the time is but they keep it
secret. You need to look for a PPS output to get the time out from the
receiver.

>



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