[ntp:questions] Stick to PPS, even if the prefer server fails

David Mills mills at udel.edu
Thu Mar 26 03:28:07 UTC 2009


Bill,If you had taken the trouble to look at the documentation , you 
would have found the "Mitigation Rules and the Prefer Peer"  
http://www.eecis.udel.edu/~mills/ntp/html/prefer.html page which clearly 
describes how the contraptoin works.

Unruh wrote:

>Dave Hart <davehart at gmail.com> writes:
>
>  
>
>>On Mar 25, 9:19=A0pm, Unruh <unruh-s... at physics.ubc.ca> wrote:
>>    
>>
>>>I have no idea why and whether kernel PPS code is any better ( or worse)
>>>than say PPS discipline using the shm PPS refclock using parallel port
>>>interrupt. Ie, both can discipline
>>>to about 1-2usec level.
>>>      
>>>
>
>
>  
>
>>Re-read the thread, then.  A kernel clock disciplined by PPS allows
>>PPS to continue to discipline the clock when ntpd's PPS implementation
>>stops doing so because of a prefer peer problem.
>>    
>>
>
>I must admit that I am completely confused by this prefer peer problem. I
>would think that ntp would use the PPS/refclock preferentially no matter
>what external servers are doing. But clearly I am not understanding
>something. If it does not that would seem to me to be a bug in ntp, not
>something one should be rewriting kernel code to overcome. 
>
>  
>
>>>The main problem is that the ntp model is too slow
>>>reacting to temperature induced drifts.
>>>      
>>>
>
>
>  
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>>I'm sure that's your main problem re ntpd, but it's completely
>>tangential to the thread.
>>    
>>
>
>I was not clear-- the main problem with ntp's ability to discipline the
>system clock to better than a few microseconds is that ntp reacts too
>slowly to temperature induced drifts. Ie, while ntp might be able to
>discipline a clock with a constant rate to sub microsecond precision, ntp
>in a running, temperature varying system cannot, in part because of ntp's
>reaction to drift rate changes. I think that is germaine to the thread, but
>opinions may vary.
>Ie, if you use ntp with the addition of temperature measurements to
>estimate the temp induced drift rate, you can discipline the clock much
>better than with ntp on its own. 
>However if the only concern was losing PPS discipline due to prefer peer
>problems, then I agree this is tangential.
>
>
>
>  
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>>>>The complete system doesn't serve time to others. It's intention is
>>>>monitoring. I'm building this as a diploma thesis for a
>>>>telecommunication company. They have a large number of ntp servers
>>>>with GPS receivers (called SSU) across the country. So my system only
>>>>watches the offsets, that these SSUs have to assure that they are OK.
>>>>        
>>>>
>>>They can monitor only to the msec (or 1/10 msec) =A0level unless the othe=
>>>      
>>>
>>r systems are at the same place.
>>    
>>
>
>
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>>I think he's monitoring the self-reported offsets between the NTP
>>disciplined clock and the local GPS receiver.  Network delay would not
>>be a factor if so.
>>    
>>
>
>I got the impression that he was using this machine to monitor a whole
>bunch of other machine distributed over the USA to make sure that their
>clocks were well disciplined by their own on board GPS receivers.
>
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