[ntp:questions] Ginormous offset and slow convergance

Richard B. Gilbert rgilbert88 at comcast.net
Wed Nov 30 13:27:50 UTC 2011


On 11/29/2011 9:21 PM, unruh wrote:
> On 2011-11-29, Richard B. Gilbert<rgilbert88 at comcast.net>  wrote:
>> On 11/29/2011 1:42 PM, Pete Ashdown wrote:
>>> Running with an Oncore GPS&   a TAPR TAC.  If I "ntpdate -b" a nearby
>>> synchronized server before I start ntpd, the offsets initially look pretty
>>> good:
>>>
>>>        remote           refid      st t when poll reach   delay   offset  jitter
>>> ==============================================================================
>>>    GPS_ONCORE(0)   .GPS.            0 l    -   16    0    0.000    0.000   0.000
>>>    time-C.timefreq .ACTS.           1 u   39   64    1   37.534  -11.785   0.001
>>>    ntp-nasa.arc.na .GPS.            1 u   38   64    1   19.868   -0.948   0.001
>>>
>>> As time goes on, things start to get crazier:
>>>
>>>        remote           refid      st t when poll reach   delay   offset  jitter
>>> ==============================================================================
>>> xGPS_ONCORE(0)   .GPS.            0 l    6   16  377    0.000  -999.97   0.003
>>>    time-C.timefreq .ACTS.           1 u   50   64    3   37.370  -16011.   0.080
>>>    ntp-nasa.arc.na .GPS.            1 u   52   64    3   19.823  -16000.   0.046
>>>
>>> More time with 100% reachability:
>>>
>>>        remote           refid      st t when poll reach   delay   offset  jitter
>>> ==============================================================================
>>> xGPS_ONCORE(0)   .GPS.            0 l    5   16  377    0.000  -15999.   0.002
>>> +time-C.timefreq .ACTS.           1 u   19   64  377   37.887  -16011.   0.122
>>> *ntp-nasa.arc.na .GPS.            1 u   24   64  377   19.760  -16000.   0.027
>>>
>>> Finally after 30 minutes, a lock and convergence, but still big offsets:
>>>
>>>        remote           refid      st t when poll reach   delay   offset  jitter
>>> ==============================================================================
>>> oGPS_ONCORE(0)   .GPS.            0 l    4   16  377    0.000  172.425  34.702
>>> +time-C.timefreq .ACTS.           1 u   29   64   77   37.579  151.544 104.339
>>> *ntp-nasa.arc.na .GPS.            1 u   22   64   77   19.779  165.811 106.581
>>>
>>>
>>> Is there anything I can do to decrease the convergence time?
>>
>> Little or nothing!  NTPD can, and sometimes does, take ten hours to
>> reach "steady state".  It needs about thirty minutes to find a
>> reasonable facsimile of the correct time.  For the next nine hours and
>> thirty minutes, it will refine that value until it's as good as it's
>> going to get.
>>
>> If you maintain a constant temperature in the room in which you keep
>> your computers, NTPD will maintain an accurate and stable clock as long
>> as you maintain that stable temperature.
>>
>> If you can't maintain that steady state you need to either fix things
>> until you can or lower your expectations.
>>
>> There is another "product" That will synchronize quickly.  I don't
>
> Possibly chrony. Except that the "price" of faster convergence is vastly
> improved accuracy and just as stable as NTPD.
> But it only runs on Linux or bsd, not windows.
>
> (If time spans, rather than time is most important to you, then chrony
> might be slightly worse than ntpd, because of the algorithm used to
> correct the clock-- a fairly rapid slew to correct the offset, and a
> frequency change to correct the drift. )
>
> Note that if you get rid of ntpd's drift file, and use the burst option,
> you can get faster convergence from ntpd apparently.
>

I think the effect of getting rid of the drift file depends on the value 
stored in the file!  If the value is reasonably close to correct, I 
think it's helpful.  If you are restarting because you have been without 
power for the last three hours, the drift file is almost certainly useless!




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