[ntp:questions] Ginormous offset and slow convergance

Richard B. Gilbert rgilbert88 at comcast.net
Tue Nov 29 22:26:03 UTC 2011


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 
recall its name. Supposedly there is a price in accuracy and stability
for that quick start.  Consult the Control Systems Theory people about
the NTP algorithms and how they differ from algorithms giving fast 
convergence.





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