[ntp:questions] Speed of ntp convergence

Martin Burnicki martin.burnicki at meinberg.de
Thu Nov 6 09:50:48 UTC 2008


David J Taylor wrote:
> Unruh wrote:
> []
>> An hour later, it was still 7ms off, another hour, 2.6ms and another
>> hour
>> later, still 1.2 ms off. Ie, only after about 6 hours was it within a
>> ms of
>> the correct time. Now, usually this PPS controls the time to within
>> about 2us (not ms, usec) but it is apparently going to take over 10
>> hours to get
>> there. That is of course completely rediculous.
> 
> There sounds to be something wrong with your system.

As mentioned in another reply, AFAIR older versions of the NTP daemon did
converge quite a bit faster than recent versions.

> As a comparison, I have a very old Pentium 133 system here running FreeBSD
> with local GPS PPS and some other Internet-based stratum 2/3 servers
> (probably NTP pool and a fixed name).  I'm sure it's well within a few
> minutes for it to reach it's full accuracy (tens of microseconds). For 
> interest, I've just (0645 UTC) switched it off and on, and we will be able
> to watch its recovery here (30 minute updates):
> 
>   http://www.satsignal.eu/mrtg/pixie_ntp.html
> 
> Here it is about a minute after startup:
> 
> ntpq -p pixie
>      remote           refid      st t when poll reach   delay   offset
> jitter
>
==============================================================================
> +calx.pulsewidth 193.120.10.3     2 u   62   64    1   22.272    1.700
> 0.743
> +admin.islay.bit 192.33.96.102    2 u   62   64    1   21.131    0.921
> 1.112
> +dnscache-london 128.250.33.242   2 u   62   64    1   22.845    3.299
> 0.666
>  88-96-233-89.ds .PPS.            1 u   14  128    7   63.431    0.044
> 2.789
> *utserv.mcc.ac.u 193.62.22.98     2 u   64   64    1   26.494    4.312
> 0.829
>  GPS_NMEA(1)     .PPS.            0 l   12   64    3    0.000   -0.137
> 1.654
> 
> .. and a few minutes later ...
> 
> ntpq -p pixie
>      remote           refid      st t when poll reach   delay   offset
> jitter
>
==============================================================================
> +calx.pulsewidth 193.120.10.3     2 u   54   64   37   22.348    2.946
> 0.877
> +admin.islay.bit 192.33.96.102    2 u   53   64   37   20.496    1.862
> 1.057
> +dnscache-london 128.250.33.242   2 u   57   64   37   23.090    3.809
> 0.662
>  88-96-233-89.ds .PPS.            1 u  134  256   17   63.431    0.044
> 2.007
> +utserv.mcc.ac.u 193.62.22.98     2 u   53   64   37   25.564    5.371
> 0.868
> *GPS_NMEA(1)     .PPS.            0 l    3   64   77    0.000   -0.001
> 0.803
> 
> It's using the out-of-the-box NTP code, and probably a rather old version
> of NTP.
> 
>   version="ntpd 4.2.0-a Sun May  8 06:01:21 UTC 2005 (1)"

Now the question is how a recent version of ntpd would converge on this
machine.

Martin
-- 
Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont
Germany




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