[ntp:questions] Speed of ntp convergence

David J Taylor david-taylor at blueyonder.neither-this-part.nor-this-bit.co.uk
Sat Nov 1 06:56:09 UTC 2008


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 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)"

It's a very simple system, described here:
  http://www.satsignal.eu/ntp/FreeBSD-GPS-PPS.htm

Cheers,
David 




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