[ntp:questions] NTP and PPS calibration interval (convergence speed)

unruh unruh at invalid.ca
Tue May 28 12:46:46 UTC 2013


Not really. What would be more helpful would be graphs of the behaviour,
of the offsets over time, including times when the machines are heavily
used and times when they are not. (See for example
www.theory.phsyics.ubc.ca/chrony/chrony.html and past graphs as well)
Unfortunately the gps attached to my one ntp machine has gone nuts
(Garmin 18-- both my Garmins failed after about about 2 years. Not a
very good piece of gear-- but you can look at older graphs from 3 or 4
years ago to see how the ntp run machine behaved.
On 2013-05-24, Miguel Gon?alves <miguel.barbosa.goncalves at gmail.com> wrote:
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> Hi!
>
> I've read a lot about the speed of NTP when reacting to changes in
> temperature to in turn affect the oscillator.
>
> I am posting my findings and thoughts to the list so it becomes
> archived for future users in need of this information.
>
> First, let me say that I have 2 GPS receivers (Garmin 18 LVC -
> 10.0.2.2 - on the roof and Sure GPS board - 10.0.2.4 - on a window
> facing South) connected to identical (both ALIX 1D boards) and
> dedicated machines just for NTP. I am running 4.2.6p5 on both. I also
> have a Meinberg NTP server (10.0.2.1) in the same LAN segment. This
> configuration gives me 3 clocks one of which (Meinberg) I use to check
> the performance of the others.
>
> The 3 servers use minpoll 4 maxpoll 4 for their refclocks and each one
> lists the other two as servers with minpoll and maxpoll set to 4.
>
> For me, it is not clear why calibration occurs every 256 seconds when
> I have a GPS receiver with a PPS output. I understand that a good
> frequency drift estimation would be good when free running but frankly
> I don't want my NTP servers to serve time when the GPS signal is not
> available. I can trust my Meinberg box to do that because it has a
> TXCO but not my regular NTP servers.
>
> I was also surprised that Meinberg's NTP server calibration interval
> never goes above 4 seconds. They must have done what I did.
>
> I changed my FreeBSD 8.3 kernel to never go above 4 seconds and the
> results are in the graphics attached.
>
> I changed PPS_FAVGDEF in the file /sys/kern/kern_ntptime.c from the
> default 8 (256 seconds) to 2 (4 seconds). The frequency is therefore
> adjusted every 4 seconds instead of 256.
>
> Here are some ntpq billboards
>
>
> (Meinberg NTP server)
>
> freebsd8# ntpq -p 10.0.2.1
>      remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
>  LOCAL(0)        .LOCL.          10 l   12   16  377    0.000    0.000   0.002
> +GENERIC(0)      .GPS.            0 l   14   16  377    0.000   -0.001   0.002
> oPPS(0)          .PPS.            0 l    1   16  377    0.000   -0.001   0.002
> -10.0.2.2        .PPS.            1 u    4   16  377    0.162   -0.005   0.011
> +10.0.2.4        .PPS.            1 u   10   16  377    0.158   -0.003   0.006
>
>
> (Garmin 18 LVC on the roof)
>
> freebsd8# ntpq -p 10.0.2.2
>      remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
> oGPS_NMEA(0)     .PPS.            0 l   11   16  377    0.000    0.000   0.004
> +10.0.2.1        .PPS.            1 u    8   16  377    0.148    0.004   0.005
> +10.0.2.4        .PPS.            1 u    7   16  377    0.132   -0.003   0.004
>
>
> (Sure GPS board on the window)
>
> freebsd8# ntpq -p 10.0.2.4
>      remote           refid      st t when poll reach   delay   offset  jitter
>==============================================================================
> oGPS_NMEA(0)     .PPS.            0 l   13   16  377    0.000    0.000   0.004
> +10.0.2.1        .PPS.            1 u   10   16  377    0.161    0.004   0.006
> +10.0.2.2        .PPS.            1 u    9   16  377    0.139    0.002   0.004
>
>
>
> Hope this helps someone.
>
> Regards,
> Miguel
>
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