Nickolay Orekhov <[email protected]> wrote:
> 1. If you mark clocks as "true" you somehow fool yourself :-). Because now
> if the clocks are real falsetickers you won't even know about it and your
> system
> will be out of sync and for ex. will show low offset from some falseticker
> maybe.
Yes, reading about the "true" option I got the impression that it is "wrong"
way to deal with this. I removed them form the clocks and added tos mindist
with large enough value, now it is set to 0.400 (I don't know if that is
insanely large for the purpose...)
>
> 2. NTPD can choose 2 of 3 clock sources but he can't choose 1 of 2 clocks.
> So you have to combine NMEA and PPS in one source
> or add another ntp server for selection algorithm to work.
With this system the only clock sources will be the local clock and gps, so
no other ntp servers are available. I guess I could try adding 127.0.0.1
as one server?
>
> 3. When starting ntpd will at first STEP time to some clock source. I can
> suppose It will be NMEA. But later ntpd will choose
> PPS because of low jitter and spike detection algorithm can take place. It
> will wait for 15 minutes and then make another STEP. To skip this
> 15 minutes, set time1 to real value.
OK, I watched the system and ntpd seems to pick up PPS either within the
first minute, or after ~15 or 16 minutes from ntpd startup. The NMEA source
is selected pretty quickly and ntpq -p shows small offset for NMEA from
the start (samples are 20 seconds apart):
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 10 16 377 0.000 0.032 2.680
PPS(0) .PPS. 0 l - 16 0 0.000 0.000 0.000
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 14 16 377 0.000 0.029 2.117
PPS(0) .PPS. 0 l - 16 0 0.000 0.000 0.000
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 2 16 377 0.000 0.032 1.059
PPS(0) .PPS. 0 l - 16 0 0.000 0.000 0.000
remote refid st t when poll reach delay offset jitter
and after about 15 minutes the PPS is selected.
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 14 16 377 0.000 3.266 3.055
PPS(0) .PPS. 0 l - 16 0 0.000 0.000 0.000
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 2 16 377 0.000 11.005 6.203
oPPS(0) .PPS. 0 l 1 16 3 0.000 -340.79 1.406
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 6 16 377 0.000 6.842 3.767
oPPS(0) .PPS. 0 l 5 16 7 0.000 -343.03 3.024
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 10 16 377 0.000 4.044 4.705
oPPS(0) .PPS. 0 l 9 16 17 0.000 -344.79 4.006
remote refid st t when poll reach delay offset jitter
==============================================================================
*GPS_NMEA(0) .GPS. 10 l 14 16 377 0.000 -0.024 7.511
oPPS(0) .PPS. 0 l 13 16 37 0.000 -346.15 4.634
remote refid st t when poll reach delay offset jitter
If I understand correctly ntpd adjusts the system clock slowly instead of
quickly setting it to desired time. But how quickly I can expect it to adjust
the system clock if it is within couple of hundred msec from "real" time and
ntpd has got NMEA and PPS available?
>
> That's just a wild guess.
> To be more specific, please, could you post your full config first of all,
> and then ntpq output
> of commands such as "pe", "rv", "cv" and so on short after restart and then
> after about half an hour? I think there's some problem in configs. C
I will post output from ntpq commands next year! :)
ntp.conf looks like this:
(I set NMEA time1 to 0.200 to see if the PPS offset would be smaller when
ntpd picks up the PPS -- so far I didn't see much impovement there)
-----------------
statsdir /tmp/
statistics loopstats peerstats clockstats
filegen loopstats file loopstats type day enable
filegen peerstats file peerstats type day enable
enable auth stats monitor
logfile /tmp/ntpd-log
server 127.127.20.0 mode 18 prefer minpoll 4
fudge 127.127.20.0 time1 0.200
fudge 127.127.20.0 stratum 10
server 127.127.22.0 minpoll 4
fudge 127.127.22.0 flag3 0 flag2 0
fudge 127.127.22.0 stratum 0
tos mindist 0.400
driftfile /tmp/ntp.drift
-------------------
Thanks for your help!
Tomi
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