[ntp:questions] Re: SHM refclock not being selected
davids at webmaster.com
Sun Mar 13 02:40:00 UTC 2005
"John Ackermann N8UR" <jra at febo.com> wrote in message
news:mailman.13.1110633714.576.questions at lists.ntp.isc.org...
> In order to use a PPS source with Linux 2.6.x, I've been running David
> Schwartz' shm_linux_clock driver with an HP3801A GPSDO as the external
> refclock. I am getting very good reported results and the SHM clock
> consistently has the lowest jitter of any of my servers, but for some
> reason ntpd doesn't select it as the source. Here's the ntpq -p output
> after a week of operation:
That's kind of strange.
> remote refid st t when poll reach delay offset
> +GPS_HP(0) .GPS. 0 l 18 64 377 0.000 -2.045
> +SHM(0) .PPS. 0 l 54 64 377 0.000 0.882
> *tock.febo.com .PPS. 1 u 411 1024 377 10.362 5.926
> +time-B.timefreq .ACTS. 1 u 411 1024 377 74.705 -5.855
> -ntp1.usno.navy. .USNO. 1 u 451 1024 175 79.246 23.077
> -ntp2.usno.navy. .USNO. 1 u 452 1024 377 79.070 26.126
Now, you have two stratum zero closk, they're both perfectly reachable,
and have good jitter.
> Selecting "tock" as the survivor isn't a bad thing, as it is a FreeBSD
> machine sitting on the local subnet, and has two stratum 0 refclocks, but
> I would think that since the SHM clock *always* has lower delay and jitter
> (as well as sane offset values), it should end up being the survivor.
That's what I would think. It's also odd that it would take a stratum
one reference over a stratum zero one.
> Should I be using the "prefer" keyword to force selection of the SHM
> clock? Or is there some other trick I should be playing with this
Can you post your configuration? I'm wondering if perhaps you have some
'restrict default' lines without the appropriate override. It may help to
restrict 127.0.0.0 mask 255.0.0.0
> By the way -- I was getting absolutely horrendous, unusable time from this
> machine until I recompiled the 2.6.9 kernel with HZ=100, and selected
> "clock=pit" at boot time (not sure which of the two changes is making the
> most significant impact; I wasn't in a position to experiment through
> multiple reboots).
Now that's strange. Does this machine have one CPU or more than one?
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