[ntp:questions] Re: SHM refclock not being selected
David Schwartz
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
> jitter
> ==============================================================================
> +GPS_HP(0) .GPS. 0 l 18 64 377 0.000 -2.045
> 3.867
> +SHM(0) .PPS. 0 l 54 64 377 0.000 0.882
> 0.033
> *tock.febo.com .PPS. 1 u 411 1024 377 10.362 5.926
> 0.192
> +time-B.timefreq .ACTS. 1 u 411 1024 377 74.705 -5.855
> 0.759
> -ntp1.usno.navy. .USNO. 1 u 451 1024 175 79.246 23.077
> 0.253
> -ntp2.usno.navy. .USNO. 1 u 452 1024 377 79.070 26.126
> 4.196
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
> configuration?
Can you post your configuration? I'm wondering if perhaps you have some
'restrict default' lines without the appropriate override. It may help to
add this:
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?
What type?
DS
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