Hello,
I'm currently running an NTP server using a Garmin 18x LVC as a
stratum-0 time source. Before I installed Linux, I used FreeBSD. Its
frequency error was quite low, usually around 1 ppm. This allowed me
to have stable, desirable times such as:
remote refid st t when poll reach delay
offset jitter
==============================================================================
+navobs1.gatech. .GPS. 1 u 32 64 377 58.966
2.622 5.055
+ntp-s1.cise.ufl .GPS. 1 u 18 64 377 69.260
2.001 1.998
*GPS_NMEA(1) .PPS. 0 l 5 16 377 0.000
-0.001 0.002
Even under heavy load, the offset for FreeBSD always seemed to be
under 20 microseconds.
Now, I'm trying to run a similar time server with Linux 2.6 and
Rudalfo's LinuxPPS patch (or shmpss or gpsd, doesn't really matter to
me).
I've installed Debian Etch (4.0) stable, and recompiled the testing
2.6.28 Linux kernel with Debian patches and the newest LinuxPPS
patch.
At the moment, I'm using the ATOM driver in ntpd and am experiencing
undesirable frequency error and offsets.
According to today's drift file,
Min offset: -68 microseconds, Max offset: 112 microseconds.
Frequency ppm Min: -18.1570, Max: -17.5150.
a sample ntpq output is
remote refid st t when poll reach delay
offset jitter
==============================================================================
+navobs1.gatech. .GPS. 1 u 21 64 377 57.631
1.882 2.206
oPPS(0) .PPS. 1 l 10 16 377 0.000
-0.018 0.001
Jitter is often as high as 0.006.
It appears there is no difference between LinuxPPS, shmpps, and gpsd
for these timings. Also, this was not tested under load (besides any
normal load for a debian machine).
So, does anyone have any suggestions for reducing frequency error in
Debian (or the linux kernel)?
Also, is there anyway to keep timings low while producing load? I'd
like to run other things on this server, but not at the expense of
time precision.
Thanks for all the support this group has provided me so far via other
posts,
Chris
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