[ntp:questions] serialPPS+gpsd+ntpd large offset & jitter

M Z mcrunch2 at yahoo.com
Wed Oct 20 14:00:50 UTC 2010

----- Original Message ----
Sent: Mon, October 18, 2010 7:00:01 AM
Subject: questions Digest, Vol 72, Issue 16

Date: Sun, 17 Oct 2010 17:52:39 +0200
From: Miroslav Lichvar <mlichvar at redhat.com>

On Sat, Oct 16, 2010 at 08:24:02AM -0700, M Z wrote:

I have both usglobalsat MR350P and garmin 18xLVC connected to serial at both 
4800 and 9600.
gpsd was 2.37 but is now 2.95. ntpd is 4.2.4p7, OS: SuSE 11.2 = 2.6.31. I have 
used bothserial ports. I set to low_latency as per instructions:
> I have since run ntp on 2 other similar systems, with and without the gps. It 
> seems that the problem is not with the gps but with ntp?as there is always a 
> large offset and jitter. I have tried some things from the ntp.org problems 
> such as setting HZ=100, setting clocksource to tsc, hpet, and ?acpi_pm. I do 
> know what to do with acpi so i didn't test that. one of my systems has drift of 
> < +-10 which is good. one of the systems is not behind my firewall.
> I have snipped all gps?info out below as i only want to know what to do with 
> now.

Any interesting ntpd messages in syslog?

One additional thing you could try is to disable the kernel discline
by adding "disable kernel" to ntp.conf. If this helps, it's a kernel
bug or possibly a nano/micro mismatch between kernel and ntpd.
Miroslav Lichvar

It was DEFINITELY the kernel with SuSE. Putting 'disable kernel' in the 
/etc/ntp.conf file did the trick. Now i have:
xSHM(0)            .GPS.   0 l  8 16  377    0.000    -190.76  35.393
*SHM(1)            .PPS.    0 l  8 16  377    0.000        0.031   0.008
+someone.else ........    1 u  5 64  377  83.123       0.132   1.868

why can't we all get along? (kernel and ntpq). *joke*

I have another question. The fudge in ntp.conf for the GPS is suggested to be 
set at .420 - how can
I calculate the best value? gpsd gives all kinds of debug information but what 
numbers tell me how delayed the GPS signal is compared to the pps? Or is the 
'offset' from ntpq the key?


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