|Re: [chrony-users] Fatal error : adjtimex() failed|
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On Tue, 21 Aug 2012, Tomalak Geret'kal wrote:
On 21/08/2012 16:31, Bill Unruh wrote:
On Mon, 20 Aug 2012, Tomalak Geret'kal wrote:
> On 20/08/2012 22:44, Bill Unruh wrote:
> > Hmm. How are you feeding the shm? The PPS source cannot give you the
> > seconds.
> > It is only accurate to the nsec, but completely
> > oblivious to seconds, so
> > you
> > have to do something to feed it the seconds. That could
> > be the gps itself,
> > or
> > some other source.
> The SHM is fed by a known-good process that works with ntpd and also
Is it a secret which program you use?
No, it's not a secret, but it's in-house so you won't have heard of it.
The code is pretty much extracted straight from gpsd, though - there's
nothing unusual in it. I can show source if required, though I'd rather
OK. Are you sure it is actually treating the shm properly?
> with chrony when I can get it to start up. As you can see from the
> syslog, the SHM source was selected successfully.
> > > > [sw200319 /root]# chronyc sources
> > > 210 Number of sources = 2
> > > MS Name/IP address Stratum Poll LastRx Last
> > sample
> > >
> > ============================================================================
> > > > #? PPS0 0 4 43m
> > -1607ms[ +400ms] +/- > 155ms
> > > #* GPS 0 4 16
> > -14ms[ -14ms] +/- > 60ms
> > That indicates that the PPS is almost 2 seconds out from the gps. a
> > few
> > 10s or
> > even 100s of ms I could understand, but this indicates
> > that the pps source
> > is
> > getting the wrong seconds information.
> > Also a fluctuation of 400ms or even 155 ms is pretty huge.
> But as you point out yourself, PPS is oblivious to time-of-day as it
> provides only *timing*. My understanding is that this value in "chronyc
> sources" is actually just an artefact of the PPS not having been used to
> discipline usage of the SHM source for a full 43 minutes, so it's
> showing the result of jitter in the NMEA input?
All sources MUST have a seconds source as well. Ie, PPS needs to be fed
seconds by some other source. For you it was the GPS source I believe.
why that 1.6 second offset is so weird. Also that line says that the last
time it got a PPS signal was 43 minutes ago.
It should be say 15 sec ago instead. Your PPS source is not working at
My PPS is a known-good 50%-on-50%-off source.
I don't experience this issue at all when "noselect" is used on the
NMEA/"GPS" source. That is, when I can launch chronyd past my
adjtimex()/shmget() issues, the PPS has so far lasted up to 16 hours (longer
tests pending) - far longer than it managed without the "noselect".
Perhaps the PPS is simply not polled any more in such a case?
Are you getting better than 43 min between readings of the PPS
I'm not really worried about this case any more - "noselect" on the GPS
source is doing its job as far as I can tell and my PPS/GPS offsets remain
sane. Again, longer tests pending.
It's really just the adjtimex()/shmget() oddity I'm confused about now. It
really does seem to occur largely randomly and then vanish when I replace the
binary with a new build which differs only by more verbose syslog output; to
me, this screams UB in my build, but yikes. My investigation continues...!
This is honestly still a million times better than working with ntpd. Kudos.
William G. Unruh | Canadian Institute for| Tel: +1(604)822-3273
Physics&Astronomy | Advanced Research | Fax: +1(604)822-5324
UBC, Vancouver,BC | Program in Cosmology | unruh@xxxxxxxxxxxxxx
Canada V6T 1Z1 | and Gravity | www.theory.physics.ubc.ca/
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