Re: [opensuse] chrony and hwclock



On Fri, 2011-07-15 at 09:14 -0400, Anton Aylward wrote:
Roger Oberholtzer said the following on 07/15/2011 07:17 AM:
That seems to be the point of chrony. Access to the time source need not
be regular. Obviously the longer it is not connected the more time will
suffer.

Isn't this what NTP does?
Only with 'layers' (?'stratum'?) of authority for fall-back.

Surely that is the idea. But when all you have is a GPS, things do not
reliably work to plan. If NTP does not find an external server, it
eventually falls back to localhost. localhost uses whatever the BIOS has
for time. If your time looks correct in the absence of a ntp server, it
is probably because of some mechanism other than ntp. Like the hwclock
stuff openSUSE does at boot and shutdown. I think that is what maintains
time between boots. It requires that the time be correct when the system
is shutdown. To whatever extent it is incorrect when it is shut down, it
will be incorrect when it boots.

Given that ntp seems to only periodically re-check available servers, if
the GPS is not on-line when it checks, it may be missed. You have to
hope that the GPS is working when ntp checks. Otherwise ntp will choose
another. In a vehicle that is not attached to anything else, that will
be localhost. Which really has little useful information to help improve
things.

This becomes a real problem when the system is turned on. ntp may not
see the GPS when it first starts. It then goes to localhost. Eventually
the GPS comes alive. Until ntp checks again, the GPS won't be used. So
the system may have incorrect time for a long time.

Systems that are on all the time or for very long periods of time do not
see the deficiency in ntp. It has become apparent in systems that are
off and on with some frequency. It is a recurring topic on the gpsd
mailing list.

Having claimed all this, I realize that I may have a few points wrong. I
can only say that I see on my laptop (on and off a lot - external time
source only available after boot is long completed) the unexpected ntp
behavior described by others. I have no investment in using chrony or
ntp. I am only exploring options at this time.

And my real question about any possible side effects of not running
hwclock at system shutdown remains unanswered.

Outside of military, an automobile is about the worse environment for
electronics you will encounter. Its bad enough to be the electronics
in, say a traffic light controller cabinet, exposed to heat
fluctuations. But auto electronics has to deal with that and more.
Vibration, salt, dirt ....

As we well know. Our systems are used anywhere from Northern Finland to
Saudi Arabia. Being on the road, they are exposed to many nasty things.
Given that we measure road quality, they find themselves on some nasty
bouncing surfaces. Environmental factors determine a large part of our
design. It is one thing that drives up the cost of such systems.


Yours sincerely,

Roger Oberholtzer

OPQ Systems / Ramböll RST

Office: Int +46 10-615 60 20
Mobile: Int +46 70-815 1696
roger.oberholtzer@xxxxxxxxxx
________________________________________

Ramböll Sverige AB
Krukmakargatan 21
P.O. Box 17009
SE-104 62 Stockholm, Sweden
www.rambollrst.se


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