Re: [chrony-dev] pps source is still marked as synchronised when no absolute clock sources are available

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On Wed, 20 Jan 2010, Hattink, Tjalling (FINT) wrote:

Hi,

This is my report of a second issue I found when experimenting with
Chrony. See the mail titled "makestep command sometimes makes chrony
stop reading its sources" for more background info.

I use two reference sources for synchronising the system clock, a SHM
interface providing absolute time from a GPS card, and a PPS interface
providing precise relative time from a GPS card.

Are those separate GPS cards or the same card?


The daemon providing the SHM interface only provides timestamps when the
GPS card is locked and its time is accurate. The PPS interface is always
providing sub-second timestamps.

When chrony is started it does not discipline the system clock until the
SHM interface is marked synchronised. After the SHM source is correct it
starts to look at the PPS clock and after a while the PPS clock is used
as main source. This is correct behaviour. Now I disconnect the antenna
from the GPS card and the SHM interface stops providing timestamps, as
the GPS clock is unreliable. You can see in the sources list that the
SHM clock is marked unreachable. But the PPS pulses are still provided
and Chrony keeps using the PPS as synchronisation source. I think this
is not correct behaviour. Chrony should check if any absolute time
source is reachable and synchronised every time when a PPS signal comes
in. If not it should also mark the PPS sources unreachable. This
behaviour is also shown by ntpd.

I disagree. chrony is able to keep time to an accuracy of 1/2 sec between the
pps pulses, even if some are missed. The local clock is assumed not be jumping
around by seconds at a time, or nothing, chrony, ntp, you, will every be able
to discipline it. Anyway, you should always have some other fallback source
from the internet listed to make sure that the the system does not suddenly
jump. Ie, the theory is that once the time is within 1/2 sec then the pps
clock can discipline the local clock, using the ability of the local clock to
discipline the time.



If you need more information or you want me to test patches regarding
this issue, don't hesitate to contact me!

Kind regards,

Tjalling Hattink



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