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

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

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

--
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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