[ntp:questions] Red Hat vote for chrony

Rob nomail at example.com
Tue Dec 9 21:22:12 UTC 2014

Charles Swiger <cswiger at mac.com> wrote:
>> The correct solution is of course to not depend on $0.10 crystals as the time base for dedicated NTP servers. :-)
> Well, yes.  You can get a PCI(e) card with a TCXO or OCXO and an
> optional GPS module like the Beagle ClockCard or a SpectraCom TSync
> for a few hundred bucks.
> That's quite a bit more than a $40 GPS puck, but these will also
> freewheel for a lot longer before losing or gaining a second in
> error: ~2 seconds/month if kept stable at 23C, I believe one said.

But how much better is such a card at stabilizing the real time clock
in the system?  Aren't these cards ultimately used to lock the cheap
crystal in the system itself?  And how fast is the control loop there?

I am using ntpd (development release) to lock the clock of a Linux
system to ntpd, and of course I can observe the excursions that result
from temperature increases and decreases.  When monitoring the offset
measured by ntpd it is clearly visible that there is an offset proportional
to the derivative of the temperature.  Apparently ntpd is not quick
enough to adapt the frequency estimate, and the offset remains nonzero
until the temperature again stabilizes.

Shortening the poll interval improves this, but there still is an offset
of up to 8us when the temperature changes 4degC/hour.
(synced to an external GPSDO via PPS)

Does such a card do better, or is it in fact "just" a GPSDO on a PCI
card that still is "only" used to lock the system time via ntpd?

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