| Re: [chrony-users] Massive difference between NTP clocks and PHC |
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On Fri, Oct 14, 2016 at 12:26:05PM +0100, Denys Rtveliashvili wrote:
> I see that "linuxptp" has configured chrony to connect to PTP0 (shared
> memory segment) rather that PHC0 (direct connection to a PHC clock). I
> wonder if that makes any real difference.
One difference is that the SHM refclock is always in UTC, while the
PHC can be in TAI or UTC, so you don't have to deal with the offset
between TAI and UTC. Another difference is that the SHM refclock will
stop when the PHC is not synchronized by PTP.
> The only unknown is whether I should add some kind of static offset as while
> PTP measures the time at the card quite precisely, there is still some delay
> for getting time from that clock on the card over PCIE express and a shared
> memory segment and then to RTC clock. I guess it would be on a scale of
> microseconds.
No, you shouldn't need to specify any offset for that unless you have
measured the amount of asymmetry in the delay. The delay is assumed to
be symmetric and the measurements are automatically corrected by half
of the delay.
--
Miroslav Lichvar
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