[ntp:questions] Attn Linux distributors - pse include PPS

Rob nomail at example.com
Sun Apr 27 16:15:59 UTC 2014

William Unruh <unruh at invalid.ca> wrote:
> But those modules give timing to one a few (5-10) usec. because of
> interrupt handling issues. Your shm solution would seem to me to be more
> than adequate for any timing requirements if they can be solved with an
> interrupt driven pps. 

Well, the kernel PPS API does the timestamping in the interrupt handler
while my gpsd/SHM solution does timestamping in a user process that is
marked to be ready to run in an interrupt handler.

Especially on a loaded system it can be less accurate due to scheduling.
I experimented with marking the process for realtime scheduling, but
the disadvantage is that it hangs the entire system when it goes in
a loop due to some bug.

On my system the offset and jitter for kernel PPS from a professional
GPS receiver (Datum) are usually 1 or 2 microseconds in the ntpq display,
but always less than 5.

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