[time-nuts] Re: PPS latency? User vs kernel mode

[Hal Murray]

folkert at vanheusden.com said:
>> "It is painfully obvious that the interrupt latency is the dominant
>> noise factor in PPS timestamping in the second case."

> I wrote something this evening that just busy-loops waiting for a raising
> edge and then writes down the current time to feed that to ntp. 

You are trading interrupt latency for user-kernel crossing delays and 
scheduler quirks.

There is probably a way to dedicate a CPU to your polling job.  I think I've 
done that on Linux but don't remember any names and it's probably different on 
FreeBSD.

What ntpd are you using?  The PPS driver may have an option to log every 
pulse.  It's FLAG4 in ntp classic and ntpsec.  That would let you compare your 
two paths.

You also need to compare the adev type statistics from the interrupt path when 
your polling is/isn't running to see if polling is distorting the interrupt 
path.


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