[ntp:questions] measuring os latency for pps

Dan Drown dan-ntp at drown.org
Tue Aug 25 17:38:17 UTC 2015


Quoting folkert <folkert at vanheusden.com>:
> Not sure if it is interesting for you guys but I wrote a simple program
> for e.g. Linux (or any other system with the pps api implemented) that
> listens on a pps source waiting for a pulse and then toggles a gpio
> pin. That way you can measure the latency introduced by the the kernel
> when listening from userspace. Note that there's a little extra latency
> due to the gpio-pin handling.
>
> You can also use it e.g. blink a led :-)
>
> It is on github: https://github.com/flok99/pps2gpio

What sort of latency have you seen?

This is a histogram (200ns bins) of interrupt latencies I've seen on  
the BeagleBone Black:

http://blog.dan.drown.org/content/images/2014/Dec/interrupt-latency.png

Over 6.5 days worth of samples, 99% were under 8.62us, and 1% were  
under 6.04us.  The mean was 6.92us and the stddev was 1.72us.


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