[ntp:questions] questions] Performance estimation

David J Taylor davidtaylor at writeme.com
Wed Jun 17 08:09:22 UTC 2020

From: Dan Drown

The system's time (kernel "clocksource") on the RPI is actually not
running at the same speed as the CPU clock.

From dmesg: arch_timer: cp15 timer(s) running at 19.20MHz (phys).

This gives you around 52ns of resolution.  I believe it's the same on
all the rpi models.

> I would also like to see whether the characteristics of the GPS and its
> location make a measurable difference to the RPi's timekeeping.  For
> example: is it better to have a GPS with 3 service capability at a
> location where the signal is poor, or is it masked by the RPi's
> performance?  All this with kernel-mode PPS.

What I've used for this is a percentile of offsets.  Looking at the 1%
and 99% values on a histogram is an estimate of the system's stability.

For instance (not an rpi):

So from that graph, I can say that 98% of the time, the system clock
is within +/-80ns of the PPS.

I believe you're using ntpd, and my code to generate that graph from
ntpd logs is here: https://github.com/ddrown/chrony-graph/tree/ntpd

Quoting William Unruh:
> The question then is how rapidly the system can respond to an
> interrupt,. This at least used to be of the order of a microsecond.
> Also, how logd does it take to read the clock with the kernel gettime
> routines. They all limit the accuracy of your clock using gps refclock
> (and also how long the wire is between the gps unit and the computer)

On different ARM hardware (beaglebone black), I've measured interrupt


I'd expect the rpi to have a similar magnitude.  Somewhere around
+10us delay and 1us jitter.


Thanks for the info on the RPi system clock.  Even at 19 MHz, it's way
better than the microsecond level in part of the ntpq -crv report.  I've now
discovered that at least one item there is at nanosecond level, so I've
updated the Perl script and MRTG presentation accordingly.

I also prefer plotting offsets, and I have a Windows program which allows
histogram plots.  Just checking the histogram on a couple of Windows PCs
they show mean 0.2 & 0.3 us and SD 7 and 10 us.

I don't know what the path is on the RPi for the PPS interrupt but I would
expect it to be similar to that on the BBblack, so that latency graph is of
interest - thanks!

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