Did a test for a while: every 5 minutes I would look at the output of
ntpq -c pe -n for jitter of the pps source.
This pps source was "measured" either in the kernel (this newly added
gpio pps support in the raspberry pi) or in user space using my own
rpi_gpio_ntp.
kernel pps interrupt handling
-----------------------------
root@detijd:/etc# while true ; do echo `date +%s` `ntpq -c pe -n | grep
127.127.22.0` ; sleep 300 ; done | tee -a /pps.log
root@detijd:/etc# cat /pps.log | awk '{ print $11; }' | awk 'BEGIN { max = -1;
min = 1000; } { if ($1 > max) { max=$1; } if ($1 < min) { min=$1; } t+=$1; n++;
} END { print min, max, t/n, n; }'
0.001 0.124 0.0131945 797
This is minimum, maximum (yes that's 124!) and average. And I took 797
samples which is almost 3 days.
average temperature of soc: 49.7 with stddev of 1.87
userspace
---------
note: I looked at ntpq every 30 seconds(!)
root@hetlicht:/var/log/ntpstats# cat /rpn.log | awk '{ print $11; }' | awk
'BEGIN { max = -1; min = 1000; } { if ($1 > max) { max=$1; } if ($1 < min) {
min=$1; } t+=$1; n++; } END { print min, max, t/n, n; }'
0.001 0.101 0.00955351 27797
This is over 9,6 days.
average temperature of soc: 51,1 with stddev of 4.01
TL;DR:
- kernel : 13.2us jitter on average
- userspace: 9.6us jitter on average
Folkert van Heusden
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