I am quite new to Xenomai, but not to realtime and embedded development.
I am trying to run Xenomai on a MVME3100 board (PPC8540 processor,
kernel version 2.6.14, Xenomai 2.3.1).
I am trying to do latency measurements on my target, and I have some
questions.
As far as I understand it, the value in /proc/xenomai/latency is the
time needed to start a timer, and this value is subtracted from the
value provided to the timer. The value calculated by Xenomai on my
platform is 9489. All latencies reported with the latency tool are
negative when this value is used.
When I write 0 to this setting (to get the real values without offset)
and run the latency test program I see different values for the
different modes: user space = 3.6 usec, kernel space = 1.8 usec and IRQ
= 0.9 usec. What is the relation between the value calculated by Xenomai
and the values measured by the latency program, and what is the correct
way to calculate the latency setting?
I also did a port of the cyclictest program to Xenomai. Instead of the
pthreads I used the RT-tasks, like the latency program. The cyclictest
program uses signals to trigger the task, but as I found out this does
not work in userspace: the applications returns to secondary mode at the
moment the thread waits for a signal. Is there another way to do this
(next to moving the whole program to kernel space)?
One option of the cyclictest program is that you can have several
threads running. I modified the output of the cyclictest tool somewhat,
to have min, max and average like the latency program.
When running multiple threads within this program (without load) the
values range from -5 usec to +5 usec, with an average of around 0 usec.
But with more RT tasks I see a very high latency once in a while (over
400 usec).
Below is the example of such a test run with 10 threads/tasks:
T: 0 ( 1305) P:80 I:1000 O: 0 Min: -2.404 Avg: -0.001 Max: 1.873
T: 1 ( 1306) P:79 I:1000 O: 0 Min: -3.052 Avg: -0.001 Max: 1.584
T: 2 ( 1307) P:78 I:1000 O: 0 Min: -5.694 Avg: 0.000 Max: 18.282
T: 3 ( 1308) P:77 I:1000 O: 0 Min: -5.166 Avg: 0.046 Max: 479.782
T: 4 ( 1309) P:76 I:1000 O: 0 Min: -2.595 Avg: 0.036 Max: 368.310
T: 5 ( 1310) P:75 I:1000 O: 0 Min: -1.731 Avg: -0.001 Max: 1.633
T: 6 ( 1311) P:74 I:1000 O: 0 Min: -481.273 Avg: -0.001 Max: 490.352
T: 7 ( 1312) P:73 I:1000 O: 0 Min: -273.947 Avg: -0.001 Max: 277.717
T: 8 ( 1313) P:72 I:1000 O: 0 Min: -47.016 Avg: -0.001 Max: 45.789
T: 9 ( 1314) P:71 I:1000 O: 0 Min: -22.968 Avg: -0.001 Max: 29.572
(P=Priority, I=interval, O=overruns)
When looking at the detailed information, I see that the latencies occur
when a higher priority task is ending. Is there some specific reason for
this, or is there some kind of configuration problem with my setup?
Kind regards,
Johan Borkhuis
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