[Xenomai] Reading /proc/xenomai/stat causes high latencies

Jeroen Van den Keybus jeroen.vandenkeybus at gmail.com
Tue Apr 22 18:02:35 CEST 2014


Using a 3.10.18 kernel with Xenomai 2.6.3, reading the stat entry of
/proc/xenomai causes high latencies in RT tasks. I've found a report
on a similar issue in
https://sites.google.com/site/manisbutareed/linuxcnc-2-5/xenomai-user-threads.
We also had this occurring on a 3.8.13 kernel.

A typical latency test run looks like:

RTT|  00:00:01  (periodic user-mode task, 100 us period, priority 99)
RTH|----lat min|----lat avg|----lat max|-overrun|---msw|---lat best|--lat worst
RTD|     -2.382|     -2.357|     -1.945|       0|     0|     -2.382|     -1.945
RTD|     -2.577|     -2.360|     -1.749|       0|     0|     -2.577|     -1.749
RTD|     -2.380|     -2.360|     -1.865|       0|     0|     -2.577|     -1.749
RTD|     -2.568|     -2.361|     -1.530|       0|     0|     -2.577|     -1.530
RTD|     -2.379|     -2.359|     -1.732|       0|     0|     -2.577|     -1.530
RTD|     -2.381|     -2.361|     -2.008|       0|     0|     -2.577|     -1.530
RTD|     -2.381|     -2.360|     -2.085|       0|     0|     -2.577|     -1.530
RTD|     -2.699|     -2.359|      2.566|       0|     0|     -2.699|      2.566
RTD|     -2.380|     -2.320|     -1.876|       0|     0|     -2.699|      2.566
RTD|     -2.381|     -2.359|      2.528|       0|     0|     -2.699|      2.566
RTD|     -2.380|     -2.360|     -1.805|       0|     0|     -2.699|      2.566
RTD|     -2.579|     -2.311|     -0.045|       0|     0|     -2.699|      2.566
RTD|     -2.380|     -2.359|     -2.072|       0|     0|     -2.699|      2.566
RTD|     -2.575|     -2.360|      2.065|       0|     0|     -2.699|      2.566
RTD|     -2.381|     19.028|   3043.067|      31|     0|     -2.699|   3043.067
RTD|     -2.566|     26.488|    105.823|      32|     0|     -2.699|   3043.067
RTD|     -2.443|     -2.276|      0.597|      32|     0|     -2.699|   3043.067
RTD|     -2.584|     -2.306|      2.032|      32|     0|     -2.699|   3043.067
RTD|     -2.377|     -2.242|      4.106|      32|     0|     -2.699|   3043.067
RTD|     -2.537|     -2.291|      4.394|      32|     0|     -2.699|   3043.067

It is obvious where I issued the 'cat /proc/xenomai/stat' command.

I will try to create an I-trace now. (config is same as under my
previous post 'Slow execution of RT task' - I'm still looking into
that issue as well).


Jeroen.




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