Latency Calibration
The material found here has been derived from a similar directory that was once distributed with the original NMT-RTLinux.
This directory contains a test module to make a fine tuning of the oneshot real-time scheduler. The idea is to measure the difference in time between the expected switch time and the time when a task is actually called by the scheduler.
Note that under the appropriate arch directory you'll find an equivalent module that gives you directly the macro to insert in rtai.h.
To run it do:
make clean
make
./run, and follow what it says you.
"run" (well, in fact the module itself) understands the following command line parameters:
overall=x Calculate overall (x=1) or per-loop (x=0) statistics period=x Period in ns; default: 100000 loops=x Averages are calculated over x loops; default: 1 use_fpu=x Do we want to use the FPU? Default: 0
start_timer=x Declares if the timer should be started or not (default: 1)
Check will print a sequence of 3 "miss" numbers (nanosecs) - min, max, average. Use these numbers to change the LATENCY value in "rtai.h" in directory "include" or to set the it at rtai_sched installation by typing
insmod modules/rtai_sched Latency=<x>"
where <x> is derived from running "check" and should make the average value printed by it approximately zero. In practice that means you should set LATENCY to the average value printed by "check" if this is greater than 500. Tuning beyond such a precision is illusory.
It must be noted that the results obtained depend somewhat on the computer load and on the number of real time tasks you are running.
There is also a provision to load the module while something else is running that has already called start_rt_timer, used for mixed up testing, also with LXRT installed.
For that insert the module by hand with the command:
insmod ./rt_process "star_timer=0"
Or use the run script with:
./run start_timer=0.
To set parameters you can use the standard parametrization at insmod or do, just an e.g.: ./run period=1000000 use_fpu=1 start_timer=0.