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Chapter 4 Installation and calibration
4.5 Changing the configuration
Each time you want to change your configuration, just run "make xconfig|gconfig|menuconfig"
in your build dir, then "make" (or "make all").
If you only want to rerun the "configure" script using the current RTAI configuration, just run
"make reconfig", there is no need to restart any GUI for that.
When invoked for the first time in an empty build directory, the default "make" goal is
"menuconfig" in a regular terminal, or "xconfig" in an emacs-term.
4.6 Modifying the autotool template files
If you have to change some template files used by any of the autotools (i.e. Makefile.am,
configure.in, acinclude.m4), then you will need the following packages to rebuild the derived
files:
- autoconf 2.59
- automake 1.9.2
- aclocal 1.9.2
- libtool 1.5.8
4.7 Checking the performance
Before using the integrated calibration tool it's a good idea to verify the overall performances of
your system and find out if it is suitable for real-time applications. Doing so it's just a matter of
running the latency test which is available for kernel and user mode. It verifies the architectural
latency and jitter, up to the scheduling of a task. When used with the hard timer in oneshot
mode it can serve as a fine tuning of the average real-time scheduling latency. In oneshot
mode 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. In periodic mode however the variations of
the task period are used as measures of the scheduling latency/jitter. The reason for such a
choice are that there will be always a drift because the timer interrupt is based on a time
baseline that is different from the one used to carry out measurement calculations; in case of a
very good calibration (see next paragraphs) such a drift can be made very small and as far as
there is no loss of timer interrupts there will be no drift in periodic mode, unless the hardware
goes astray, so period variations are valid measures. The execution is fully customizable at
insmod time as it is possible to set up, the timer mode, the timer period, the averaging time,
the per average time or overall worst cases and the use of the FPU (with some calculations).
This test is located, after having installed RTAI under install_dir/testsuite/kern/latency or
install_dir/testsuite/user/latency. Running the ./run script will start it and the following results
will appear:
RTAI User Manual 3.4 – October 2006 – rev 0.3 - pag 10