[rtl] Jitter test

[Stuart Hughes <sehughes@zentropix.com>]

Hi all,

I have attached a simple portable jitter test.  This I have tested this
on rtlinux-2.0pre2 and RTAI-0.7.

The basis of the test is that each cycle is independant of each other,
so that clock drift errors do not accumalate.

If you make this, and type ./run_test, it will install all the needed
rt-modules, as well as the target jitter_rt.o module.

Once you have done this, you can the leave it runnning and at any point
type:
cat /proc/jitter to see the max/min recorded jitter values in
nanoseconds.

Please note, that in order to get a true appreciation of worst case, you
should stress the system.  I usually run a ping -f to the target machine
and on the target run top with an interval of zero. You may have some
other methods you prefer.

Any comments about the module, even bad ones are welcome (as it can
always be improved.

Regards, Stuart.

PS: An interesting note, on my laptop (Dell Inspiron 7k), normally I
will see worst case jitters on RTL/RTAI of about +/- 15 usec.  But, if
you invoke any of the 'special functions' such as changing the screen
brightness or plugging in the power cord, you will see jitter up to 53
milliseconds.

jitter.tgz