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Re: [rtl] Jitter test
Hi Jan,
The test is very simple. A task runs every 100usecs and measures the
current time, using the time stamp counter, which is a free running
clock on Pentiums that has a resolution to the clock frequency.
The previously calculated expected time is used to calculate how
late/early this pass ran, and if it is the largest so far, it overwites
the current max/min value (min being the earliest time which will be
negative).
Next, the expected next wakeup time is caculated (now+100usec).
Finally the task suspends until its next wakeup.
The results are available at any time by doing a cat /proc/jitter. The
results give a good idea of the amount of jitter. Please remember the
limits of measument accuracy that occur due to the resolution of the
8254 interrupt clock source (1/1193180 ~ 0.838 usecs).
Regards, Stuart
Jan Borgosz wrote:
>
> Hi
>
> I'm solving problems with measurment devices for jitter - may you write
> something more about your program, methods and algorithms
>
> Sincerely
> Jan Borgosz
>
> Stuart Hughes wrote:
>
> > 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.
> >
> > ------------------------------------------------------------------------
> > Name: jitter.tgz
> > jitter.tgz Type: WinZip File (application/x-compressed)
> > Encoding: base64
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