Re: [Xenomai-help] timing, scheduling. some theory?

[Wolfgang Grandegger <wg@domain.hid>]

roland Tollenaar wrote:
> Hi
> 
>> Xenomai kernel implements the so-called "FIFO" scheduling policy, that
>> is, the task which runs at any time is the task with the highest
>> priority that has been ready for the longest time. Calling
>> rt_task_wait_period suspends the calling task, and let other tasks
>> run.
>>
>> If I understand correctly, what you want is to set some outputs at some
> You understand correctly.
> 
>> precise deadline, I see two ways to do what you want:
>> * either the time when you compute the output has no importance, and the
>>   only thing that matters is when you set the output, in which case you
>>   can call rt_task_wait_period right before setting the outputs, your
>>   task will do
>>   compute next output
>>   go to sleep
>>   set output
>>   compute next output
>>   go to sleep
>>   set output
>>   etc...
> Actually this is exactly what I had in mind only it is important
> (discrete state space controller computations) that the input is also
> read at exact intervals. So maybe:
> 
> compute next output k+1
> go to sleep
> read input for k+2 computations
> set output k+1
> compute next output k+2
> go to sleep
> read input for k+3 computations
> set output k+2
>>   etc...
> 
>> * or the time between reading input and setting output has to be as
>>   fixed as possible, in which case what you will have to do is to
>>   calibrate the WCET (worst case execution time) of the computations on
>>   the machine you are running, setup rt_task_set_periodic so that your
>>   task is woken up at (deadline - WCET), and in each loop, do the
>>   computations, then busy wait until the deadline then set the
>>   output. In order to busy wait, you may use rt_timer_tsc.
> 
> Off hand I am not exactly sure why this second technique would give me
> a more fixed interval between reading input and writing output than
> the modified technique 1 I suggest above?
> 
> Obviously one problem i have is the duration of obtaining input and
> setting output. Both will be coming in and going out over a CANOpen
> connection but from the moment the messages are on the bus I have no
> control over what happens. Of course this has nothing to do with
> xenomai but only with the deterministic characteristics of the CAN
> protocol. And AFAIK that is not good?

What do you mean with a CANopen connection? CANopen is a protocol stack 
which must run on your system to communicate with CANopen nodes through 
the CAN bus.

Wolfgang.