roland Tollenaar wrote:
> Hi,
>
> Ok I have the framework of my application up. Qt, user-interface, 2
> tasks running in parallel, one @ 1ms the other @ 2ms.
>
> Next step is to start incorporating the work that must be performed in
> the tasks but before I do so it will be useful to have some
> understanding of how the scheduling takes place.
>
> I fear the above might not convey my question clearly. I find it hard
> to explain exactly what I mean by this so I will pose the question
> differently.
>
> What does
>
> rt_task_wait_period();
>
> do exactly? It will be followed by the work that must be done in the
> task but does the work get performed while rt_task_wait_period(); is
> "waiting" or does nothing happen while the program is on that call?
>
> In the latter case I will have no real control over when the task is
> complete. On a fast machine the task will be finished sooner than on a
> slower machine.
>
> For example say I am setting outputs based on calculations (e.g. QR
> decompositions, singular value decompositions etc) processing certain
> inputs. If I read the inputs first after calling
> rt_task_wait_period() then do the calculations and finally set the
> outputs the time difference between reading the inputs and setting
> outputs will (in the best case) depend on the the speed of the
> machine.
>
> In this scenario there are obviously more optimal manners to structure
> the program but I am using it as an example.
>
> Anyhow is there any documentation that elucidates these aspects of xenomai?
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
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...
* 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.
--
Gilles Chanteperdrix.
_______________________________________________
Xenomai-help mailing list
[email protected]
https://mail.gna.org/listinfo/xenomai-help
