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Roll of Priorities !

Posted by Sachin Bhujbal on November 19, 2008
We are using FreeRTOS for some development.
Following Observations are found.

---------------------------------------------------------
Preem- Priority(Max) No. Priority ption of Tasks of
status each Task
---------------------------------------------------------
0 10 10 9
Result :- Only Higher Priority Task Runs
---------------------------------------------------------
0 10 10 0 to 9
Result :- Only Higher Priority Task Runs
---------------------------------------------------------
1 10 10 9
Result :- Every Task running
---------------------------------------------------------
1 10 10 0 to 9
Result :- Only Higher Priority Task Runs
---------------------------------------------------------
Above observations shows that if we want every task to run, Kernel should preemtive & priorities of all Tasks should be equal. Is it right ?
How can I run all tasks in Round Robin Design with different priorities in picture ?
Please help me to understand the roll of Priorities in
Cooperative & Preemtive mode of kernel.


RE: Roll of Priorities !

Posted by Richard on November 19, 2008
Priorities ---

The scheduler will always run the highest priority task that is able to run. A task is able to run if it is in the Read state (it is not Blocked or suspended).

If you have a high priority task that never blocks or suspends then it will always be chosen to run, starving lower priority tasks of any processing time.


Pre-emption/Co-operation ---

When using the preemptive scheduler the kernel will select a new task to run each time the RTOS tick interrupt occurs. It may choose the same task to run as was already running if all the other tasks are of lower priority.

If you use the cooperative scheduler then a new task will only ever be chosen to run when a task calls taskYIELD(). So unless you call taskYIELD() [or making a blocking API call] then the same task will always run.



I'm writing an application note on this at the moment, but its not ready yet.

Regards.

RE: Roll of Priorities !

Posted by Sachin Bhujbal on November 26, 2008
Thanks Dear Rechards.

Sachin D. Bhujbal


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