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Processing time between tasks with the same priority

Posted by orifai01 on May 9, 2017

Hi, I wonder how the scheduler decides between two tasks with the same priority, when configUSEPREEMPTION equals to 1 and configUSETIME_SLICING equals to 0 (i.e. a context switch should not occur on a tick interrupt). Thank you in advanced.


Processing time between tasks with the same priority

Posted by rtel on May 9, 2017

...the same way it does when configUSE_PREEMPTION is 1, it will always choose the task of that priority that has been waiting the longest.


Processing time between tasks with the same priority

Posted by rtel on May 9, 2017

Sorry, I meant the same way it does when configUSETIMESLICING is 1.


Processing time between tasks with the same priority

Posted by heinbali01 on May 9, 2017

And so :

the [way the] scheduler decides between two tasks with the same priority

is

...the same way it does when configUSETIMESLICING is 1, it will always choose the task of that priority that has been waiting the longest.

FreeRTOS has a principle of "fairness":

● The runnable task with the highest priority will be selected ● If the are more runnable tasks with the highest priority, the one which is waiting the longest will go first

( where "runnable" means "ready to run", enlisted in the pxReadyTasksLists[] )

As you probably know, configUSE_TIME_SLICING needs configUSE_PREEMPTION to be effective.

With time-slicing enabled, the clock-tick will force a task-switch, in case there are more runnable tasks of the same priority. Without time-slicing enabled, a task must yield explicitly in order to switch to the next same priority-task.


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