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Pre-emptive scheduling issue

Posted by Chandra V on June 19, 2009
Dear All

I am getting round robin scheduling ( equal time slice between tasks ) behavior even though i have configured to
Preemptive scheduling mechanism. I tried to explain my sample application below.

Microcontroller : NEC V850
Tick rate : 8ms

Sample Application : 1
Two tasks were created with priority 1 and 2. In each task,seperate test pin (I/O pin configured as Output) is toggled continuesly for DSO measurement.
I expected that highest priority task always executes continously without allowing low priority task. But it executes by round robin scheduling manner.

Now i have changed both tasks priority to 1. Still it executes by round robin scheduling manner irrespective of priority changes.

Is my expectation correct?

If anything is wrong then please reply!


RE: Pre-emptive scheduling issue

Posted by Richard Damon on June 19, 2009
Does the higher priority task wait for anything? (Like a call to vTaskDelay?) If so, when it does so it tells the kernel it doesn't need the processor at the moment and it gives time to the lower priority task.

RE: Pre-emptive scheduling issue

Posted by Chandra V on June 22, 2009

Thanks for your feedback.

I didn't use delay routine. I am new to freeRTOS usage. So i want to test pre-emptive schedular feature between tasks and also round robin scheduler feature.

As per my understanding, Low priority will not get chance to get execute if higher task is executing without any possibilities of block/suspend state. Thats what i expected in sample application.


RE: Pre-emptive scheduling issue

Posted by incrediball on June 22, 2009
Your assumption is correct. The low priority task will not run as long as the high priority task is not suspended. It can be suspended by a delay command, when taking a semaphore, waiting for queued data, etc.

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