Scheduling algorithms and priority inversion

1. Scheduling algorithms and priority inversion

2. Scheduling algorithms and priority inversion • What is real-time? • Scheduling algorithms: • Rate monotonic • Deadline driven • The priority inversion problem • Conclusion

3. Scheduling algorithms • A real-time system is one where the right results must be delivered on time. • Scheduling algorithm: the set of rules with which the activities are ordered for execution.

4. Scheduling algorithms • Rate monotonic: Assign the priority of each task according to its period, so that the shorter the period the higher the priority. Bad: Good:

5. Scheduling algorithms • Deadline driven: The next task to run is based on finding the task with the earliest deadline.

6. Priority inversion problem Conditions: • Concurrent tasks share a resource that is protected by e.g. a semaphore. • At least one intermediate priority task exists.

7. 2 3 1 Priority inversion problem High Medium Low System Interrupt

8. Priority inversion problem Solutions: • Avoid sharing resources between tasks of differing priorities. • Turn off preemption of the low-priority task for the critical section. • The low-priority task could raise its own priority while in the critical section.

9. Conclusion The understanding of scheduling algorithms is of fundamental significance to developers of real-time applications.