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Chapter 4 – Periodic Task Scheduling

Chapter 4 – Periodic Task Scheduling. In many real-time systems periodic tasks dominate the demand. Three classic periodic task scheduling algorithms: Time Line (Cyclic) Rate Monotonic Earliest Deadline First. What is the landscape here? What causes the challenges?.

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Chapter 4 – Periodic Task Scheduling

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  1. Chapter 4 – Periodic Task Scheduling In many real-time systems periodic tasks dominate the demand. Three classic periodic task scheduling algorithms: Time Line (Cyclic) Rate Monotonic Earliest Deadline First What is the landscape here? What causes the challenges?

  2. Periodic Task Scheduling Notation Ti denotes the period of task i Cidenotes the Computation time for one instance of task i

  3. Periodic Task Scheduling Assumptions

  4. Additional Periodic Task Parameters

  5. < 1 if set is scheduable Processor Utilization Factor - U Let be the upper bound of the processor utilization factor for a set under a given algorithm A. When the set is said to fully utilize the processor. 1

  6. Time Line Scheduling (Cyclic Scheduling) 40 Hz 20 Hz 10 Hz Time Line Scheduling (Off-line scheduling strategy)– Divide the time line into time slices for scheduling tasks, e.g. use the Greatest Common Divisor of the Task Periods as the time slice: Schedulability is guaranteed if: Time line scheduling is sensitive to changes in the task parameters. Can time line scheduling also accommodate aperiodic tasks ?

  7. Rate Monotonic Scheduling RM is optimal among all fixed Priority assignment schedules. Why? How can you how show that it must be optimal?  Which is clearly non-optimal

  8. Rate Monotonic – Calc of ULUB for Two Tasks Let , the number of Then the schedule produced by RM satisfies one of two conditions: The critical time zone is for Uub

  9. Earliest Deadline First Proof: Assume U > 1 and the task set is schedulable. This leads to a contradiction:

  10. Example: Comparing RM to EDF Since U > ln 2 = 0.69, Schedulability can’t be guaranteed using Rate Monotonic

  11. Homework #4 Lecture 5/1 1) Verify the schedulability under RM of the following task set: 2) Verify the schedulability under EDF, and then construct the corresponding schedule. Due May 8

  12. Mutual Exclusion Problems

  13. Mutual Exclusion Problems

  14. Mutual Exclusion Problems

  15. Mutual Exclusion Problems

  16. Mutual Exclusion Problems • Priority Ceiling protocol: • The priority ceiling PC(S) is the highest priority of all processes that may lock S. • A task P that attempts to lock a semaphore will be suspended unless its priority is higher than PC(S) for all S currently locked by all tasks Q not= P. • If P is suspended, then the task Q that holds the lock with the highest PC is considered to be blocking P. Q then inherits P’s priority.

  17. Mutual Exclusion Problems

  18. Mutual Exclusion Problems Again, using Priority Ceiling

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