cs6235-3B-feedback-edf

cs6235-3B-feedback-edf - CS 4220/6235 Feedback-Based EDF 1...

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CS 4220/6235 Feedback-Based EDF 1 1 CS4220 Embedded Systems CS6235 Real CS6235 Real-Time Systems Time Systems 3B: Feedback 3B: Feedback-Based EDF Based EDF Instructor: Calton Pu [email protected] TAs: Qinyi Wu ([email protected]) Peter Budny ([email protected]) 2 Earliest Deadline First Assumptions The schedule is feasible Tasks are independent Dynamic priority scheduler Look at all tasks in the queue Compare their deadlines Earliest Deadline First (EDF) Minimizes deadline failures when feasible Another story when schedule is infeasible
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CS 4220/6235 Feedback-Based EDF 2 3 EDF Basics Optimal dynamic priority scheduling A task with a shorter deadline has a higher priority Executes a job with the earliest deadline (4,1) (5,2) (7,2) 5 5 10 10 15 15 T 1 T 2 T 3 4 EDF Example When the schedule is feasible, EDF can schedule it No need for preemption (4,1) (5,2) (7,2) 5 5 10 10 15 15 T 1 T 2 T 3
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CS 4220/6235 Feedback-Based EDF 3 5 EDF Optimality Optimal scheduling algorithm Up to 100% WCAU (Worst Case Achievable Utilization) Intuition: can t lose the most urgent job (4,1) (5,2) (3,1) 5 5 10 10 15 15 T 1 T 2 T 3 6 Discussion of EDF Optimal dynamic scheduler Why do we need anything else? Several practical problems Overhead of dynamic scheduling Instability under overload (over the cliff) “Priority inversion”
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CS 4220/6235 Feedback-Based EDF 4 7 Minimum Laxity First Similar to EDF, improved Laxity = time to latest feasible start time (when the task can still complete) Run the task closest to failing first Optimal in minimizing deadline failures More graceful degradation Comparison with EDF MLF = EDF when all tasks have same length 8 Control Systems Systems with changes Human-controlled: bicycle, cars, airplane Inherently unstable airplane wings
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