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Real-time/Multimedia CPU Scheduling Policies: RMS and EDF

This lecture covers the concepts of real-time and multimedia CPU scheduling, including evaluation criteria, scheduling algorithms, and the admission control process. It focuses on two key scheduling policies: Rate Monotonic Scheduling (RMS) and Earliest Deadline First (EDF).

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Real-time/Multimedia CPU Scheduling Policies: RMS and EDF

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  1. CS 414 – Multimedia Systems DesignLecture 31 – Multimedia OS (Part 1) Klara Nahrstedt Spring 2011 CS 414 - Spring 2011

  2. Administrative • MP3 is out • MP3 Help Session • Wednesday, April 13, 7-8pm in 1304 Siebel Center • mi-clicker discussion CS 414 - Spring 2011

  3. CPU Process/Thread Scheduling • Maintain many programs in memory • Determine how to best schedule them • Requires information about the processes and an understanding of the system goals • Switch among processes rapidly so each user perceives direct ownership of system CS 414 - Spring 2011

  4. CPU Scheduling Evaluation Criteria • Throughput: processes completed per unit time • Turnaround: from submission to termination • Wait: time waiting in the ready queue • Response: from user request to system response • Utilization: how busy the CPU is over time • Want predictable system behavior CS 414 - Spring 2011

  5. Scheduling Algorithms • FCFS • Shortest Job First • Priority scheduling • Round-robin • Multi-level queue • Rate monotonic • Earliest deadline first For General Purpose Process CPU Scheduling For RT/Multimedia Process CPU Scheduling CS 414 - Spring 2011

  6. Real-time/Multimedia Processing Requirements • Need to process continuous multimedia data • Processing occurs in predetermined, usually periodic intervals • Processing must be completed by certain deadlines • Need RT process manager • Perform admission control • Determine schedule • Perform reservation • Schedule to give processing guarantees CS 414 - Spring 2011

  7. RT/Multimedia Processing Requirements • Main Problem: How to find a feasible schedule? • Conflicting Goals/Problems: • How do we schedule multimedia (RT) processes so that • non-RT processes do not starve when RT process is running • RT process is not subject to priority inversion CS 414 - Spring 2011

  8. Model in RT Scheduling • Task (Process) – schedulable unit • Task characterized by • Timing constraints • Resource requirements • Assumptions • Periodic tasks without precedence relations • Time constraints • s – task starting point • e – task processing time • d – task deadline • p – task period CS 414 - Spring 2011

  9. Periodic Constant Processing Time Tasks • Example Usage Pattern: Peak Processing Time=10ms Period = 100ms 0 10 110 210 100 200 Time(ms) CS 414 - Spring 2011

  10. Periodic Variable Processing Time Tasks • Example Usage Pattern: Period = 100ms Peak Processing Time = 30ms Sustainable Processing Time = 15ms Burst Tolerance = 7ms 0 15 120 210 100 200 Time(ms) CS 414 - Spring 2011

  11. Model of RT Scheduling • Congestion avoidance deadline • If period at (k-1) step is equal to ready (start) time of period k • Tasks: preemptive vs. non-preemptive • Main goal of RT Scheduling: • find feasible schedule of all periodic tasks so that newly arriving task and all previous admitted tasks finish processing in every period according to their deadline CS 414 - Spring 2011

  12. Model of RT Scheduling • Must have Schedulability (Admission) Test for RT tasks • What is the performance metric for RT tasks? • Guarantee ratio := number of guaranteed tasks/total number of tasks • Process utilization (U): CS 414 - Spring 2011

  13. Scheduling Policies of RT Tasks • Rate- Monotonic Scheduling (RMS) • Designed/proved by C.L. Liu and Layland 1973 • Policy: task with highest rate has highest priority • Static and optimal, priority-driven • Optimal means that there no other static algorithm that is able to schedule a RT task which can’t be scheduled by RMS algorithm • Assumptions: • Tasks are periodic • Each task must complete before next request • All tasks are independent • Run-time of each task request is constant • Any non-periodic task has no required deadline CS 414 - Spring 2011

  14. Example of RMS CS 414 - Spring 2011

  15. Scheduling Policies for RT Tasks • Earliest Deadline First (EDF) Policy • Optimal dynamic algorithm • Produces valid schedule if one exists • Complexity O(n2) • Upper bound of process utilization 100% • Policy: task with earliest deadline has highest priority CS 414 - Spring 2011

  16. Example of EDF CS 414 - Spring 2011

  17. Comparison between RMS and EDF CS 414 - Spring 2011

  18. Admission Control (for preemptive tasks) • Schedulability test for RMS • Schedulability test for EDF CS 414 - Spring 2011

  19. Example • Consider the following preemptive RT tasks and their characteristics • T1: p1 = 50ms, e1=10ms • T2: p2 = 100ms, e2=20ms • T3: p3 = 200ms, e3=50ms • T4: p4 = 100ms, e4=20ms • Are these tasks schedulable via RMS? • If yes, what is the feasible schedule? • Are these tasks schedulable via EDF? • If yes, what is the feasible schedule? CS 414 - Spring 2011

  20. Preemptive vs. Non-preemptiveScheduling CS 414 - Spring 2011

  21. Conclusion RMS and EDF are basic policies for real-time scheduling policies Most of the Real-time tasks are guided by these scheduling policies CS 414 - Spring 2011

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