Chapter 3: Real-Time Scheduling and Schedulability Analysis

1. Chapter 3: Real-Time Scheduling and Schedulability Analysis Albert M. K. Cheng

2. Real-Time Scheduling • Schedulers: compile-time, run-time • Systems: uniprocessor, multiprocessor • Characterization of a task: c computation time (WCET) s start (release or ready) time d deadline (relative to start time) p period or minimum separation

3. Optimal Scheduler • One which may fail to meet the deadline of task only if no other scheduler can • Examples include uniprocessor earliest-deadline-first (EDF) and least-laxity-first (LLF) schedulers for independent tasks with no synchronization constraints and no resource requirements

4. EDF vs FIFO (non-real-time scheduler) Example: task s d c 1 0 5 3 2 0 12 4 3 0 10 4 ready queue: head 2, 3, 1 tail FIFO schedule: 2, 3, 1 (task 1 misses deadline) EDF schedule: 1, 3, 2 (all tasks meet deadlines)

5. Rate-Monotonic (RM) Scheduler:Basic Assumptions • Compile-time/static • Preemptive tasks • No precedence constraints • Negligible context-switching time • Periodic tasks

6. Schedulability Test for RM • Liu and Layland’s sufficient condition: Utilization U = sum c_I / p_I Number of tasks n A task set is schedulable if U <= n(2^(1/n) – 1) Necessary and sufficient condition requires checking inequalities or plot of W vs t

7. Sporadic Tasks • Can be modeled by periodic tasks with periods equal to the minimum separation • Deferred server (DS) is less wasteful • Tasks with critical sections (non-preemptive) must be handled differently

8. Scheduling Tasks with Constraints • Deterministic Rendezvous Model: tasks with rendezvous (as in Ada) • Kernelized Monitor Model: tasks with critical sections • Dataflow Model: tasks with precedence and data flow constraints

9. Multiprocessor Scheduling • Scheduling game board representation: shows a tasks’ statuses dynamically • Non-optimality of uniprocessor-optimal schedulers • Need to consider special cases to improve scheduling efficiency • Need to avoid conflict-free task sets