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

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

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

EDF vs FIFO (non-real-time scheduler) Example: task s d c 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)

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

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

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

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

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