Книга: Real-Time Concepts for Embedded Systems

14.4 Schedulability Analysis-Rate Monotonic Analysis

After an embedded application has been decomposed into ISRs and tasks, the tasks must be scheduled to run in order to perform required system functionality. Schedulability analysis determines if all tasks can be scheduled to run and meet their deadlines based on the deployed scheduling algorithm while still achieving optimal processor utilization.

Note that schedulability analysis looks only at how systems meet temporal requirements, not functional requirements.

The commonly practiced analytical method for real-time systems is Rate Monotonic Analysis (RMA). Liu and Layland initially developed the mathematical model for RMA in 1973. (This book calls their RMA model the basic RMA because it has since been extended by later researchers.) The model is developed over a scheduling mechanism called Rate Monotonic Scheduling (RMS), which is the preemptive scheduling algorithm with rate monotonic priority assignment as the task priority assignment policy. Rate monotonic priority assignment is the method of assigning a task its priority as a monotonic function of the execution rate of that task. In other words, the shorter the period between each execution, the higher the priority assigned to a task.

A set of assumptions is associated with the basic RMA. These assumptions are that:

· all of the tasks are periodic,

· the tasks are independent of each other and that no interactions occur among tasks,

· a task's deadline is the beginning of its next period,

· each task has a constant execution time that does not vary over time,

· all of the tasks have the same level of criticality, and

· aperiodic tasks are limited to initialization and failure recovery work and that these aperiodic tasks do not have hard deadlines.

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