RTEMS C User's Guide
The Rate Monotonic Scheduling Algorithm (RMS) is important to real-time systems designers because it allows one to guarantee that a set of tasks is schedulable. A set of tasks is said to be schedulable if all of the tasks can meet their deadlines. RMS provides a set of rules which can be used to perform a guaranteed schedulability analysis for a task set. This analysis determines whether a task set is schedulable under worst-case conditions and emphasizes the predictability of the system's behavior. It has been proven that:
RMS is an optimal static priority algorithm for scheduling independent, preemptible, periodic tasks on a single processor.
RMS is optimal in the sense that if a set of tasks can be scheduled by any static priority algorithm, then RMS will be able to schedule that task set. RMS bases it schedulability analysis on the processor utilization level below which all deadlines can be met.
RMS calls for the static assignment of task priorities based upon their period. The shorter a task's period, the higher its priority. For example, a task with a 1 millisecond period has higher priority than a task with a 100 millisecond period. If two tasks have the same period, then RMS does not distinguish between the tasks. However, RTEMS specifies that when given tasks of equal priority, the task which has been ready longest will execute first. RMS's priority assignment scheme does not provide one with exact numeric values for task priorities. For example, consider the following task set and priority assignments:
| Task | Period (in milliseconds) | Priority |
| 1 | 100 | Low |
| 2 | 50 | Medium |
| 3 | 50 | Medium |
| 4 | 25 | High |
RMS only calls for task 1 to have the lowest priority, task 4 to have the highest priority, and tasks 2 and 3 to have an equal priority between that of tasks 1 and 4. The actual RTEMS priorities assigned to the tasks must only adhere to those guidelines.
Many applications have tasks with both hard and soft deadlines. The tasks with hard deadlines are typically referred to as the critical task set, with the soft deadline tasks being the non-critical task set. The critical task set can be scheduled using RMS, with the non-critical tasks not executing under transient overload, by simply assigning priorities such that the lowest priority critical task (i.e. longest period) has a higher priority than the highest priority non-critical task. Although RMS may be used to assign priorities to the non-critical tasks, it is not necessary. In this instance, schedulability is only guaranteed for the critical task set.
RTEMS C User's Guide
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