A periodic task is one which must be executed at a regular interval. The interval between successive iterations of the task is referred to as its period.

Periodic tasks can be characterized by the length of their period and execution time. The period and execution time of a task can be used to determine the processor utilization for that task. Processor utilization is the percentage of processor time used and can be calculated on a per-task or system-wide basis.

Typically, the task's worst-case execution time will be less than its period. For example, a periodic task's requirements may state that it should execute for 10 milliseconds every 100 milliseconds. Although the execution time may be the average, worst, or best case, the worst-case execution time is more appropriate for use when analyzing system behavior under transient overload conditions.

In contrast, an aperiodic task executes at irregular intervals and has only a soft deadline. In other words, the deadlines for aperiodic tasks are not rigid, but adequate response times are desirable. For example, an aperiodic task may process user input from a terminal.

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:

1. RMS is an optimal static priority algorithm for scheduling independent, preemptible, periodic tasks on a single processor.

2. 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.

3. 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, than 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.