RTEMS 4.0.0 On-Line Library

Rate Monotonic Manager Processor Utilization Rule

RTEMS C User's Guide

18.2.6: Processor Utilization Rule

The Processor Utilization Rule requires that processor utilization be calculated based upon the period and execution time of each task. The fraction of processor time spent executing task index is Time(index) / Period(index). The processor utilization can be calculated as follows:

Utilization = 0

for index = 1 to maximum_tasks
  Utilization = Utilization + (Time(index)/Period(index))

To insure schedulability even under transient overload, the processor utilization must adhere to the following rule:

Utilization = maximum_tasks * (2(1/maximum_tasks) - 1)

As the number of tasks increases, the above formula approaches ln(2) for a worst-case utilization factor of approximately 0.693. Many tasks sets can be scheduled with a greater utilization factor. In fact, the average processor utilization threshold for a randomly generated task set is approximately 0.88.

RTEMS C User's Guide