RTEMS 4.9.5 On-Line Library

Scheduling Concepts Task Priority and Scheduling

RTEMS Ada User's Guide

18.2.1: Task Priority and Scheduling

The most significant of these mechanisms is the ability for the user to assign a priority level to each individual task when it is created and to alter a task's priority at run-time. RTEMS provides 255 priority levels. Level 255 is the lowest priority and level 1 is the highest. When a task is added to the ready chain, it is placed behind all other tasks of the same priority. This rule provides a round-robin within priority group scheduling characteristic. This means that in a group of equal priority tasks, tasks will execute in the order they become ready or FIFO order. Even though there are ways to manipulate and adjust task priorities, the most important rule to remember is:

The RTEMS scheduler will always select the highest priority task that is ready to run when allocating the processor to a task.

RTEMS Ada User's Guide