RTEMS C User's Guide
The concept of scheduling in real-time systems dictates the ability to provide immediate response to specific external events, particularly the necessity of scheduling tasks to run within a specified time limit after the occurrence of an event. For example, software embedded in life-support systems used to monitor hospital patients must take instant action if a change in the patient's status is detected.
The component of RTEMS responsible for providing this capability is appropriately called the scheduler. The scheduler's sole purpose is to allocate the all important resource of processor time to the various tasks competing for attention. The RTEMS scheduler allocates the processor using a priority-based, preemptive algorithm augmented to provide round-robin characteristics within individual priority groups. The goal of this algorithm is to guarantee that the task which is executing on the processor at any point in time is the one with the highest priority among all tasks in the ready state.
There are two common methods of accomplishing the mechanics of this algorithm. Both ways involve a list or chain of tasks in the ready state. One method is to randomly place tasks in the ready chain forcing the scheduler to scan the entire chain to determine which task receives the processor. The other method is to schedule the task by placing it in the proper place on the ready chain based on the designated scheduling criteria at the time it enters the ready state. Thus, when the processor is free, the first task on the ready chain is allocated the processor. RTEMS schedules tasks using the second method to guarantee faster response times to external events.
RTEMS C User's Guide
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