Software & Updates

RTOS Scheduling Algorithms

Real-time operating systems (RTOS) work through various mechanisms to insure that the tasks complete in a specified time window 100% of the time.  This is known as determinism.  Most RTOSs rely on pre-emptive scheduling to insure that the highest priority thread runs to completion without interruption.  That being said, an RTOS can support different scheduling techniques.  Generally, the scheduling of algorithms vary according to their usage. These scheduling algorithms include;

Cooperative Scheduling

Computer multitasking occurs when scheduling is not generated from an initiated system switch as is the case under cooperative scheduling. The processes involuntarily trigger subsequent events that are needed for the process. In essence, a generated process leads to other related sequences. Alternatively, a process can be blocked for other multiple processes to commence. Therefore, this scheduling requires cooperation among the various processes controlled by the operating system. Given the dependencies among the various systems, a downturn in one leads to a general breakdown in the entire scheduling. The cooperative scheduling is used in timed events as in the case of JavaScript or Python.

Pre-emptive Scheduling

The presence of context switches enables the interruption of sequences in the computer system that changes the output from the scheduling. This process does not require the cooperation of the sequences but instead operates independently. The pre-emptive scheduler enjoys the privilege of tampering with any of the events without stopping the others. In Kernel functions, pre-emption may not be permissible leading to system breakdown. The Kernel-mode operates opposite to the User mode, the latter allowing system interruptions with ease. The processes are either input or output bound which makes them simpler to execute or dependent on the CPU. The scheduler runs once for each time slice hence choosing the next sequence.

Fixed priority pre-emptive Scheduling

Tasks subjected to the processor carry different weight in terms of urgency and value. Therefore, the fixed priority scheduling is made to ensure only the most preferred tasks are handled first. The system operates a time slice facilitated by a clock interruption input to ensure the tasks only take the time allocated. However, the prioritization of tasks poses the challenge of thread lockout, where the lower priority tasks may never be attended to at all. Innovations have made it possible to allow the lesser priority tasks to accrue priority through aging hence can be processed with time. A majority of RTOS have pre-emptive schedulers which can be converted to the non-pre-emptivemode by turning off time slicing.

Earliest Deadline First (EDF)

Tasks can be operated depending on their deadlines. EDF is a dynamic priority scheduling based on optimal delivery. The tasks with the earliest deadline are queued for processing. Through such operations, there is an assurance that all deadlines shall be met.


While pre-emptive scheduling is the preferred method to guarantee determinism, the RTOS scheduling algorithms can be based on the nature of tasks and the kind of results needed. The tasks can are operated within the desired timeframe and through pre-emptive scheduling when necessary. Cooperative and non-cooperative scheduling are both necessary to deal with programmed tasks in most systems.  In other words, not every task can be the highest priority task that runs to completion.

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