Scheduling Algorithms Used in Real-Time Operating Systems (RTOS) with Examples

Real-Time Operating Systems (RTOS) are crucial for time-sensitive applications that require precise and predictable task execution. One of the essential components of an RTOS is its scheduling algorithm, which determines the order in which tasks are executed on the CPU. In this article, we will explore some common scheduling algorithms used in RTOS and provide examples to illustrate their functionality.

First-Come-First-Serve (FCFS) Scheduling:

FCFS is a simple and intuitive scheduling algorithm where tasks are executed in the order they arrive. The first task that enters the system is the first one to be served. However, FCFS is not suitable for real-time applications as it does not consider task priorities or deadlines, leading to potential priority inversion or missed deadlines.

Example: Consider three tasks with different execution times – Task A (10 ms), Task B (5 ms), and Task C (15 ms). In FCFS, they will be executed in the order A → B → C, regardless of their importance or timing constraints.

Round-Robin (RR) Scheduling:

RR is a preemptive scheduling algorithm where each task is given a fixed time slice or quantum to execute. Once a task’s time slice expires, it is suspended, and the next task in the queue is given a chance to run. This algorithm ensures fairness among tasks and can prevent any single task from monopolizing the CPU. However, it may not be ideal for real-time applications with strict timing requirements.

Example: Suppose three tasks with the same execution time quantum of 5 ms – Task X, Task Y, and Task Z. In RR, they will be executed in a circular manner – X → Y → Z → X → Y → Z, and so on.

Priority-Based Scheduling

In this scheduling algorithm, tasks are assigned priorities, and the CPU executes the task with the highest priority first. If multiple tasks have the same priority, they are scheduled using a predetermined order (e.g., FCFS). Priority-based scheduling can be either preemptive or non-preemptive, depending on whether higher-priority tasks can interrupt lower-priority ones.

Example: Consider three tasks with different priorities – High priority (Task H), Medium priority (Task M), and Low priority (Task L). In priority-based scheduling, the CPU will execute tasks in the order H → M → L, with the highest priority task getting precedence.

Earliest Deadline First (EDF) Scheduling:

EDF is a dynamic priority scheduling algorithm that assigns priorities based on task deadlines. The task with the earliest deadline is given the highest priority and is scheduled first. EDF ensures that tasks with impending deadlines receive attention first, making it well-suited for real-time applications with hard deadlines.

Example: Assume three tasks with different deadlines – Task D1 (deadline: 20 ms), Task D2 (deadline: 15 ms), and Task D3 (deadline: 25 ms). In EDF, the CPU will execute tasks in the order D2 → D1 → D3, as Task D2 has the earliest deadline.

Conclusion: RTOS scheduling algorithms play a vital role in determining the behavior and efficiency of time-sensitive applications. The choice of scheduling algorithm depends on the specific requirements of the application. While FCFS and RR are straightforward, priority-based scheduling and EDF are more suitable for real-time applications. Understanding the strengths and limitations of each algorithm helps developers make informed decisions when designing real-time systems to meet stringent timing constraints.