Difference between Preemptive and Non-Preemptive Scheduling in Operating Systems with Examples

Scheduling is a fundamental aspect of operating systems that determines how tasks or processes are allocated CPU time. Two commonly used scheduling strategies are preemptive scheduling and non-preemptive (or cooperative) scheduling. Understanding the differences between these approaches is essential for designing efficient and responsive systems. In this article, we will explore the distinctions between preemptive and non-preemptive scheduling and provide examples to illustrate their functionalities.

Preemptive Scheduling:

In preemptive scheduling, the operating system can interrupt a running task and allocate CPU time to a higher-priority task if it becomes available. This means that a task can be forcibly removed from the CPU before it completes its execution. Preemptive scheduling ensures that critical or time-sensitive tasks receive immediate attention, even if a lower-priority task is currently running.

Example:
Consider a preemptive scheduling scenario with two tasks: Task A and Task B. Task A has a higher priority than Task B. With preemptive scheduling, if Task B is currently executing, and Task A with higher priority becomes ready, the operating system will interrupt Task B and allocate CPU time to Task A. This guarantees that high-priority tasks are promptly attended to.

Non-Preemptive (Cooperative) Scheduling:

In non-preemptive (cooperative) scheduling, a task voluntarily relinquishes CPU control once it completes or enters a waiting state. The task is responsible for explicitly yielding control to the operating system, allowing other tasks to execute. Non-preemptive scheduling relies on tasks cooperating and adhering to timing constraints to ensure fairness and efficient resource utilization.

Example:
Consider a non-preemptive scheduling scenario with two tasks: Task X and Task Y. Task X starts executing first and continues until it voluntarily yields control to Task Y. Task Y will not gain access to the CPU until Task X explicitly gives up control by completing its execution or entering a waiting state. Non-preemptive scheduling requires tasks to be cooperative and relinquish control to maintain fairness among all running tasks.

Key Differences:

1. Task Interruption:
   Preemptive scheduling allows the operating system to interrupt a running task and allocate CPU time to higher-priority tasks. In non-preemptive scheduling, tasks need to voluntarily yield control to other tasks.
2. Responsiveness:
   Preemptive scheduling provides better responsiveness since high-priority tasks can be immediately executed, even if lower-priority tasks are running. Non-preemptive scheduling relies on tasks cooperatively yielding control, which may result in delayed execution of higher-priority tasks.
3. Priority Inversion:
   Preemptive scheduling can experience priority inversion, where a low-priority task holds a resource needed by a high-priority task, causing delays. Non-preemptive scheduling reduces the likelihood of priority inversion since tasks need to voluntarily release resources.
4. Complexity:
   Preemptive scheduling introduces additional complexity due to task preemption and context switching. Non-preemptive scheduling is simpler as tasks explicitly control their execution time.

Conclusion:

Preemptive and non-preemptive scheduling are two distinct approaches used in operating systems. Preemptive scheduling enables the operating system to interrupt running tasks and allocate CPU time to higher-priority tasks, ensuring responsiveness and timely execution. Non-preemptive scheduling relies on tasks cooperatively yielding control and requires task cooperation and adherence to timing constraints. The choice of scheduling strategy depends on the specific requirements of the system, the nature of the tasks, and the need for responsiveness or resource sharing.