Difference Between Preemptive and Non-Preemptive Scheduling

In today’s world, computers manage multiple tasks at the same time, deciding which process runs and when. Process scheduling ensures smooth system operation by managing CPU time based on process priorities. Among the various methods available, preemptive and non-preemptive scheduling are the two main types. Preemptive scheduling allows the CPU to switch between tasks based on priority, while non-preemptive scheduling lets a task finish before moving to the next. Each method uses a different approach and affects system performance in unique ways. In this blog, you will understand what preemptive and non-preemptive scheduling are, how they work, and their key differences in detail.

Table of Contents:

• What is Preemptive Scheduling?
• What is Non-Preemptive Scheduling?
• Differences Between Preemptive and Non-Preemptive Scheduling
• Key features of Preemptive Scheduling
• Working of Preemptive Scheduling
• Advantages and Disadvantages of Preemptive Scheduling
• Key features of Non-Preemptive Scheduling
• Working of Non-Preemptive Scheduling
• Advantages and Disadvantages of Non-Preemptive Scheduling
• Common mistakes for Preemptive and Non-Preemptive Scheduling
• Best Practices for Preemptive and Non-Preemptive Scheduling
• Real-World Example for Preemptive and Non-Preemptive Scheduling
• Conclusion

What is Preemptive Scheduling?

Preemptive scheduling is a CPU scheduling strategy in which the currently executing process may be interrupted and the CPU may be given to another process that is demanding attention. It is similar to your memory of a strict manager being able to come and take a worker off a task if something more urgent needs to be addressed. A preemption can happen for many reasons, such as a different, higher-priority process becoming available for execution or when the time slice for the currently running process has expired. The primary purpose of preemptive scheduling is simply to ensure that every process gets access to the CPU and high-priority processes do not have to wait indefinitely.

What is Non-Preemptive Scheduling?

Non-preemptive scheduling is an alternative to preemptive scheduling. In non-preemptive scheduling, once a process begins executing, it continues to execute until it finishes or the process releases the CPU. A non-preemptive scheduling example would be a worker who is given a task that they complete until they finish, or when they decide to take a break. There is no external force that can interrupt a running process in non-preemptive scheduling. The process must finish or give up the CPU on its own before another process can use it. It can’t be stopped or interrupted while it’s running.

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Differences Between Preemptive and Non-Preemptive Scheduling

Key features of Preemptive Scheduling

1. Faster Response: Preemptive scheduling allows a system to respond faster to new or higher-priority events.

2. Emphasizes Fairness in CPU Usage: It is focused on fairer use of the CPU time slice, so that no single process can monopolize the CPU time.

3. Allows Task Interruption: The primary distinction of preemptive scheduling is that the operating system can suspend any task that is currently running.

Preemption Ensures Efficient CPU Usage: It prevents one process from holding on to the CPU for too long while also allowing other tasks to run in the system.

5. Allows Process Switching: This type of scheduling is intended for quick switching between processes. Preemptive scheduling is highly useful for time-sharing systems with multiple users, applications, and tasks actively running on the system at the same time.

Working of Preemptive Scheduling

The operating system uses a method, typically using a timer interrupt or priority queue, to know when to switch processes. When one process is executing, it is possible that a timer is set. If the timer expires while the process is executing, the operating system will suspend the process and then save the state of the running process, and choose another process to run. The operating system will also preempt the currently running lower-priority process if a new process arrives with a higher priority than the lower-priority process. Since preemptive scheduling means there is almost always a potential for interruption in any current task, the scheduling system must constantly monitor for process switching and interrupts.

Advantages and Disadvantages of Preemptive Scheduling

Preemptive scheduling has both advantages and disadvantages that affect system performance and complexity. Understanding these helps in selecting the best approach for different computing environments.

Advantages of Preemptive Scheduling

• Increased responsiveness for interactive applications.
• Better distribution of CPU time among processes.
• Suitable for real-time systems where timely execution is required.
• Prevents any single process from holding CPU control for too long.
• Ensures critical tasks receive prompt attention.

Disadvantages of Preemptive Scheduling

• Frequent context switching may lead to thrashing, reducing performance.
• Frequent context switching results in higher overhead.
• It can reduce overall system throughput.
• Adds complexity to the design of the operating system.
• More difficult to implement properly.

Key features of Non-Preemptive Scheduling

1. Simple to Implement: Non-preemptive scheduling is, in general, easier to design or implement than preemptive scheduling since it does not have to implement complex interruption designs.

2. Less Context Switching Overhead: Once a process begins, it runs until it finishes or voluntarily gives up the CPU, which reduces the overhead of the operating system, saving and restoring the process state.

3. Task Efficiency: This model enables a task to run on the CPU uninterrupted, so for those long-running, CPU-bound type jobs, this works very efficiently.

4. Predictable Task Completion: As the tasks run without interruptions or preemptions, their completion times are more predictable due to the absence of external interference.

5. Appropriate for Batch System: This method works more effectively in environments where it is a batch processing way to execute a queue of jobs, where the jobs will run sequentially one after the other, and the disruptive element of immediate user interaction is not a high priority in real time.

Working of Non-Preemptive Scheduling

The operation of a non-preemptive scheduling model is straightforward. Once a process is selected to run, it retains full control of the CPU and continues execution until it either completes or performs an I/O operation. (For example, during a read from disk) the process willingly gives up the CPU when it finishes or needs input or output. In non-preemptive scheduling, the scheduler can assign the CPU to another process only when the current one finishes or voluntarily yields. It operates on the principle of “run to completion or yield.”

Advantages and Disadvantages of Non-Preemptive Scheduling

Non-preemptive scheduling has both advantages and disadvantages that impact system efficiency and simplicity. Knowing these helps in choosing the right method for various system requirements.

Advantages of Non-Preemptive Scheduling

1. Minimal context switching, which means low overhead.
2. More CPU time is available to run the actual important tasks.
3. Better for systems with tasks that require consistent and uninterrupted execution.
4. Easier to implement and maintain.
5. No chance of race conditions in critical sections since processes run to completion.

Disadvantages of Non-Preemptive Scheduling

1. Not suitable for real-time or time-sharing systems.
2. No responsiveness, especially for interactive work.
3. Long-running processes can block higher-priority tasks from running.
4. Increased wait times for other processes.
5. Can lead to a poor user experience.

Common mistakes for Preemptive and Non-Preemptive Scheduling

1. Improper Preemption Interval (Preemptive): If the interval is too small, it leads to excessive context switching. If the interval is too large, it reduces responsiveness and the system starts behaving like a non-preemptive one.

2. Lack of Shared Resource Coordination (Preemptive): Interrupting a process using shared resources may cause race conditions, data corruption, or system crashes.

3. Poor Algorithm Fit for Interactive Workloads (Non-Preemptive): Long-running tasks can block the CPU and delay time-sensitive or high-priority operations.

4. Uneven Task Execution Time (Non-Preemptive): Assuming all tasks have similar durations can lead to CPU monopolization by longer processes while others remain idle.

5. No Recovery from Stuck Processes (Non-Preemptive): If a process enters an infinite loop or gets stuck in a deadlock, it can freeze the system because there is no way to interrupt it.

Best Practices for Preemptive and Non-Preemptive Scheduling

1. Maximize Responsiveness (Preemptive): Implement effective priority management and optimize time slices to ensure high-priority and interactive tasks receive timely CPU access and maintain system responsiveness.

2. Ensure Data Consistency (Preemptive): Use synchronization mechanisms like mutexes and semaphores to protect shared resources and prevent race conditions and data corruption during context switches.

3. Match Scheduling to Workload Type (Non-Preemptive): Apply non-preemptive scheduling to systems with predictable, long-running tasks such as batch jobs or embedded systems that rely on precise timing and minimal context switching.

4. Minimize Starvation and Delays (Non-Preemptive): Break long tasks into shorter ones or have them yield the CPU periodically. Use watchdog timers to detect and recover from stuck or unresponsive processes.

5. Balance Throughput and Latency (Both): Select a scheduling strategy that aligns with system goals by weighing task completion rate (throughput) against the responsiveness required for real-time or interactive applications.

Real-World Example for Preemptive and Non-Preemptive Scheduling

Let’s now see how the preemptive and non-preemptive scheduling works with the help of an example.

Processes:

Goal to reach: To schedule the process based on priority.

1. Preemptive Priority Scheduling

Rules to follow: CPU in preemptive priority scheduling will always allocate the task that is on a high-priority process immediately on arrival. This will be working on high priority even if there is another process running on the CPU.

Steps:

1. Time 0: P1 arrives and immediately starts executing, as there are no other processes in the system.
2. Time 2: P2 arrives with a higher priority (1) than P1. Therefore, P2 will preempt P1, pausing its execution, and P2 will start running.
3. Time 5: P2 will complete its process. Then again, P1 will resume its work. At this point, P1 still has 4 units of burst time remaining.
4. Time 6: Now, P3 arrives with priority = 3, which is lower than P1’s priority = 2, so P1 continues executing.
5. Time 9: P1 completes. Now, P3 will start running.
6. Time 13: At the end, P3 completes its processing.

Gantt Chart:

Note: Here, the P1 was interrupted at t=2, when P2 arrived.

2. Non-Preemptive uses Shortest Job First (SJF)

Rules to follow: This method follows the Shortest Job First (SJF) principle by selecting the task with the shortest burst time among the available ones. It does not reconsider shorter jobs that arrive later.

Steps:

1. Time 0: P1 arrived and started running the process.
2. Time 2: Then, P2 arrived with a burst time of 3, which is less than P1’s, so the remaining time will be 4, but P1 continues in the non-preemptive method.
3. Time 6: Then P1 has completed the task.
4. Queue: Now, P2, which has a burst time of 3, and P3 has a burst time of 4. This makes the P2 run first, as P2 is the shortest.
5. Time 9: Now, P2 has completed its task. Then P3 starts to run.
6. Time 13: In the end, P3 completes its process.

Gantt Chart:

Note: Here, P2 arrived earlier but had to wait for P1 to finish its task.

Conclusion

The core difference between preemptive scheduling and non-preemptive scheduling in operating systems is responsiveness and efficiency. Preemptive scheduling focuses on quickly switching tasks to ensure fairness and meet real-time needs. Non-preemptive scheduling allows tasks to finish without interruption, making it easier to implement and better suited for batch processing. Knowing this key difference helps system designers pick the right scheduling method based on their system’s needs.

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