CPU Scheduling in Operating Systems 

When you use a computer or mobile device, many tasks run at the same time, such as opening apps, playing music, or downloading files. The operating system manages these activities to keep everything running smoothly. One important way it does this is through CPU scheduling. This process decides which task gets CPU time at the right moment so the system stays fast and responsive. In this blog, you will learn what CPU scheduling is, why it matters, the different types of scheduling, and the advantages and disadvantages of each

Table of Contents:

• What is CPU Scheduling in Operating System?
• Importance of CPU Scheduling in Operating System
• Types of CPU Scheduling in Operating System
• Advantages and Disadvantages of CPU Scheduling in Operating System
• Important Terminologies Used in CPU Scheduling
• Criteria for Evaluating CPU Scheduling Algorithms
• Comparison of CPU Scheduling Algorithms in Operating System
• Common CPU Scheduling Mistakes in Operating Systems
• Conclusion

What is CPU Scheduling in Operating System?

CPU scheduling is the process in an operating system that decides which process or program gets access to the CPU at any given time. Since each CPU core can handle only one task at a time, the operating system uses scheduling to manage multiple tasks efficiently, especially in systems with multi-core processors or multithreading. When several programs are waiting, the scheduler selects one based on specific rules and algorithms, giving that program control of the CPU. This ensures the system runs smoothly and efficiently. This process is also known as process scheduling in OS.

Importance of CPU Scheduling in Operating System

Let’s explore some reasons why CPU scheduling matters. In fact, efficient process scheduling in OS ensures better multitasking and fairness for all processes.

• Optimized CPU Utilization: It ensures that the CPU is always engaged in productive work, rather than idling. If one task is waiting for input, the CPU can go to another ready task. 
• Fairness for Processes: Every process should receive a fair CPU time. Scheduling works towards the goal of ensuring that no single task monopolizes all the CPU time, thereby leaving other processes unattended.
• Faster System Response: For tasks that require quick response times, scheduling can improve the responsiveness of the system. Examples include clicking a button, playing a video, and updating a web page. 
• Support Multi-tasking: It supports multitasking by quickly switching between tasks, allowing multiple applications to run at the same time.

Become a Job-Ready Software Engineer

Master coding, system design, and real-world projects with expert mentors. Get certified and land your dream tech job

Explore Program

Types of CPU Scheduling in Operating System

There are two main types of CPU scheduling techniques used in an operating system.

1. Preemptive Scheduling

In preemptive scheduling, the CPU can take control from a running process if a higher-priority task arrives. This ensures that ivmportant tasks are handled quickly, even if it means pausing another task in progress.

Common examples of preemptive scheduling are:

• Round Robin (RR) Scheduling: Each process gets a fixed time to run. If it doesn’t finish in that time, it is paused and placed back in the queue, allowing the next process to run.
• Shortest Remaining Time First (SRTF) Scheduling: The process with the least time left to complete is given the CPU next.
• Priority Scheduling (Preemptive): Processes with higher priority can interrupt and replace lower priority ones already using the CPU.

Real-world example: CPU scheduling examples here include online games.

2. Non-Preemptive Scheduling

In non-preemptive scheduling, once a task gets control of the CPU, it continues to use it until it either finishes or moves to a waiting state. The CPU does not interrupt this task, even if a higher-priority task arrives while it is still running.

There are a few examples of non-preemptive scheduling:

• First-Come, First-Served (FCFS): The process that arrives first is given the CPU first and runs until it finishes.
• Shortest Job Next (SJN): The process with the shortest total execution time is selected to run before others.
• Priority Scheduling (Non-Preemptive): The CPU is assigned to the highest priority process among those waiting, but once a process starts running, it will not be interrupted until it completes.

Real-world example: CPU scheduling examples of this type include batch processing systems.

If you’re interested in learning more, check out our detailed blog on Preemptive and Non-Preemptive Scheduling!

Advantages and Disadvantages of CPU Scheduling in Operating System

CPU scheduling has advantages and disadvantages. It improves multitasking, responsiveness, and efficiency, but is also responsible for complexities and dependencies on the OS and processes. Let’s look at the main advantages and disadvantages of CPU scheduling in an operating system.

Advantages

Let’s explore some advantages of CPU scheduling:

• Enhances CPU Utilization: CPU scheduling is used to effectively keep the CPU busy by assigning it a process that is ready. This minimizes idle time and improves the overall efficiency of the system.
• Facilitates Multitasking: Scheduling enables the system to handle multiple tasks by quickly switching the CPU between them, creating the illusion that all tasks are running simultaneously.
• Decreases Waiting Time: Scheduling helps reduce the time processes spend waiting in the queue by choosing the most suitable process at the right moment.
• Provides Fairness: Scheduling algorithms aim to give each process a fair amount of CPU time, preventing any one process from waiting too long.

Disadvantages

Let’s explore the disadvantages of CPU scheduling:

• Difficult Implementation: Some scheduling methods are complex and use advanced logic, making them hard to implement, especially in systems with many tasks.
• Overhead Due to Task Switching: Frequently switching between tasks uses system time and resources. If switching happens too often, it can slow down overall performance.
• Potential Starvation: In some scheduling algorithms, lower-priority tasks may never get CPU time if higher-priority tasks keep arriving.
• Mismanaged Task Priorities: Incorrectly setting task priorities can cause important processes to be ignored, leading to delays and affecting the system’s overall operation.

Important Terminologies Used in CPU Scheduling

Let’s explore some important terms that are used in CPU scheduling:

1. Arrival Time: Arrival time refers to the time when a process enters the ready queue. The arrival time is therefore the time when the process is ready to be executed on the CPU.

2. Burst Time: Burst time (or the execution time) is the total time a process requires from the CPU to complete its execution. Time spent waiting is excluded.

3. Completion Time: Completion time is the time when a process has finished its execution. It contains both the waiting time and the burst time.

4. Turnaround Time: Turnaround time is the total time to completion of a process from its arrival.

Formula:

Turnaround Time = Completion Time – Arrival Time

5. Waiting Time: Waiting time is the time the process has spent waiting in the ready queue to gain access to the CPU.

Formula:

Waiting Time = Turnaround Time – Burst Time

6. Response Time: Response time is the duration from when a task arrives to when it first gets the CPU. It is especially important in interactive systems that need quick reactions.

7. Context Switch: A context switch is when the system saves the current process’s state and loads the state of a different process. This lets the CPU switch from one task to another smoothly.

Criteria for Evaluating CPU Scheduling Algorithms

An efficient CPU scheduling algorithm should meet certain important criteria to ensure the system runs efficiently. The better an algorithm fulfills these goals, the more effective it will be. Here are the key criteria used to evaluate CPU scheduling:

1. Fairness: The scheduling method should give all processes a fair chance to use the CPU and avoid starvation, where some tasks never get executed.
2. CPU Utilization: The algorithm should keep the CPU as busy as possible by minimizing idle time and making full use of the processor.
3. Throughput: It should maximize the number of processes completed within a given time period, improving overall system productivity.
4. Turnaround Time: The algorithm should minimize the total time taken for a process from start to finish.
5. Waiting Time: Reducing the time a process spends waiting in the ready queue is important for faster execution and system fairness.
6. Response Time: For interactive systems, the algorithm should minimize the delay between a request and the start of processing to improve user experience.

Comparison of CPU Scheduling Algorithms in Operating System

Common CPU Scheduling Mistakes in Operating Systems

Let’s explore some common mistakes in CPU scheduling : 

• Excessive Context Switching: Using too many switches between tasks can waste time and decrease efficiency.
• Starvation: If the high-priority task keeps coming, the low-priority task might never get CPU time, and this will lead to starvation issues.
• Utilizing the Wrong Algorithm for the Type of System: Using complex algorithms for simple systems, or relying on basic scheduling for real-time environments, can lead to poor performance.
• Unbalanced Time Quantum in Round Robin Scheduling: If the time slice is too short, the CPU wastes time switching between tasks instead of executing them. If it is too long, the scheduling behaves like First-Come, First-Served (FCFS), reducing responsiveness.

By avoiding these pitfalls, process scheduling in OS can be more effective, leading to better overall system performance and user experience.

Conclusion

CPU scheduling in an operating system plays a vital role in managing tasks efficiently. It determines which task uses the CPU and for how long, leading to better performance, reliability, and user satisfaction. Various scheduling algorithms, such as First-Come First-Served, Round Robin, Priority, and Shortest Remaining Time First, offer different advantages that are suitable for specific system needs. Understanding the key concepts and criteria of scheduling helps in selecting the best method for any situation, ensuring smooth and effective system operation.

Take your skills to the next level by enrolling in the Software Engineering Course today and gaining hands-on experience. Also, prepare for job interviews with Software Engineering Interview Questions prepared by industry experts.

Explore other blogs related to Operating System by Intellipaat:

CPU Scheduling in Operating Systems – FAQs