Process Scheduling Algorithms: FCFS, SJF, Round Robin, and More

Introduction

In an operating system, process scheduling ensures the CPU is efficiently utilized by managing the order in which processes execute. This is vital for multitasking environments, where multiple processes compete for CPU time. Process scheduling algorithms dictate how these processes are prioritized, balancing factors like throughput, latency, and fairness. In this article, we’ll explore popular algorithms such as First-Come, First-Served (FCFS), Shortest Job First (SJF), Round Robin (RR), and more.

What is Process Scheduling?

Process scheduling is the method by which an operating system determines the execution order of processes in the CPU. Its primary goals are to maximize CPU utilization, improve response time, and ensure fairness.

Key Concepts in Scheduling:

• CPU Burst: The time a process spends executing on the CPU.
• IO Burst: The time a process spends waiting for I/O operations.
• Preemptive Scheduling: A process can be interrupted and moved to a waiting state.
• Non-Preemptive Scheduling: A process runs to completion before the CPU is reassigned.

Types of Process Scheduling Algorithms

1. First-Come, First-Served (FCFS)

How it Works: Processes are executed in the order they arrive in the ready queue.

• Advantages: Simple and easy to implement.
• Disadvantages: Can lead to the convoy effect, where long processes delay others.

2. Shortest Job First (SJF)

How it Works: The process with the shortest CPU burst time is executed first.

• Advantages: Minimizes average waiting time.
• Disadvantages: Requires knowledge of future burst times, leading to impracticality.

Variants:

• Non-Preemptive SJF: The process runs to completion.
• Preemptive SJF (Shortest Remaining Time First): A shorter process can interrupt the current process.

3. Round Robin (RR)

How it Works: Each process gets a fixed time slice or quantum. Processes are cycled through the ready queue.

• Advantages: Ensures fairness, suitable for time-sharing systems.
• Disadvantages: Choosing an inappropriate time quantum can lead to inefficiency.

4. Priority Scheduling

How it Works: Processes are assigned priorities; the highest-priority process executes first.

• Advantages: Allows important tasks to finish quickly.
• Disadvantages: Can cause starvation, where low-priority processes are indefinitely delayed.

5. Multi-Level Queue Scheduling

How it Works: Processes are classified into queues based on priority or type. Each queue has its own scheduling algorithm.

• Advantages: Specialization for different process types.
• Disadvantages: Complex implementation.

Comparing Scheduling Algorithms

Conclusion

Choosing the right process scheduling algorithm depends on the system’s goals. For example, Round Robin is ideal for fairness, while SJF minimizes waiting time. Understanding these algorithms enables system designers to optimize CPU utilization, response times, and overall system performance.