Scheduling Algorithms in Operating System

One of the most important jobs of any operating system is to decide how and when processes get to use the CPU. For this task, operating systems use various types of scheduling algorithms. A scheduling algorithm decides the order in which processes will be given access to the CPU, with the goal of increasing system efficiency, using resources correctly, and allowing processes to run with minimal waiting time.

Scheduling algorithms are a critical part of any OS. Different algorithms work better in different situations. For example, FCFS is simple but not suitable for long processes, while SJF reduces turnaround time but is hard to implement. Round Robin is good for quick user responses, but setting the right time quantum is important. Priority scheduling gives preference to important processes but can lead to starvation. The right algorithm is chosen based on the system's needs, the nature of the processes, and user priorities.

Scheduling algorithms can be divided into two main categories:

In this article, we will understand various types of scheduling algorithms in detail, their benefits, limitations, and in which situation each algorithm is suitable.

Introduction: FCFS is the simplest scheduling algorithm. Processes are executed in the same order they arrive in the system. This means the process that comes first gets the CPU first.

Benefits: Simple and easy to understand. No complexity and its implementation is straightforward.

Drawbacks: Short processes might have to wait a long time because of long processes, which is known as the Convoy Effect. The average waiting time can be high.

Suitability: It is useful when the execution time of processes is similar, but less effective when there is a big difference in execution times.

Introduction: The SJF algorithm allocates the CPU to the process that has the shortest execution time. This means the smallest process runs first.

Benefits: Reduces average waiting time and turnaround time.

Drawbacks: It is difficult to implement because it has to estimate the correct execution time of a process. Long processes might wait for a very long time, leading to starvation.

Suitability: It is more effective when the execution times of all processes are known and there isn't much difference in their times.

Introduction: The Round Robin algorithm is especially used in time-sharing systems. In this, each process is allocated the CPU for a fixed amount of time (called a time quantum). If the process is not finished by the end of that time, it is sent to the back of the queue, and the CPU is given to the next process.

Benefits: All processes get a fair share of the CPU time. The average response time is low, so the user gets a quick response.

Drawbacks: If the time quantum is too small, the frequency of context switching increases, which can increase overhead. If the time quantum is too large, it starts to work like FCFS.

Suitability: In multi-user systems where it is necessary to give equal time to all processes.

Introduction: In this algorithm, each process is given a priority, and the CPU is allocated to the process with the highest priority. Priority scheduling can be implemented as both preemptive and non-preemptive.

Benefits: Important processes are executed quickly.

Drawbacks: Low-priority processes might suffer from starvation, causing them to wait for a very long time. This problem can be solved through aging, where the priority of low-priority processes is increased over time.

Suitability: It is suitable when some processes need to be executed quickly to perform more important tasks.

Introduction: In this algorithm, processes are divided into different queues based on their type, and a separate scheduling algorithm is used for each queue. For example, there can be separate queues for interactive and batch processes.

Benefits: Can handle different types of processes better.

Drawbacks: It can be complex and might put more load on the system.

Suitability: It is used when there are different types of processes in the system with different scheduling requirements.