Scheduling Criteria and Algorithms

In an operating system, the scheduling of processes is a crucial task for ensuring efficient utilization of the CPU and meeting the requirements of various processes. Several scheduling algorithms have been developed over the years, each with its own set of criteria and benefits. This article will explore some of the commonly used scheduling criteria and algorithms, including First-Come-First-Serve (FCFS), Shortest Job First (SJF), and Round-Robin (RR).

First-Come-First-Serve (FCFS)

The FCFS scheduling algorithm is the simplest and most intuitive approach. As the name suggests, it prioritizes the processes based on their arrival time, with the first process arriving being the first to get executed. FCFS is non-preemptive, meaning a process will continue execution until it completes or enters the blocked state.

One significant advantage of FCFS is its simplicity and fairness. However, it suffers from the "convoy effect," where a long-running process can hold up subsequent processes, resulting in higher waiting times for later arrivals. Additionally, FCFS is not suitable for time-sharing systems or scenarios where response time is critical.

Shortest Job First (SJF)

The SJF algorithm aims to minimize the waiting time by selecting the process with the shortest burst time for execution. It can be either preemptive or non-preemptive. In the preemptive version, if a process with a shorter burst time arrives while another process is executing, it preempts the executing process and starts executing the new process.

SJF can significantly reduce waiting time and improve turnaround time for processes. However, it is challenging to predict the burst time accurately, especially for long processes, which can lead to poor performance due to inaccuracies in estimating the duration.

Round-Robin (RR)

In Round-Robin scheduling, each process is assigned a fixed time slice or quantum. The processes are executed in a circular order, with each process getting a chance to execute for the designated time slice. If a process does not complete within its time slice, it is preempted and moved to the back of the queue, allowing the next process in line to execute.

RR is particularly suited for time-sharing systems as it provides fairness and prevents any single process from monopolizing the CPU for an extended period. However, it may suffer from high context switching overhead, especially with small quantum sizes.

Other Scheduling Algorithms

Apart from FCFS, SJF, and RR, there are several other scheduling algorithms used in various operating systems. Some of them include:

• Priority Scheduling: Assigns priorities to processes and executes higher-priority processes first.
• Multi-Level Queue Scheduling: Divides processes into multiple queues based on priority or other criteria and executes them in a predefined order.
• Shortest Remaining Time: A dynamic version of SJF where processes with the shortest remaining time are given preference.
• Lottery Scheduling: Assigns tickets to processes, and a random lottery determines the winner for execution.

These algorithms have their own advantages and limitations, making them suitable for different scenarios based on factors like system workload, responsiveness, fairness requirements, and resource utilization.

Conclusion

Scheduling algorithms play a crucial role in the efficient management of processes in an operating system. Different criteria and algorithms are used to prioritize and assign CPU time to processes. While FCFS is simple and fair, SJF reduces waiting time, and RR ensures fair time-sharing. Other algorithms like priority scheduling, multi-level queue scheduling, shortest remaining time, and lottery scheduling offer additional variations to cater to specific system requirements. Choosing the most suitable algorithm depends on understanding the system's needs and optimizing various factors like turnaround time, response time, fairness, and resource utilization.