Scheduling in Operating Systems Overview

Scheduling in Operating Systems

Introduction

Task scheduling is a crucial aspect of any operating system. It plays a vital role in efficiently allocating CPU time to multiple processes running on a system. This process ensures that the CPU is utilized effectively, no process monopolizes the CPU, and tasks are completed in a timely manner. In this article, we will discuss the types of schedulers, CPU scheduling, process scheduling, scheduling algorithms, and real-time scheduling.

Types of Schedulers

Schedulers in operating systems can be categorized into three types: long-term, medium-term, and short-term schedulers.

1. Long-term Scheduler (Job Scheduler): This scheduler determines which processes from the job queue, a pool of all processes waiting to be executed, should be loaded into memory. Its goal is to maximize CPU utilization and system throughput while maintaining a balance between I/O-bound and CPU-bound processes.

2. Medium-term Scheduler: In some systems, especially those with limited memory, a medium-term scheduler exists to swap processes in and out of memory. When a process is moved out of memory, it's placed in a suspended state until it can be reloaded, reducing the pressure on physical memory.

3. Short-term Scheduler (CPU Scheduler): The short-term scheduler is the one most people think of when discussing scheduling. It decides which process in the ready queue, the set of processes that are currently in memory and ready to execute, should run next. The goal here is to minimize response time, maximize CPU utilization, and ensure fairness.

CPU Scheduling

CPU scheduling is the process of deciding which process will own the CPU to use while another process is suspended. The main function of the CPU scheduling is to ensure that whenever the CPU remains idle, the OS has at least selected one of the processes available in the ready-to-use line.

Process Scheduling

Process scheduling is the act of selecting and assigning the CPU to a particular process from the pool of available processes. In a multitasking environment, multiple processes compete for CPU time, and the operating system must decide which process to execute next.

Scheduling Algorithms

Operating systems employ different scheduling algorithms, each with its own set of rules and priorities. Some common scheduling algorithms include:

1. First-Come, First-Served (FCFS): A process is executed in the order it arrives in the ready queue.

2. Shortest Job Next (SJN) / Shortest Job First (SJF): The process with the shortest job is executed next.

3. Round Robin (RR): Each process is cyclically assigned a fixed time slot. It is the preemptive version of First come First Serve CPU Scheduling.

4. Priority Scheduling: Processes are assigned different priorities, and the process with the highest priority is executed next.

5. Multilevel Queue Scheduling: Multiple queues are used, each with a different priority.

6. Multilevel Feedback Queue Scheduling: A combination of multilevel queue scheduling and priority scheduling, where processes can move between queues based on their behavior.

Real-time Scheduling

Real-time scheduling is a scheduling technique used in real-time systems. These systems require precise and timely execution of tasks, such as those found in control systems or multimedia applications.

Priority Scheduling

Priority scheduling is a scheduling algorithm in which processes are assigned different priorities, and the process with the highest priority is executed next.

In conclusion, scheduling in operating systems is a complex and critical task that impacts the performance and user experience of computer systems. It involves a delicate balance between optimizing CPU utilization, ensuring fairness among processes, and adapting to changing workloads. Operating systems employ various scheduling algorithms and policies to meet these objectives, and the choice of which one to use depends on the specific requirements of the system.