Process Scheduling in Operating Systems: Algorithms and Techniques for Efficient CPU Utilization

Explore the complexities of process scheduling in operating systems. This guide explains different scheduler types (long-term, short-term, medium-term), various scheduling algorithms, and their impact on system performance, response times, and resource utilization.

Process Scheduling in Operating Systems

Introduction to Process Scheduling

In an operating system, process scheduling is the activity of selecting a process from the ready queue (a list of processes ready to run) and assigning it to the CPU for execution. The scheduler aims to utilize the CPU effectively and provide reasonable response times to various processes. The choice of scheduling algorithm significantly impacts overall system performance and responsiveness. There are three main types of schedulers in an operating system.

Types of Process Schedulers

1. Long-Term Scheduler (Job Scheduler)

The long-term scheduler selects processes from a pool of processes (usually on secondary storage) and moves them into the ready queue in main memory. This scheduler's main task is to control the degree of multiprogramming (the number of processes in the ready queue). The long-term scheduler is responsible for selecting processes which provide the best mix of CPU-bound and I/O-bound processes. If too many I/O-bound processes are selected, the CPU might spend too much time idle while it waits for I/O operations to complete, thereby reducing the degree of multiprogramming.

2. Short-Term Scheduler (CPU Scheduler)

The short-term scheduler (CPU scheduler) selects a process from the ready queue and assigns it to the CPU for execution. This is done using a scheduling algorithm (like Round Robin, Shortest Job First, etc.). The CPU scheduler's choices have a direct and immediate impact on system performance because a poorly chosen scheduler can lead to situations where some processes wait for a very long time, especially if it selects a long-running process.

3. Medium-Term Scheduler

The medium-term scheduler manages processes that have been swapped out of main memory. When a process needs I/O, it's moved to a waiting state. The medium-term scheduler might swap out some waiting processes to free up space in main memory for other processes, making more memory available to other processes. This reduces the degree of multiprogramming. Swapping processes helps to maintain a more efficient mix of processes in the ready queue to improve the system’s overall performance.