Exploring Process Scheduling
Overview
This document explores and organizes logical considerations and research on process scheduling techniques used in operating systems.
Introduction
Today, we can run multiple programs on a single computer and switch between them seamlessly. While this may seem natural, it is in fact enabled by a key function of operating systems called process scheduling. As such, process scheduling is a fundamental concept that, though often taken for granted, is essential to the functionality of modern operating systems.
Scheduling techniques differ slightly depending on the goals of the operating system, such as efficiency and responsiveness. This document does not focus on a specific scheduling method, but rather investigates the common objectives and principles that process scheduling in operating systems seeks to achieve.
What is Process Scheduling?
Process scheduling refers to the mechanism by which the operating system decides which process in the ready state should be assigned the CPU, when, and for how long.
Although operating systems vary in specific design goals, most generally aim to:
- Improve throughput
- Minimize turnaround time
- Enhance availability
- Increase reliability
To support these goals, process scheduling typically aims to:
- Maximize CPU utilization
- Increase throughput
- Minimize response time
- Optimize waiting time and turnaround time
- Ensure fairness
In essence, process scheduling determines when and how much CPU time each process should receive, based on the operating system’s performance goals.
Non-Preemptive vs. Preemptive Scheduling
One of the most critical distinctions in process scheduling is between non-preemptive and preemptive scheduling. Nearly all scheduling algorithms fall into one of these two categories, and this classification heavily influences their behavior and performance.
Non-Preemptive Scheduling
In non-preemptive scheduling, once a process is assigned the CPU, it cannot be interrupted by another process. It retains control until it either finishes execution or voluntarily yields the CPU (e.g., by making an I/O request).
Advantages:
- Low context switching overhead (longer average CPU usage time)
- Simple and easy to implement
Disadvantages:
- Hard to predict response times
- May not suit real-time systems
- Risk of starvation if a process monopolizes the CPU
Preemptive Scheduling
Preemptive scheduling allows the operating system to interrupt and replace a running process with another, typically higher-priority one. CPU ownership can be forcibly transferred at any time.
Advantages:
- More suitable for multi-user or real-time systems
- Allows high-priority tasks to be handled more promptly
Disadvantages:
- Higher context switching overhead if CPU control changes frequently
How Process Scheduling Is Implemented
Given the diversity of modern operating systems (due to varied application demands), implementation details vary. However, the core objectives remain similar across most systems.
Process scheduling is generally implemented based on the following metrics:
-
CPU Utilization
- Ratio of time CPU is actively used during operation
- Ideal utilization: typically 40%–90%+
-
Throughput
- Number of processes completed per unit time
- E.g., 5 processes completed in one second → throughput = 5 processes/sec
-
Waiting Time
- Total time a process spends in the ready queue
- Lower average waiting time implies faster system responsiveness
-
Turnaround Time
- Time from submission to completion of a process
- Formula:
T u r n a r o u n d T i m e = C o m p l e t i o n T i m e − A r r i v a l T i m e Turnaround Time = Completion Time - Arrival Time
-
Response Time
- Time from request submission until first CPU allocation
- Lower response time improves user-perceived responsiveness
-
Fairness
- Degree to which CPU time is evenly distributed among processes without starvation
-
Context Switching Overhead
- Cost incurred during process switch
- Minimizing this overhead is typically beneficial
Additional factors may apply depending on the system environment (e.g., mobile, server), but these seven metrics are universally important in process scheduling.
Conclusion
The research shows that although each operating system may implement scheduling differently, all share a common objective: to mediate between hardware and software, maximizing user experience. Accordingly, scheduling strategies are tailored to deliver the best possible experience to users under specific conditions.
Final Thoughts
While studying backend development, I took time to understand process scheduling—a foundational concept in operating systems. This gave me a new appreciation for how seamless multitasking, which we often take for granted, is underpinned by deliberate scheduling strategies that aim to enhance user experience.