Introduction

Process scheduling is one of the core functions of an operating system (OS) that directly impacts system performance and user experience. A process scheduler decides the order in which processes will be executed by the CPU, managing how efficiently the CPU allocates its time among competing tasks. Understanding how process schedulers work is crucial for both system administrators and software developers, especially those preparing for competitive exams like GATE and UGC NET in Computer Science.

This article delves into the concepts of process scheduling, its types, the different types of schedulers, and their roles within an operating system. Additionally, we’ll explore how modern operating systems like Windows, UNIX/Linux, and macOS implement these concepts, providing insights that can be beneficial for your exams and real-world applications.

What is Process Scheduling?

Process scheduling is the activity of the process manager that handles the removal of the running process from the CPU and the selection of another process based on a particular strategy. It is an integral part of a multiprogramming operating system, where more than one process is loaded into executable memory, and these processes share the CPU using time multiplexing.

Key Objectives of Process Scheduling:

• Maximizing CPU Utilization: Ensure that the CPU is always busy executing tasks and not sitting idle.
• Minimizing Turnaround Time: Reduce the time taken for a process to complete from submission to termination.
• Fair Allocation of Resources: Ensure all processes get fair access to CPU resources, avoiding starvation.
• Maximizing Throughput: Increase the number of processes completed per unit time.

Types of Process Schedulers

Process schedulers are categorized based on their roles in managing the life cycle of a process:

• Long-Term Scheduler (Job Scheduler)
• Short-Term Scheduler (CPU Scheduler)
• Medium-Term Scheduler

1. Long-Term Scheduler (Job Scheduler)

The long-term scheduler is responsible for controlling the degree of multiprogramming by selecting processes from the job pool and loading them into the ready queue. It operates at a high level and primarily focuses on balancing I/O-bound and CPU-bound processes.

• Function: Brings new processes to the ‘Ready’ state.
• Role: Determines which processes are worthy of being executed based on their priority and type (I/O-bound vs. CPU-bound).
• Example: In batch processing systems, the job scheduler decides which batch jobs will be brought into the system for execution.

Real-Life Application: In Windows, the Task Scheduler can be seen as an example of a job scheduler that allows users to automate tasks, such as starting programs or displaying messages, at predefined times.

2. Short-Term Scheduler (CPU Scheduler)

The short-term scheduler, often referred to as the CPU scheduler, selects one of the processes from the ready queue and allocates the CPU to it.

• Function: Manages process execution by selecting processes from the ready queue to be executed next by the CPU.
• Role: Ensures efficient execution of processes, implementing algorithms like First-Come, First-Served (FCFS), Shortest Job Next (SJN), Priority Scheduling, and Round Robin.
• Example: In time-sharing systems like UNIX, the short-term scheduler frequently switches between processes to ensure all active users get a share of the CPU.

Real-Life Application: In Linux, the Completely Fair Scheduler (CFS) is an advanced CPU scheduler that dynamically adjusts process priorities to ensure fair access to CPU time.

3. Medium-Term Scheduler

The medium-term scheduler is responsible for swapping processes in and out of the main memory to balance the load on the CPU and the system’s available memory.

• Function: Suspends and resumes processes, effectively managing the degree of multiprogramming.
• Role: Handles swapping, where processes are moved between the main memory and secondary storage to free up memory for active processes.
• Example: In a system running low on memory, the medium-term scheduler may swap out a process that is waiting for I/O to free up space for another process in the ready queue.

Real-Life Application: In UNIX systems, swapping is often used in combination with paging to manage memory more effectively, ensuring that the most critical processes stay in memory while others are temporarily moved to disk.

Other Types of Schedulers

I/O Schedulers:

Role: Manages the execution of I/O operations such as reading and writing to disks or networks. Algorithms like FCFS (First-Come, First-Served) or SSTF (Shortest Seek Time First) are used to decide the order of I/O operations.

Real-Time Schedulers:

Role: Ensures that critical tasks in real-time systems are completed within a specified deadline. Algorithms such as EDF (Earliest Deadline First) and RM (Rate Monotonic) are commonly used in real-time scheduling.

Comparison Among Schedulers

Two-State Process Model

In a basic two-state process model, processes can either be in a running state or a not running state.

• Running: The process is currently being executed by the CPU.
• Not Running: The process is not currently executing and is in the ready or waiting queue.

Context Switching

Context switching is the process where the state of a currently running process is saved so that it can be resumed later, and another process is loaded onto the CPU. This is essential in multitasking operating systems to ensure that multiple processes can share a single CPU effectively.

Mechanism: The current state of the CPU, including the program counter, registers, and other process-specific data, is saved into the Process Control Block (PCB). When the process resumes, this state is restored from the PCB.

Real-Life Application: In modern operating systems like Linux, context switching is optimized to minimize overhead, ensuring smooth multitasking even when multiple processes compete for CPU time.

Practical Examples in Operating Systems

Windows:

• Long-Term Scheduling: Task Scheduler for automating tasks.
• Short-Term Scheduling: Preemptive multitasking using a priority-based scheduling algorithm.
• Medium-Term Scheduling: Virtual memory management using swapping and paging.

UNIX/Linux:

• Long-Term Scheduling: Batch job scheduling in enterprise environments.
• Short-Term Scheduling: Completely Fair Scheduler (CFS) for managing CPU time.
• Medium-Term Scheduling: Swapping combined with paging for memory management.

macOS:

• Long-Term Scheduling: Managed through system processes like launchd.
• Short-Term Scheduling: Hybrid of preemptive multitasking and cooperative multitasking.
• Medium-Term Scheduling: Efficient memory management using swapping and paging.

Conclusion

Process schedulers are critical components of any operating system, ensuring that processes are executed in a manner that optimizes system performance, maximizes CPU utilization, and maintains system stability. By understanding the various types of schedulers and their roles, you can better appreciate how operating systems manage tasks, which is essential knowledge for GATE and UGC NET exam preparation.

Frequently Asked Questions (FAQs)

1. What is CPU scheduling in an OS?

CPU scheduling refers to the technique that allows one process to utilize the CPU while others are either waiting or are put on hold.

2. What is Inter-Process Communication (IPC)?

IPC is a mechanism that allows processes to communicate with each other and synchronize their actions.

3. What is PCB in an OS?

The Process Control Block (PCB) is a data structure used by the operating system to store all the information about a process.