What is a process scheduler in OS?

A process scheduler is a core component of the Operating System that manages the allocation of CPU time among multiple processes. It determines which process should run next when the CPU becomes available and ensures efficient utilization of system resources by preventing the CPU from remaining idle.

The scheduler continuously monitors processes in different states (ready, waiting, running) and makes intelligent decisions about CPU allocation. When a process completes its execution or enters a waiting state, the scheduler immediately assigns the CPU to another ready process, maintaining optimal system performance.

How Process Scheduling Works

Characteristics of Good Process Scheduler

An effective process scheduler exhibits the following characteristics −

• High CPU Utilization − Keeps the CPU busy by continuously assigning ready processes, minimizing idle time.

• Flexibility − Allows dynamic adjustment of scheduling policies based on system requirements.

• Fairness − Ensures all processes receive appropriate CPU time without starvation.

• Deadline Management − Supports real-time constraints and time-critical applications.

• Process Interdependency Handling − Manages relationships and dependencies between related processes.

Types of Scheduling

Process scheduling is categorized into two main types based on how the CPU is allocated −

Preemptive Scheduling

In preemptive scheduling, the scheduler can forcibly remove a currently running process from the CPU to allocate it to a higher-priority process. The interrupted process is moved back to the ready queue and resumes execution later when the CPU becomes available.

Key Features:

• Processes are assigned CPU for fixed time slices

• Higher-priority processes can interrupt lower-priority ones

• Provides better response time for interactive systems

• Requires timer hardware for implementation

Non-Preemptive Scheduling

In non-preemptive scheduling, once a process is allocated the CPU, it retains control until it voluntarily releases the CPU by completing execution, entering a waiting state, or terminating.

Key Features:

• No forced context switching during execution

• Simpler implementation without special hardware requirements

• Lower overhead due to fewer context switches

• May lead to poor response time for shorter processes

Comparison

Conclusion

The process scheduler is essential for efficient multitasking in operating systems, managing CPU allocation among competing processes. The choice between preemptive and non-preemptive scheduling depends on system requirements, with preemptive scheduling offering better responsiveness and non-preemptive scheduling providing simplicity and lower overhead.