Process Queues in Operating Systems: Managing Process States

Learn how operating systems use process queues to manage the different states of a process. Understand the types of process queues, how they organize Process Control Blocks (PCBs), and their role in process scheduling and resource allocation.

Process Queues

Introduction

The operating system (OS) manages several types of queues corresponding to the different states of a process. Each queue contains the Process Control Blocks (PCBs) of processes in a specific state. When a process transitions from one state to another, its PCB is removed from the current state queue and added to the queue of the new state.

Types of Process Queues

The OS maintains the following queues to manage processes:

1. Job Queue

  • All newly created processes are initially stored in the Job Queue.
  • It is maintained in the secondary memory (e.g., hard disk).
  • The long-term scheduler (Job Scheduler) selects some jobs from this queue and loads them into primary memory for execution.

2. Ready Queue

  • The Ready Queue resides in primary memory and contains processes ready for execution.
  • The short-term scheduler (CPU Scheduler) selects processes from the ready queue and dispatches them to the CPU for execution.

3. Waiting Queue

  • Processes requiring input/output (I/O) operations are moved to the Waiting Queue.
  • When a process transitions from the running state to the waiting state (e.g., waiting for an I/O operation), its PCB is added to this queue.
  • Once the I/O operation is complete, the process is moved back to the ready queue for execution.

Queue Management in Process Lifecycle

The movement of processes between these queues is managed by the OS schedulers:

  • The long-term scheduler controls the job queue, determining which processes are admitted into the system for processing.
  • The short-term scheduler selects processes from the ready queue for CPU execution.
  • The I/O scheduler manages processes in the waiting queue, ensuring smooth handling of I/O operations.

Process Lifecycle Overview

A process typically moves through the following states and queues:

  1. New: Process is created and added to the job queue.
  2. Ready: Process is loaded into primary memory and waits in the ready queue.
  3. Running: Process is dispatched by the short-term scheduler to the CPU for execution.
  4. Waiting: If the process requires I/O, it is moved to the waiting queue.
  5. Terminated: After execution, the process is removed from the system.

Summary

The efficient management of process queues by the OS ensures optimal use of system resources, balanced scheduling, and smooth process transitions. Each type of queue plays a critical role in handling the lifecycle and scheduling of processes.