Instructor: Neerja Mhaskar

Course: COMP SCI 3SH3 – Operating Systems

Based on: Chapter 5 of the textbook Operating Systems Concepts

Overview

This chapter focuses on CPU Scheduling, an essential component of operating system design, responsible for managing the system's most critical resource: the CPU. Effective scheduling optimizes CPU utilization, turnaround time, waiting time, and overall system throughput.

1. Basic Concepts

• Resource Scheduling:
  • The operating system schedules almost all resources in the system, but CPU scheduling is central to its design.
  • Goal: Maximize CPU utilization since processes alternate between CPU usage and I/O waiting.
• Process Execution Cycle:
  • A process alternates between CPU bursts (execution on the CPU) and I/O bursts (waiting for I/O).
  • I/O-bound programs: Many short CPU bursts.
  • CPU-bound programs: Few long CPU bursts.
  • The balance of I/O-bound and CPU-bound processes is key in selecting the appropriate CPU scheduling algorithm.

2. Process Scheduling

• The operating system maintains several scheduling queues:
  • Job queue: Contains all processes in the system.
  • Ready queue: Holds processes that reside in memory and are ready to execute.
  • Device queues: Contains processes waiting for I/O devices. Each device typically has its own queue.
• Process Scheduler: Selects processes from these queues based on the selected scheduling algorithm.

3. Ready Queue and I/O Device Queues

• Queues are a central part of the process scheduling system.
• A queueing diagram can be used to visually represent the movement of processes between these queues.

4. CPU Scheduler