CPU Scheduling Times in OS

Introduction

Imagine you’re at a bank trying to deposit money. Your experience mirrors how an operating system (OS) schedules processes on the CPU. Just like you wait in line, processes wait in a queue, and the OS manages their execution time.

Let’s break down the critical times in CPU scheduling using this bank analogy to solve numerical problems effortlessly.

Key CPU Scheduling Times

1. Arrival Time

• What it is: The time when a process enters the ready queue.
• Bank Analogy: You walk into the bank at 11:00 AM—this is your arrival time.
• Technical Definition: The point in time when a process becomes eligible for CPU execution.

2. Burst Time

• What it is: The time a process actually needs to execute on the CPU.
• Bank Analogy: Depositing money takes 15 minutes (the clerk’s active work time).
• Formula: \(Burst Time = Actual CPU time needed\)
• Note: Burst time is a duration, not a point in time.

3. Completion Time

• What it is: The time when a process finishes execution.
• Bank Analogy: You leave the bank at 12:00 PM—this is your completion time.
• Technical Definition: The point in time when a process exits the system.

4. Turnaround Time

• What it is: The total time a process spends in the system.
• Formula: \( Turnaround Time = Completion Time - Arrival Time \)
• Bank Analogy:
  • Arrival: 11:00 AM
  • Completion: 12:00 PM
  • Turnaround Time = 1 hour (60 minutes)

5. Waiting Time

• What it is: The time a process spends waiting (not executing).
• Formula: \( Waiting Time = Turnaround Time - Burst Time \)
• Bank Analogy:
  • Turnaround Time: 60 minutes
  • Burst Time: 15 minutes
  • Waiting Time = 45 minutes (time spent in line, filling forms, or waiting)

6. Response Time

• What it is: The time until a process gets the CPU for the first time.
• Formula: \( Response Time = First CPU Access Time - Arrival Time \)
• Bank Analogy:
  • Arrival: 11:00 AM
  • First interaction with clerk: 11:30 AM
  • Response Time = 30 minutes

Common Mistakes to Avoid

• Confusing Duration vs. Point in Time:
  • Burst Time is a duration (e.g., 15 mins).
  • Arrival/Completion Time are points in time (e.g., 11:00 AM).
• Waiting Time ≠ Idle Time:
  • Waiting Time includes delays due to queues, I/O operations, or interruptions.

Why These Times Matter

Understanding these times helps you:

• Solve CPU scheduling numericals (common in exams like GATE).
• Compare algorithms like FCFS, SJF, and Round Robin.
• Optimize OS performance (e.g., reducing waiting time).

Example

• Scenario: Editing a video while downloading a file.
• CPU-bound process (video editing): Requires long CPU bursts.
• I/O-bound process (file download): Frequently waits for network input.
• The OS uses scheduling times to balance their execution.

Summary Table

FAQ

• Q: Why does waiting time occur?

  • A: Processes wait due to queues, I/O operations, or higher-priority tasks.

• Q: Is burst time fixed?

  • A: No! It depends on the process’s requirements (e.g., complex code = longer burst time).

• Q: Can response time be zero?

  • A: Only if a process gets the CPU immediately (rare in busy systems).