Case Studies and GATE-Level Numericals – Operating Systems

Section 1: Case Studies (Real-World Applications)

These examples help students understand how OS concepts are applied in real-life systems.

Case Study 1: Linux Process Scheduling

Scenario:

The Linux OS uses a Completely Fair Scheduler (CFS). Instead of assigning strict time slices, it uses a red-black tree to ensure fairness across tasks.

Learning Insight:

• Unlike Round Robin, Linux prioritizes tasks based on how long they’ve waited.
• CFS dynamically adjusts process priorities for interactive responsiveness.

Case Study 2: Windows Virtual Memory Management

Scenario:

Windows uses demand paging and a working set model to manage memory. It adjusts the number of pages allocated to each process depending on recent usage.

Learning Insight:

• The working set model avoids thrashing.
• Page faults are minimized by prefetching and page-replacement algorithms (modified Clock algorithm).

Case Study 3: Android Low Memory Killer (LMK)

Scenario:

On Android devices, when memory becomes scarce, the LMK framework kills background apps to free up space.

Learning Insight:

• Real-time memory management based on app priority (foreground, visible, hidden).
• Reflects importance of resource prioritization in mobile OS.

Case Study 4: Real-Time OS in Automotives

Scenario:

Embedded OS like VxWorks or QNX run in cars’ ECU (Electronic Control Unit) systems to manage sensors and brakes.

Learning Insight:

• Hard real-time constraints.
• Scheduling algorithms like Rate Monotonic or EDF (Earliest Deadline First) are commonly used.

Section 2: GATE-Level Numericals (With Solutions)

These problems are structured to match GATE’s level of difficulty and format, ideal for advanced learners.

Problem 1: Turnaround Time and Waiting Time

Question:

Given 3 processes with burst times 4, 3, and 2 respectively. Assume arrival time is 0 for all and FCFS is used.

Solution:

Average Waiting Time: (0+4+7)/3 = 3.67

Problem 2: Page Replacement – LRU

Question:

Page reference string: 7, 0, 1, 2, 0, 3, 0, 4.
Frame size = 3. How many page faults using LRU?

Solution:

Step-by-step replacement:

• Faults at: 7, 0, 1, 2, 3, 4 → 6 faults

Problem 3: Turnaround and Waiting Time (SJF Non-Preemptive)

Burst Times: P1=6, P2=8, P3=7, P4=3
Order Executed: P4 → P1 → P3 → P2

Avg Waiting Time: 7

Problem 4: Page Faults using FIFO

Reference String: 1, 2, 3, 4, 1, 2, 5, 1, 2, 3, 4, 5
Frame Size: 3
Answer: 9 page faults (illustrated using a sliding window FIFO queue)

Problem 5: Banker's Algorithm – Safe State Check

Given:

• Processes = 5, Resources = 3 types
• Allocation, Max, Available matrices given.
  Task:
  Apply Banker’s Algorithm to check if system is in a safe state and list safe sequence.

Concepts Covered:

• Need Matrix = Max – Allocation
• Work vector, Finish array
• Step-by-step safe sequence validation