LOOK and C-LOOK variants in Operating Systems - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding the LOOK Disk Scheduling Algorithm

Which of the following best describes how the LOOK disk scheduling algorithm operates?

A The disk arm moves to the end of the disk in one direction, servicing requests along the way, then reverses direction to service remaining requests.

B The disk arm moves in one direction servicing requests until it reaches the last request in that direction, then reverses direction without going to the end of the disk.

C The disk arm moves in a circular manner, always moving in one direction and jumping back to the start after reaching the last request.

D The disk arm randomly selects the next request to service regardless of its position.

💡 Hint

Think about how LOOK differs from SCAN by not going all the way to the disk's end.

LOOK moves the disk arm only as far as the last request in the current direction before reversing, unlike SCAN which goes to the disk's end.

🧠 Conceptual

intermediate

Key Difference Between LOOK and C-LOOK

What is the main difference between the LOOK and C-LOOK disk scheduling algorithms?

A LOOK reverses direction after reaching the last request in one direction; C-LOOK jumps back to the first request without reversing direction.

B LOOK always moves to the disk's end; C-LOOK moves only to the last request in the current direction.

C LOOK services requests randomly; C-LOOK services requests in order of arrival.

D LOOK uses a circular queue; C-LOOK uses a linear queue.

💡 Hint

Consider how the arm moves after reaching the last request in the current direction.

LOOK reverses direction after servicing the last request in one direction, while C-LOOK jumps back to the first request at the opposite end and continues in the same direction.

🔍 Analysis

advanced

Analyzing Disk Arm Movement in C-LOOK

Given the disk requests at tracks [10, 22, 20, 2, 40, 6] and the disk arm starting at track 15 moving towards higher track numbers, what is the order in which C-LOOK services the requests?

A 20, 22, 40, 10, 6, 2

B 22, 20, 10, 6, 2, 40

C 20, 22, 40, 2, 6, 10

D 10, 6, 2, 20, 22, 40

💡 Hint

Remember that C-LOOK moves in one direction and jumps back to the lowest request after reaching the highest.

The arm moves from 15 upwards servicing 20, 22, 40, then jumps back to 2 and services 2, 6, 10 in order.

❓ Comparison

advanced

Comparing Total Head Movement: LOOK vs C-LOOK

For the request queue [5, 8, 12, 20, 25] and the disk arm starting at track 10 moving towards higher tracks, which algorithm results in less total head movement?

A Neither algorithm is efficient; FCFS would result in less head movement.

B C-LOOK results in less total head movement because it jumps back to the lowest request without reversing.

C Both LOOK and C-LOOK result in the same total head movement for this request queue.

D LOOK results in less total head movement because it reverses direction after the last request in one direction.

💡 Hint

Calculate the total distance the arm moves in each algorithm.

LOOK moves from 10 to 25 then back to 5, total movement is less than C-LOOK which jumps back causing more movement.

❓ Reasoning

expert

Choosing Between LOOK and C-LOOK for Real-Time Systems

In a real-time system where minimizing the maximum wait time for any disk request is critical, which disk scheduling algorithm is more suitable and why?

A C-LOOK is more suitable because it provides a more uniform wait time by servicing requests in a single direction and jumping back quickly.

B LOOK is more suitable because reversing direction reduces the maximum wait time for requests at the opposite end.

C Neither LOOK nor C-LOOK is suitable; FCFS is better for real-time systems.

D Both LOOK and C-LOOK are equally suitable as they have similar wait times.

💡 Hint

Consider how the arm movement pattern affects wait times for requests at different positions.

C-LOOK moves in one direction only and jumps back, which helps keep wait times more uniform and predictable, important for real-time systems.