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.

Practice

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

easy

A. LOOK moves only in one direction, while C-LOOK moves back and forth.

B. LOOK uses a queue, while C-LOOK uses a stack.

C. LOOK schedules requests randomly, while C-LOOK schedules them sequentially.

D. LOOK moves the disk head back and forth, while C-LOOK moves in one direction and jumps back.

Solution

1. Step 1: Understand LOOK algorithm movement

   LOOK moves the disk head back and forth servicing requests only as far as needed in each direction.

2. Step 2: Understand C-LOOK algorithm movement

   C-LOOK moves the disk head in one direction only and jumps back to the start without servicing requests on the return.

3. Final Answer:

   LOOK moves the disk head back and forth, while C-LOOK moves in one direction and jumps back. -> Option D

4. Quick Check:

   LOOK vs C-LOOK movement = back/forth vs one-way jump [OK]

Hint: LOOK moves back/forth; C-LOOK jumps back after one-way move [OK]

Common Mistakes:

• Confusing direction of head movement
• Thinking C-LOOK moves back and forth
• Mixing up scheduling order

2. Which of the following is the correct way to describe the C-LOOK algorithm's disk head movement?

easy

A. The head moves back and forth servicing all requests.

B. The head moves in one direction and then jumps back to the first request.

C. The head moves randomly to service requests.

D. The head moves in a circular queue servicing requests in order.

Solution

1. Step 1: Identify C-LOOK movement pattern

   C-LOOK moves the disk head in one direction only, servicing requests along the way.

2. Step 2: Understand jump back behavior

   After reaching the last request in that direction, the head jumps back to the first request without servicing on the return.

3. Final Answer:

   The head moves in one direction and then jumps back to the first request. -> Option B

4. Quick Check:

   C-LOOK = one-way move + jump back [OK]

Hint: C-LOOK moves one way, then jumps back to start [OK]

Common Mistakes:

• Assuming head moves back and forth
• Confusing C-LOOK with LOOK
• Thinking head services requests on return jump

3. Given the disk requests at positions [10, 22, 20, 2, 40, 6] and the disk head starting at position 15 moving towards higher numbers, what is the order of servicing requests using the LOOK algorithm?

medium

A. [20, 22, 40, 10, 6, 2]

B. [20, 22, 40, 6, 2, 10]

C. [20, 22, 40, 6, 2]

D. [20, 22, 40, 10, 2, 6]

Solution

1. Step 1: Move head towards higher requests

   Starting at 15, LOOK moves up servicing 20, 22, and 40.

2. Step 2: Reverse direction and service lower requests

   After 40, head moves back down servicing 10, 6, and 2. But LOOK only goes as far as the last request in each direction. Since 10 is before 15, it will be serviced on the return.

3. Step 3: Verify options

   [20, 22, 40, 6, 2] lists only requests serviced in one direction (20, 22, 40) without the return. LOOK services both directions, so [20, 22, 40, 6, 2] is incomplete. [20, 22, 40, 10, 6, 2] includes 10, 6, 2 after 40, which matches LOOK behavior.

4. Final Answer:

   [20, 22, 40, 10, 6, 2] -> Option A

5. Quick Check:

   LOOK = up then down servicing all requests [OK]

Hint: LOOK moves up then down servicing requests in both directions [OK]

Common Mistakes:

• Ignoring requests on return path
• Assuming C-LOOK behavior
• Ordering requests incorrectly

4. Identify the error in this description of the C-LOOK algorithm: "The disk head moves from the lowest to the highest request, then moves back servicing requests on the return path."

medium

A. C-LOOK does not move back to service requests; it jumps back without servicing.

B. C-LOOK services requests randomly, not in order.

C. C-LOOK moves back and forth like LOOK, so the description is correct.

D. C-LOOK only services the lowest request and stops.

Solution

1. Step 1: Understand C-LOOK movement

   C-LOOK moves the head in one direction servicing requests from lowest to highest.

2. Step 2: Check return path behavior

   Unlike LOOK, C-LOOK does not service requests on the return jump; it jumps back to the start without servicing.

3. Final Answer:

   C-LOOK does not move back to service requests; it jumps back without servicing. -> Option A

4. Quick Check:

   C-LOOK jump back skips servicing requests [OK]

Hint: C-LOOK jump back skips servicing requests [OK]

Common Mistakes:

• Assuming servicing on return jump
• Confusing C-LOOK with LOOK
• Misunderstanding head movement direction

5. A disk has requests at positions [5, 15, 25, 35, 45] and the head is currently at position 20 moving towards higher numbers. Using the C-LOOK algorithm, what is the total distance the disk head will move to service all requests?

hard

A. 50

B. 40

C. 35

D. 60

Solution

1. Step 1: Service requests moving up from 20

   The head moves from 20 to 25, then 35, then 45. Distance = 45 - 20 = 25.

2. Step 2: Jump back to lowest request and service upwards

   After 45, the head jumps back to 5 (distance = 45 - 5 = 40, but jump is not counted as movement in C-LOOK). Then it services 5 and 15 moving up. Distance from 5 to 15 = 10.

3. Step 3: Calculate total movement

   Total movement = 25 (upwards) + 10 (from 5 to 15) = 35. But jump back is considered instantaneous (no movement counted). So total movement is 25 + 10 = 35.

4. Step 4: Re-examine options

   Total movement of 35 matches 35.

5. Final Answer:

   35 -> Option C

6. Quick Check:

   C-LOOK counts only servicing moves, jump back ignored [OK]

Hint: Count only servicing moves; ignore jump back distance [OK]

Common Mistakes:

• Counting jump back as movement
• Ignoring requests before head position
• Adding distances incorrectly