LOOK and C-LOOK variants in Operating Systems - Mini Project: Build & Apply

Understanding LOOK and C-LOOK Disk Scheduling Algorithms

📖 Scenario: You are working as a system administrator managing disk operations. You want to understand how the LOOK and C-LOOK disk scheduling algorithms work to optimize disk head movement and reduce waiting time.

🎯 Goal: Build a step-by-step explanation and example of how the LOOK and C-LOOK algorithms schedule disk requests, showing the order in which disk requests are serviced.

📋 What You'll Learn

Create a list of disk requests with exact values

Set the initial position of the disk head

Explain the LOOK algorithm by ordering requests based on head movement

Explain the C-LOOK algorithm by ordering requests with circular scanning

💡 Why This Matters

🌍 Real World

Disk scheduling algorithms like LOOK and C-LOOK help operating systems manage how the disk head moves to read or write data efficiently, reducing wait times and improving performance.

💼 Career

Understanding these algorithms is important for roles in system administration, operating system development, and performance optimization.

1

DATA SETUP: Create the list of disk requests

Create a list called disk_requests with these exact integer values: 95, 180, 34, 119, 11, 123, 62, 64.

Operating Systems

# Create the list of disk requests
# Your code here

Hint

Use square brackets [] to create a list and separate numbers with commas.

2

CONFIGURATION: Set the initial disk head position

Create a variable called head_position and set it to the integer 50.

Operating Systems

disk_requests = [95, 180, 34, 119, 11, 123, 62, 64]
# Set the initial disk head position
# Your code here

Hint

Assign the number 50 directly to the variable head_position.

3

CORE LOGIC: Explain the LOOK algorithm order

Create a list called look_order that shows the order in which the disk head services requests using the LOOK algorithm starting from head_position. The head moves towards higher requests first, then reverses direction to service lower requests. Use the exact variable names disk_requests and head_position.

Operating Systems

disk_requests = [95, 180, 34, 119, 11, 123, 62, 64]
head_position = 50
# Create the LOOK algorithm order list
# Your code here

Hint

First service requests greater than 50 in ascending order, then service requests less than 50 in descending order.

4

COMPLETION: Explain the C-LOOK algorithm order

Create a list called clook_order that shows the order in which the disk head services requests using the C-LOOK algorithm starting from head_position. The head moves towards higher requests first, then jumps back to the lowest request without servicing in reverse order. Use the exact variable names disk_requests and head_position.

Operating Systems

disk_requests = [95, 180, 34, 119, 11, 123, 62, 64]
head_position = 50
look_order = [62, 64, 95, 119, 123, 180, 34, 11]
# Create the C-LOOK algorithm order list
# Your code here

Hint

Service requests greater than 50 in ascending order, then jump to the lowest requests and service them in ascending order.

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