LOOK and C-LOOK variants in Operating Systems - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the main purpose of the LOOK disk scheduling algorithm.

Operating Systems

The LOOK algorithm moves the disk arm towards the last request in one direction and then [1].

Drag options to blanks, or click blank then click option'

Aignores requests

Bstops permanently

Creverses direction

Dmoves randomly

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming the arm stops permanently after reaching the last request.

Thinking the arm moves randomly without direction.

The LOOK algorithm moves the disk arm towards the last request in one direction and then reverses direction to service requests on the return path.

2fill in blank

medium

Complete the code to describe how C-LOOK differs from LOOK.

Operating Systems

Unlike LOOK, C-LOOK moves the disk arm in one direction and then [1] to the beginning without servicing requests on the return.

Drag options to blanks, or click blank then click option'

Ajumps back

Breverses direction

Cstops

Dservices all requests

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking C-LOOK reverses direction like LOOK.

Assuming C-LOOK services requests on the return trip.

C-LOOK moves the disk arm in one direction servicing requests, then jumps back to the beginning without servicing requests on the return.

3fill in blank

hard

Fix the error in the statement about LOOK algorithm behavior.

Operating Systems

LOOK algorithm always moves the disk arm to the very end of the disk before [1].

Drag options to blanks, or click blank then click option'

Areversing direction

Bservicing requests

Cstopping permanently

Dignoring requests

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming LOOK always goes to the disk's end.

Confusing LOOK with SCAN algorithm.

The LOOK algorithm does not move to the very end of the disk unless there is a request there; it moves only as far as the last request before reversing direction.

4fill in blank

hard

Fill both blanks to complete the description of C-LOOK's disk arm movement.

Operating Systems

C-LOOK moves the disk arm in one direction until the last request, then [1] to the [2] request without servicing requests on the way back.

Drag options to blanks, or click blank then click option'

Ajumps back

Breverses

Cbeginning

Dend

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'reverses' instead of 'jumps back'.

Selecting 'end' instead of 'beginning' for the return point.

C-LOOK moves the disk arm in one direction until the last request, then jumps back to the beginning request without servicing requests on the return.

5fill in blank

hard

Fill both blanks to complete the dictionary comprehension representing LOOK algorithm's request servicing order.

Operating Systems

look_order = {req: req for req in requests if req {BLANK_2}} current_pos and req {{BLANK_2}} direction_limit{{D}}

Drag options to blanks, or click blank then click option'

A{

B>

C<

D}

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators for filtering requests.

Omitting braces in the comprehension.

The dictionary comprehension starts with '{', filters requests greater than current position and less than direction limit, and ends with '}'.

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