LOOK and C-LOOK variants in Operating Systems - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind the LOOK disk scheduling algorithm?

LOOK moves the disk arm towards the nearest request in one direction and reverses direction only when there are no more requests in that direction. It 'looks' ahead to avoid unnecessary travel to the disk's end.

beginner

How does C-LOOK differ from LOOK in handling disk requests?

C-LOOK moves the disk arm in one direction only, servicing requests until the last one in that direction, then jumps back to the first request at the other end without servicing requests on the return trip.

intermediate

Why does LOOK reduce unnecessary disk arm movement compared to SCAN?

LOOK stops moving the arm when there are no more requests in the current direction, instead of going all the way to the disk's end like SCAN, saving time and reducing wear.

intermediate

What is the advantage of C-LOOK over LOOK in terms of disk arm movement?

C-LOOK only moves the arm in one direction, which can lead to more uniform wait times and less total arm movement by skipping the return trip servicing.

advanced

In what scenario might LOOK be preferred over C-LOOK?

LOOK might be preferred when requests are evenly spread and servicing requests in both directions reduces wait time, as it services requests on the return trip as well.

What does the LOOK algorithm do when there are no more requests in the current direction?

A It stops and waits for new requests

B It continues to the end of the disk

C It reverses direction to service requests on the other side

D It jumps to the first request on the disk

LOOK reverses direction only when no more requests exist in the current direction.

How does C-LOOK handle the return trip after servicing the last request in one direction?

A Jumps directly to the first request without servicing

B Reverses direction gradually

C Stops and waits

D Services requests on the way back

C-LOOK jumps back to the first request without servicing requests on the return trip.

Which algorithm moves the disk arm all the way to the disk's end regardless of requests?

A SCAN

B C-LOOK

C LOOK

D FCFS

SCAN moves the arm to the disk's end even if no requests are there, unlike LOOK.

What is a key benefit of C-LOOK over LOOK?

A Services requests in both directions

B Reduces total arm movement by moving in one direction only

C Always moves to the disk's end

D Waits for new requests before moving

C-LOOK reduces arm movement by servicing requests in one direction and jumping back.

Which disk scheduling algorithm is more likely to provide uniform wait times?

A FCFS

B LOOK

C Random

D C-LOOK

C-LOOK tends to provide more uniform wait times by moving in one direction only.

Explain how the LOOK disk scheduling algorithm optimizes disk arm movement compared to SCAN.

Think about how LOOK 'looks' ahead before moving.

Describe the main difference between LOOK and C-LOOK algorithms and how it affects disk arm movement.

Consider the direction of arm movement and servicing on return.

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