Scheduling algorithm (SCAN, LOOK) in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the SCAN scheduling algorithm?

SCAN is a disk scheduling algorithm where the disk arm moves in one direction servicing requests until it reaches the end, then reverses direction and services requests on the way back, like an elevator.

beginner

How does the LOOK algorithm differ from SCAN?

LOOK is similar to SCAN but the disk arm only goes as far as the last request in each direction before reversing, instead of going to the physical end of the disk.

beginner

Why is SCAN called the elevator algorithm?

Because it moves the disk arm back and forth across the disk like an elevator moving up and down floors, servicing requests in one direction before reversing.

intermediate

What is the main advantage of LOOK over SCAN?

LOOK reduces unnecessary movement by not going to the disk's physical end if there are no requests there, improving efficiency and reducing seek time.

intermediate

In what scenario would SCAN or LOOK scheduling be preferred?

They are preferred when we want to reduce the average seek time and avoid starvation by servicing requests in a fair order, especially in systems with many disk I/O requests.

What does the SCAN algorithm do when it reaches the end of the disk?

A Skips requests at the end and continues forward

B Reverses direction and services requests on the way back

C Jumps back to the start of the disk immediately

D Stops and waits for new requests

SCAN moves the disk arm to the end, then reverses direction to service requests on the return trip.

How does LOOK improve efficiency compared to SCAN?

A By moving faster across the disk

B By skipping requests at the disk edges

C By servicing requests randomly

D By only moving as far as the last request in each direction

LOOK stops at the last request in each direction, avoiding unnecessary travel to the disk's physical end.

Which scheduling algorithm is also known as the elevator algorithm?

A SCAN

B LOOK

C FCFS

D SSTF

SCAN is called the elevator algorithm because it moves back and forth like an elevator.

What is a key benefit of using SCAN or LOOK over FCFS in disk scheduling?

A They reduce average seek time and avoid starvation

B They guarantee the shortest seek time for every request

C They process requests in random order

D They only service requests at the disk edges

SCAN and LOOK reduce average seek time and prevent starvation by servicing requests in a fair order.

If there are no requests beyond a certain point on the disk, which algorithm avoids unnecessary travel?

A SCAN

B FCFS

C LOOK

D SSTF

LOOK avoids traveling to the disk's physical end if no requests exist beyond the last request.

Explain how the SCAN scheduling algorithm works and why it is called the elevator algorithm.

Think about how an elevator moves up and down floors.

Describe the difference between SCAN and LOOK algorithms and the advantage of LOOK.

Consider how far the disk arm travels in each algorithm.

Practice

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

easy

A. SCAN stops at the last request in the direction, LOOK goes to the disk edge

B. LOOK moves the head randomly, SCAN moves sequentially

C. LOOK stops at the last request in the direction, SCAN goes to the disk edge

D. SCAN only moves in one direction, LOOK moves back and forth

Solution

1. Step 1: Understand SCAN behavior

   SCAN moves the disk head to the end of the disk in one direction, servicing requests along the way.

2. Step 2: Understand LOOK behavior

   LOOK moves the disk head only as far as the last request in the current direction, then reverses.

3. Final Answer:

   LOOK stops at the last request in the direction, SCAN goes to the disk edge -> Option C

4. Quick Check:

   LOOK stops early, SCAN goes to edge [OK]

Hint: LOOK stops at last request, SCAN goes to disk edge [OK]

Common Mistakes:

• Confusing which algorithm stops at disk edge
• Thinking LOOK moves randomly
• Assuming SCAN moves only one way

2. Which of the following is the correct order of servicing requests using the SCAN algorithm if the head starts at 50 and requests are at [10, 20, 35, 70, 90] with disk size 100?

easy

A. 50 -> 70 -> 90 -> 35 -> 20 -> 10

B. 50 -> 70 -> 90 -> 100 -> 35 -> 20 -> 10

C. 50 -> 35 -> 20 -> 10 -> 0 -> 70 -> 90

D. 50 -> 90 -> 70 -> 35 -> 20 -> 10

Solution

1. Step 1: SCAN moves towards higher end first

   Starting at 50, SCAN moves up servicing 70 and 90, then reaches disk edge 100.

2. Step 2: SCAN reverses direction

   After reaching 100, it moves down servicing 35, 20, and 10.

3. Final Answer:

   50 -> 70 -> 90 -> 100 -> 35 -> 20 -> 10 -> Option B

4. Quick Check:

   SCAN goes to edge 100 before reversing [OK]

Hint: SCAN always goes to disk edge before reversing [OK]

Common Mistakes:

• Not including disk edge in the path
• Reversing direction too early
• Skipping requests on the way

3. Given the LOOK algorithm with head starting at 40 and requests at [10, 20, 35, 70, 90], what is the order of servicing requests if the head moves towards higher track numbers first?

medium

A. [40, 90, 70, 35, 20, 10]

B. [40, 70, 90, 100, 35, 20, 10]

C. [40, 35, 20, 10, 70, 90]

D. [40, 70, 90, 35, 20, 10]

Solution

1. Step 1: LOOK moves towards higher requests first

   Starting at 40, it services 70 and 90, stopping at last request 90.

2. Step 2: LOOK reverses direction

   Then it moves down servicing 35, 20, and 10, stopping at last request in that direction.

3. Final Answer:

   [40, 70, 90, 35, 20, 10] -> Option D

4. Quick Check:

   LOOK stops at last request, no disk edge [OK]

Hint: LOOK stops at last request, no disk edge [OK]

Common Mistakes:

• Including disk edge in LOOK path
• Reversing direction too early
• Skipping requests on the way

4. Identify the error in this SCAN algorithm implementation snippet where the head moves from 30 to 90 with requests at [20, 40, 60, 80]:

requests = [20, 40, 60, 80]
head = 30
for track in range(head, 100):
    if track in requests:
        print(f"Servicing {track}")
for track in range(head-1, -1, -1):
    if track in requests:
        print(f"Servicing {track}")

medium

A. The first loop should go to 101, not 100

B. The first loop should include the head position

C. The second loop should start from head, not head-1

D. The second loop should start from 99, not head-1

Solution

1. Step 1: Check range end in first loop

   range(head, 100) goes from 30 to 99 (exclusive end), missing disk edge at 100.

2. Step 2: Confirm disk convention

   As per standard problems (disk size 100 includes edge 100), first loop should be range(head, 101) to reach 100.

3. Final Answer:

   The first loop should go to 101, not 100 -> Option A

4. Quick Check:

   range exclusive, edge 100 needs 101 [OK]

Hint: Range end exclusive, use 101 for disk edge 100 [OK]

Common Mistakes:

• Confusing inclusive vs exclusive range ends
• Starting second loop incorrectly
• Assuming head position is included twice

5. You have a disk with size 200 tracks and requests at [10, 50, 120, 180]. The head is at 100 and moves using LOOK algorithm. What is the total head movement if the head moves towards lower track numbers first?

hard

A. 260 tracks

B. 160 tracks

C. 180 tracks

D. 120 tracks

Solution

1. Step 1: Initial direction to lower tracks

   Lower requests: 50, 10. Path: 100 -> 50 (50 tracks), 50 -> 10 (40 tracks). Subtotal: 90.

2. Step 2: Reverse direction to higher tracks

   Higher requests: 120, 180. Path: 10 -> 120 (110 tracks), 120 -> 180 (60 tracks). Subtotal: 170.

3. Step 3: Total head movement

   90 + 170 = 260 tracks.

4. Final Answer:

   260 tracks -> Option A

5. Quick Check:

   Sum of |current - next| over servicing sequence [OK]

Hint: Sum distances in servicing order: 100-50-10-120-180 [OK]

Common Mistakes:

• Including disk edges in LOOK movement
• Adding extra distances beyond last requests
• Misordering request servicing