Why Scheduling algorithm (SCAN, LOOK) in LLD? - Purpose & Use Cases

The Big Idea

What if your elevator or disk could magically know the best path to serve everyone quickly?

The Scenario

Imagine you are managing a busy elevator in a tall building. Without a smart plan, the elevator moves randomly, going up and down without order, making people wait long and wasting energy.

The Problem

Manually deciding which floor to visit next can be slow and confusing. It causes delays, wastes time, and frustrates users because the elevator might zigzag inefficiently, just like a disk head moving back and forth without a plan.

The Solution

Scheduling algorithms like SCAN and LOOK organize the elevator's movement by sweeping in one direction, serving requests in order, then reversing. This reduces wait times and travel distance, making the system smooth and efficient.

Before vs After

✗ Before

while requests:
  next_floor = random.choice(requests)
  move_to(next_floor)
  requests.remove(next_floor)

✓ After

direction = 'up'
while requests:
  floors_in_direction = get_floors_in_direction(current_floor, direction)
  serve_floors_in_order(floors_in_direction)
  direction = reverse(direction)

What It Enables

It enables fast, predictable, and fair servicing of requests, improving overall system performance and user satisfaction.

Real Life Example

Disk scheduling in operating systems uses SCAN and LOOK to decide the order of reading or writing data, minimizing the disk arm movement and speeding up access.

Key Takeaways

Manual scheduling causes inefficiency and delays.

SCAN and LOOK algorithms organize requests by direction to optimize movement.

This leads to faster, fairer, and more predictable service.

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