Operating System - C-LOOK Disk Scheduling

What is C-LOOK Disk Scheduling?

C-LOOK (Circular LOOK) is a disk scheduling algorithm that is an optimized version of the LOOK algorithm. It is designed to reduce disk head movement and improve performance by avoiding unnecessary traversal.

How C-LOOK Works

The disk arm moves in one direction (like in LOOK), servicing all requests until it reaches the furthest request in that direction.
Instead of going to the end of the disk, it "looks ahead" and stops at the last request.
Then it jumps back to the lowest request without servicing any in between (like a circular movement).
This reduces total seek time compared to SCAN/LOOK.

Difference Between LOOK and C-LOOK

Feature	LOOK	C-LOOK
Movement	Goes to both ends of request range	Goes only as far as the last request in one direction, then jumps
Return	Reverses direction	Jumps to the start (like a circle)
Efficiency	Slightly less efficient	More efficient on average

Example

Given:

Disk head starts at: 50
Requests in queue: 95, 180, 34, 119, 11, 123, 62
Direction: Moving upwards (toward higher-numbered cylinders)

Step 1: Sort the queue

Sorted requests:
11, 34, 62, 95, 119, 123, 180

Step 2: Split based on current head (50)

Requests greater than 50 (to the right):
62, 95, 119, 123, 180
Requests less than 50 (to the left):
11, 34

Step 3: C-LOOK Scheduling Order

Start at 50
Go up to: 62 → 95 → 119 → 123 → 180
Jump to the lowest request: 11 → 34

Final order:

62 → 95 → 119 → 123 → 180 → 11 → 34

Calculate Total Head Movement

Starting at 50:

50 → 62 = 12
62 → 95 = 33
95 → 119 = 24
119 → 123 = 4
123 → 180 = 57
Jump: 180 → 11 = 169 (C-LOOK jump, no servicing during jump)
11 → 34 = 23

✅ Total head movement =

12 + 33 + 24 + 4 + 57 + 169 + 23 = **322 cylinders**

Key Advantages of C-LOOK

Reduces average seek time
Avoids unnecessary head movement to disk extremes
Efficient for large request sets with variable distribution