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
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The disk arm moves in one direction (like in LOOK), servicing all requests until it reaches the furthest request in that direction.
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Instead of going to the end of the disk, it "looks ahead" and stops at the last request.
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Then it jumps back to the lowest request without servicing any in between (like a circular movement).
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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:
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Disk head starts at: 50
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Requests in queue: 95, 180, 34, 119, 11, 123, 62
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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)
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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
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Start at 50
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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:
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50 → 62 = 12
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62 → 95 = 33
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95 → 119 = 24
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119 → 123 = 4
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123 → 180 = 57
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Jump: 180 → 11 = 169 (C-LOOK jump, no servicing during jump)
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11 → 34 = 23
✅ Total head movement =
12 + 33 + 24 + 4 + 57 + 169 + 23 = **322 cylinders**
Key Advantages of C-LOOK
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Reduces average seek time
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Avoids unnecessary head movement to disk extremes
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Efficient for large request sets with variable distribution
