GATE CS 2016 Set 1 Q48 — I/O Scheduling

In the C-LOOK (Circular LOOK) disk scheduling algorithm, the disk arm moves in one direction servicing requests until there are no more requests in that direction. Then, it immediately jumps back to the furthest request in the opposite direction (without servicing any requests on the return trip) and resumes servicing in the initial direction.

1. Given Data

• Pending requests: $10, 11, 38, 47, 87, 92, 121, 191$ (sorted order)
• Initial head position: $63$
• Direction: Towards larger cylinder numbers (upwards)

2. Step-by-Step Execution

Starting at $63$, the head moves towards the larger numbers:

1. Forward Pass: Services requests in increasing order up to the largest request: $$\text{Path: } 63 \to 87 \to 92 \to 121 \to 191$$\n $$\text{Head movement} = 191 - 63 = 128$$
2. Circular Return: From the largest request ($191$), the head jumps directly to the smallest pending request ($10$) without servicing any intermediate cylinders: $$\text{Path: } 191 \to 10$$\n $$\text{Head movement} = 191 - 10 = 181$$
3. Second Pass: Resumes moving towards larger numbers, servicing the remaining requests: $$\text{Path: } 10 \to 11 \to 38 \to 47$$\n $$\text{Head movement} = 47 - 10 = 37$$

3. Total Head Movement Calculation

$$\text{Total Head Movement} = 128 + 181 + 37 = 346$$

Thus, the total head movement incurred is 346 cylinders.