Disk scheduling with 150 cylinders

Cylinders 0–149, head at 35, previous request at 120. Since the head came from 120 to 35, it is moving towards lower cylinders. Queue = 98, 103, 38, 122, 10, 128, 65, 75.

1. SSTF

35 → 38 → 65 → 75 → 98 → 103 → 122 → 128 → 10

3 + 27 + 10 + 23 + 5 + 19 + 6 + 118 = 211

2. SCAN

Move down serving 10, go to 0, reverse, then serve the rest going up (the arm stops at 128, the last request, since nothing lies beyond).

35 → 10 → 0 → 38 → 65 → 75 → 98 → 103 → 122 → 128

25 + 10 + 38 + 27 + 10 + 23 + 5 + 19 + 6 = 163

3. C-SCAN

Move down to 0, jump to the other end (149), then continue downwards. The return jump is counted.

35 → 10 → 0 → 149 → 128 → 122 → 103 → 98 → 75 → 65 → 38

25 + 10 + 149 + 21 + 6 + 19 + 5 + 23 + 10 + 27 = 295

4. LOOK

Same as SCAN, but reverses at the last request (10), not at cylinder 0.

35 → 10 → 38 → 65 → 75 → 98 → 103 → 122 → 128

25 + 28 + 27 + 10 + 23 + 5 + 19 + 6 = 143

5. FCFS

35 → 98 → 103 → 38 → 122 → 10 → 128 → 65 → 75

63 + 5 + 65 + 84 + 112 + 118 + 63 + 10 = 520

Summary

Answer: SSTF = 211, SCAN = 163, C-SCAN = 295, LOOK = 143, FCFS = 520 cylinders. LOOK is the best.