Question

Disk requests come in to the disk driver for cylinders 10, 22, 20, 2, 40, 6, and 38, in that order. A seek takes 6 msec per cylinder. How much seek time is needed for: (a) First-come, first served (b) Closest cylinder next (c) Elevator algorithm (initially moving upward) In all cases, the arm is initially at cylinder 20.

Answer 1

Answer:

a) 876 ms

b) 360 ms

c) 348 ms

Step-by-step explanation:

a) When using the  First-come, first serve order, the starts at the initial cylinder  and is served in the same order as it arrives.

For  First-come, first serve, the order is served from (20), 10, 22, 20, 2, 40, 6, and 38.

Seek time = 10 + 12 + 2+ 18 + 38 + 34 + 32 = 146 cylinder

The request uses 146 cylinders, therefore the seek time = 146 cylinder * 6 ms/cylinder = 876 ms

b) When using the Closest cylinder next the  request starts at the initial cylinder  and is served using the shortest movement from current position.

For Elevator algorithm, the order is served from (20), 20, 22, 10, 6, 2, 38, 40

Seek time = 0 + 2 + 12+ 4 + 4 + 36 + 2 = 60 cylinder

The request uses 146 cylinders, therefore the seek time = 60 cylinder * 6 ms/cylinder = 360 ms

c) When using the Elevator algorithm the  request starts at the initial cylinder and moves upwards, then reverses direction.

For  Closest cylinder next, the order is served from (20), 20, 22, 38, 40, 10, 6, 2

Seek time = 0 + 2 + 16+ 2 + 30 + 4 + 4 = 58 cylinder

The request uses 146 cylinders, therefore the seek time = 58 cylinder * 6 ms/cylinder = 348 msa) When using the  First-come, first serve order, the starts at the initial cylinder  and is served in the same order as it arrives.

For  First-come, first serve, the order is served from (20), 10, 22, 20, 2, 40, 6, and 38.

Seek time = 10 + 12 + 2+ 18 + 38 + 34 + 32 = 146 cylinder

The request uses 146 cylinders, therefore the seek time = 146 cylinder * 6 ms/cylinder = 876 ms

b) When using the Closest cylinder next the  request starts at the initial cylinder  and is served using the shortest movement from current position.

For Elevator algorithm, the order is served from (20), 20, 22, 10, 6, 2, 38, 40

Seek time = 0 + 2 + 12+ 4 + 4 + 36 + 2 = 60 cylinder

The request uses 146 cylinders, therefore the seek time = 60 cylinder * 6 ms/cylinder = 360 ms

c) When using the Elevator algorithm the  request starts at the initial cylinder and moves upwards, then reverses direction.

For  Closest cylinder next, the order is served from (20), 20, 22, 38, 40, 10, 6, 2

Seek time = 0 + 2 + 16+ 2 + 30 + 4 + 4 = 58 cylinder

The request uses 146 cylinders, therefore the seek time = 58 cylinder * 6 ms/cylinder = 348 ms