Question

The outstanding disk requests are for tracks 6,10,4,20,36,8, and 40 in that order. Assume that the seek time speed is 5 msec/track, there are 50 cylinders total, and presently the read-write head is at track 15 and moving up towards 50. Calculate the seek time for each of the following scheduling methods.
a. FCFS
b. SSTF
c. C-LOOK

Answer 1

Answer:

a. FCFS

seek: 9+4+6+16+16+28+32 = 111 cylinders, 555 ms

b. SSTF

seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms

c. C-LOOK

seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms

Explanation:

a. FCFS: requests are served in the order they are received. It is used when load is low and has long waiting time for long requests queues.

order: (15), 6, 10, 4, 20, 36, 8, 40

seek: 9+4+6+16+16+28+32 = 111 cylinders, 555 ms

b. SSTF: requests are served using shortest seek time first. It minimizes arm movement, and maximizes request rate

order: (15), 20, 36, 40, 10, 8, 6, 4   15 10 8 6 4 40 36 20

seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms

c. C-LOOK: requests are served in one direction until when done then reverses. It does not go to end of disk.

Assume maximum track is 50 then reverses

order: (15), 20, 36, 40, (50), 10, 8, 6, 4

seek: 5+16+4+30+2+2+2 = 61 cylinders, 305 ms