Example 4:

Given the following: Total number of cylinders in the disk = 200 (numbered 0 to 199)

Current head position = cylinder 23

Current requests in order of arrival = 20, 17, 55, 35, 25, 78, 99

Assume the same details about the disk as in Example 4 above.The request queue does not remain the same but it changes as new requests are added to the queue. At any point of time the algorithm uses the current requests to make its decision as to the next request to be serviced. Consider the following request sequence:

Initially (at time 0) the queue contains requests for cylinders: 99, 3, 25

At the time the next decision has to be taken by the algorithm, one new requests has joined the queue: 46

Next decision point one more request has joined the queue: 75

Next decision point one more request has joined the queue: 55

Next decision point one more request has joined the queue: 85

Next decision point one more request has joined the queue: 73

Next decision point one more request has joined the queue: 50

Assume that the disk head at time 0 is just completing a request at track 55.

a) Show the schedule for FCFS, SSTF, SCAN, C-SCAN, LOOK, and C-LOOK.

b) Compute the response time (in units of head traversal) for each of the above requests.

c) What is the average response time and throughput observed for each of the algorithms?

Solution for b) and c):

Request	FCFS	SSTF	SCAN	CSCAN	LOOK	CLOOK
R1 (cyl 99)
R2 (cyl 3)
R3 (cyl 25)
R4 (cyl 46)
R5 (cyl 75)
R6 (cyl 55)
R7 (cyl 85)
R8 (cyl 73)
R9 (cyl 50)
Avg Response
Throughput

Please, do not answer these questions unless you give a complete correct answer