Question

The outstanding disk requests are for cylinders 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 that presently the read-write head is athe regime has largely consolidated control in the west, where only weeks earlier his rule seemed track 15, moving up.

Calculate the seek time for each of the following:

a). FCFS

b). SSTF

Answer 1

Explanation:

Disk Scheduling: is when the operating system request for data that arrives on the disk. when there are multiple request,the new request are placed on queue as pending request, as one request is completed, the system chooses which of the pending request to be served next.

FCFS (first come first served): this is the service requests in the order of arrival.

SSTF (shortest seek time first): this is the service requests in the queue that is closest  to the current position of the disk head.

Given:

The outstanding disk requests are for cylinders 6, 10, 4, 20, 36, 8, and 40

Seek time speed: 5 msec/track

Initial track: 15

To Calculate seek time for;

a) FCFS (first come first served):

seek time:

= (15-6)+(6-10)+(10-4)+(4-20)+(20-36)+(36-8)+(8-40)

= 9+4+6+16+16+28+32

= 111 tracks × 5 ms/track

=555 ms

b) SSTF (shortest seek time first):

seek time:

= (15-10)+(10-8)+(8-6)+(6-4)+(4-20)+(20-36)+(36-40)

= 5+2+2+2+16+16+16

= 59 tracks × 5 ms/track

= 295 ms