Question

Task 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Consider a disk drive with 2,000 tracks, numbered 0 to 1,999. The request queue has

the following composition: 1045 750 932 878 1365 1787 1245 664 1678 1897

The current position is 1167 and the previous request was served at 1250. For each

of the following disk scheduling algorithms, compute the total distance (in tracks)

that the disk arm would move. Explain how you arrive at your answers.

4.1       First-In-First-Out (FIFO)                                                                                (2)

4.2      Shortest-Service-Time-First (SSTF)                                                             (2)

4.3      SCAN (Without LOOK variation)                                                                     (2)

4.4      C-SCAN (Without C-LOOK variation) (2)

Answer 1

4.1)

FIFO -

Current position is 1167

In FIFO, the requests will be served in order that is 1045 750 932 878 1365 1787 1245 664 1678 1897

Number of movements -

1167 -> 1045 - 122

1045 -> 750 - 295

750 -> 932 - 182

932 -> 878 - 54

878 -> 1365 - 487

1365 -> 1787 - 422

1787 -> 1245 - 542

1245 -> 664 - 581

664 -> 1678 - 1014

1678 -> 1897 - 219

Now take sum of all these results

4.2)

SSTF - This algorithm services that request next which requires least number of head movements from its current position

Hence requests will be served in following order -

1167 -> 1245 -> 1365 -> 1678 -> 1787 -> 1897 -> 1045 -> 932 -> 878 -> 750 -> 664

I hope you can perform addition by yourself

4.3)

SCAN - Head starts from one end of the disk and move towards the other end servicing all the requests in between.After reaching the other end, head reverses its direction and move towards the starting end servicing all the requests in between.

Hence requests will be served in following order -

1167 -> 1245 -> 1365 -> 1678 -> 1787 -> 1897 -> 1045 -> 932 -> 878 -> 750 -> 664

I hope you can perform addition by yourself

4.4)

C - SCAN - Head starts from one end of the disk and move towards the other end servicing all the requests in between.After reaching the other end, head reverses its direction.It then returns to the starting end(that is 0) without servicing any request in between.The same process repeats.

Hence requests will be served in following order -

1167 -> 1245 -> 1365 -> 1678 -> 1787 -> 1897 -> 664 -> 750 -> 878 -> 932 -> 1045

I hope you can perform addition by yourself

I have tried to explain it in very simple language and I hope that i have answered your question satisfactorily.Leave doubts in comment section if any.