GATE Overflow Test Series | Operating Systems | Test 2 | Question: 29

Question

Suppose that a disk drive has $5000$ cylinders, numbered $0$ to $4999.$ The drive is currently serving a request at cylinder $1243,$ and previously it was at cylinder $125.$ The queue of pending requests in FIFO order is: $$86, 1470, 913, 1774, 948, 1509, 1022$$
Starting from the current head position, if the total distance (in cylinders) that the disk arm moves to satisfy all the pending requests for First Come First Served (FCFS) and Shortest Seek Time First (SSTF) are $\alpha$ and $\beta$ respectively, then $2\alpha+3\beta =$ _______

Answer 1

$125-1243.$
In FCFS, requests will be serviced in order as $86, 1470, 913, 1774, 948, 1509, 1022$

Total head movements $\alpha =(1243-86)+(1470-86)+(1470-913)+(1774-913)+(1774-948)+(1509-948)+(1509-1022)$

$\quad = 5833. $

In SSTF, requests will be serviced in the following order

$1022,948,913,1470,1509,1774,86.$

Total head movements $\beta = (1243-1022) + (1022-948)+(948-913)+(1470-913)+(1509-1470)+(1774-1509)+(1774-86)$

$\qquad = 221+ 74+ 35+ 557+ 39+265+ 1688=2879.$

So, $2\alpha + 3\beta =20303. $