GATE CS 2020 Q35 — I/O Scheduling

To find the order of service under the Shortest Seek Time First (SSTF) scheduling algorithm, we calculate the absolute distance from the current head position to all pending requests and choose the closest one at each step.

The initial head position is $100$. The pending requests are $P(155)$, $Q(85)$, $R(110)$, $S(30)$, and $T(115)$.

1. From 100: Distances are $P(55)$, $Q(15)$, $R(10)$, $S(70)$, $T(15)$. The closest is $R(110)$.
   Head moves to $110$.
2. From 110: Distances are $P(45)$, $Q(25)$, $S(80)$, $T(5)$. The closest is $T(115)$.
   Head moves to $115$.
3. From 115: Distances are $P(40)$, $Q(30)$, $S(85)$. The closest is $Q(85)$.
   Head moves to $85$.
4. From 85: Distances are $P(70)$, $S(55)$. The closest is $S(30)$.
   Head moves to $30$.
5. From 30: Only $P(155)$ remains.
   Head moves to $155$.

The servicing sequence is: $R o T o Q o S o P$.

Let's evaluate the options:

• A: $T$ is serviced before $P$. (True, $T$ is 2nd and $P$ is 5th)
• B: $Q$ is serviced after $S$, but before $T$. (False, $Q$ is serviced before $S$ and after $T$)
• C: The head reverses direction between servicing $Q$ and $P$. (True, head moves left from $Q(85) o S(30)$, then reverses right to $P(155)$)
• D: $R$ is serviced before $P$. (True, $R$ is 1st and $P$ is 5th)