GATE CS 2012 Q.31 (CS Q.31): CPU Scheduling

Consider the 3 processes, $P_1$ , $P_2$ and $P_3$ shown in the table.

The completion order of the 3 processes under the policies FCFS and RR2 (round robin scheduling with CPU quantum of 2 time units) are

FCFS: $P_1, P_2, P_3$ ; RR2: $P_1, P_2, P_3$

FCFS: $P_1, P_3, P_2$ ; RR2: $P_1, P_3, P_2$

FCFS: $P_1, P_2, P_3$ ; RR2: $P_1, P_3, P_2$

FCFS: $P_1, P_3, P_2$ ; RR2: $P_1, P_2, P_3$

Official answer and solution

Solution

Simulate process execution and trace completion times under FCFS and RR2 scheduling policies.

At $t = 0$ : $P_1$ arrives (burst 5). Runs from $t = 0$ to $2$ (remaining: 3). $P_2$ arrives at $t = 1$ . Ready queue: $[P_2, P_1]$ .
At $t = 2$ : $P_2$ runs from $t = 2$ to $4$ (remaining: 5). $P_3$ arrives at $t = 3$ . Ready queue: $[P_1, P_3, P_2]$ .
At $t = 4$ : $P_1$ runs from $t = 4$ to $6$ (remaining: 1). Ready queue: $[P_3, P_2, P_1]$ .
At $t = 6$ : $P_3$ runs from $t = 6$ to $8$ (remaining: 2). Ready queue: $[P_2, P_1, P_3]$ .
At $t = 8$ : $P_2$ runs from $t = 8$ to $10$ (remaining: 3). Ready queue: $[P_1, P_3, P_2]$ .
At $t = 10$ : $P_1$ runs for its remaining 1 unit from $t = 10$ to $11$ . $P_1$ completes at $t = 11$ .
At $t = 11$ : $P_3$ runs for its remaining 2 units from $t = 11$ to $13$ . $P_3$ completes at $t = 13$ .
At $t = 13$ : $P_2$ runs for its remaining 3 units from $t = 13$ to $16$ . $P_2$ completes at $t = 16$ .
Completion order: $P_1, P_3, P_2$ .

Final answer

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