GATE CS 2012 Q31 — CPU Scheduling

1. First-Come, First-Served (FCFS):

Since FCFS schedules processes strictly in the order of their arrival times ($P1$ at $0$, $P2$ at $1$, and $P3$ at $3$), they execute and complete in the same order:

• $P1$ runs from $0$ to $5$ (completes at $5$)
• $P2$ runs from $5$ to $12$ (completes at $12$)
• $P3$ runs from $12$ to $16$ (completes at $16$)

Thus, the FCFS completion order is $P1, P2, P3$.

2. Round Robin with Quantum = 2 (RR2):

We track the ready queue (RQ) and execution step-by-step:

• $[0 - 2]$: $P1$ runs. At $t=1$, $P2$ arrives. At $t=2$, $P1$ is preempted.
  $\text{RQ} = [P2, P1]$
• $[2 - 4]$: $P2$ runs. At $t=3$, $P3$ arrives. At $t=4$, $P2$ is preempted.
  $\text{RQ} = [P1, P3, P2]$
• $[4 - 6]$: $P1$ runs. At $t=6$, $P1$ is preempted.
  $\text{RQ} = [P3, P2, P1]$
• $[6 - 8]$: $P3$ runs. At $t=8$, $P3$ is preempted.
  $\text{RQ} = [P2, P1, P3]$
• $[8 - 10]$: $P2$ runs. At $t=10$, $P2$ is preempted.
  $\text{RQ} = [P1, P3, P2]$
• $[10 - 11]$: $P1$ runs for its remaining $1$ unit and completes at $11$.
  $\text{RQ} = [P3, P2]$
• $[11 - 13]$: $P3$ runs for its remaining $2$ units and completes at $13$.
  $\text{RQ} = [P2]$
• $[13 - 16]$: $P2$ runs for its remaining $3$ units and completes at $16$.

Thus, the RR2 completion order is $P1, P3, P2$.