ITPEC FE Subject A April 2025 Question 14

Preemptive Priority Scheduling — calculate average waiting time.

Given four processes with arrival times, burst times, and priorities (lower number = higher priority), simulate preemptive scheduling:

Execution timeline:
- t=0–2: P1 runs (priority 2, only process)
- t=2–3: P2 arrives (priority 1), preempts P1
- t=3–9: P2 done, P1 resumes (6 ms remaining)
- t=9–11: P3 runs (priority 3, next highest)
- t=11–14: P4 runs (priority 4)

Waiting time = completion_time − arrival_time − burst_time:
- P1: 9 − 0 − 8 = 1
- P2: 3 − 2 − 1 = 0
- P3: 11 − 4 − 2 = 5
- P4: 14 − 6 − 3 = 5

Average = (1 + 0 + 5 + 5) / 4 = 2.75

Key rule: In preemptive scheduling, a running process is interrupted when a higher-priority process arrives. Waiting time excludes the time a process is actually executing.

AI-generated — may contain errors