ITPEC FE Morning October 2022 Question 18

Preemptive Priority Scheduling — CPU idle time — simulate a timeline with priority-based CPU allocation and parallel I/O.

Given three tasks with priorities and sequences CPU → I/O → CPU:

• •A (High): CPU(3) → I/O(5) → CPU(2)
• •B (Medium): CPU(2) → I/O(6) → CPU(2)
• •C (Low): CPU(1) → I/O(5) → CPU(1)

Rules:
- CPU goes to the highest-priority ready task (preemptive)
- I/O runs in parallel with CPU (separate device)
- No OS overhead, no I/O contention

Timeline:

• •t=0–3: A runs CPU(3); B, C wait
• •t=3–5: A starts I/O(5); B runs CPU(2)
• •t=5–6: B starts I/O(6); C runs CPU(1)
• •t=6–8: C starts I/O(5); CPU idle (2ms) — all tasks on I/O
• •t=8–10: A returns from I/O, runs CPU(2) → A done
• •t=10–11: CPU idle (1ms) — B and C still on I/O
• •t=11–13: B returns, runs CPU(2) → B done; C also returns but waits
• •t=13–14: C runs CPU(1) → C done

CPU idle = 2 + 1 = `3` ms

Why not others:
- (a) 2 — misses the idle gap at t=10–11
- (c) 4 — overcounts idle time
- (d) 5 — likely ignored parallel I/O

Key rule: I/O device operates independently from CPU. Track each task's I/O return time to know when it becomes ready for its next CPU burst.

AI-generated — may contain errors