ITPEC FE Subject A April 2024 Question 13

Priority Scheduling — Mean Turnaround Time — sort by priority, compute each completion time.

Given five processes arriving at time 0:

• •A: runtime 10, priority 3
• •B: runtime 6, priority 5
• •C: runtime 2, priority 2
• •D: runtime 4, priority 1
• •E: runtime 8, priority 4

Step 1 — Sort by priority (1 = highest):

• •D (pri 1) → C (pri 2) → A (pri 3) → E (pri 4) → B (pri 5)

Step 2 — Compute completion times:

• •D: 0 + 4 = 4
• •C: 4 + 2 = 6
• •A: 6 + 10 = 16
• •E: 16 + 8 = 24
• •B: 24 + 6 = 30

Step 3 — Mean turnaround time:

• •(4 + 6 + 16 + 24 + 30) / 5 = 80 / 5 = 16

Answer: b) 16

Why not others:
- (a) 14 — arithmetic error or wrong execution order
- (c) 20 — result of running in alphabetical order (A→B→C→D→E) instead of by priority
- (d) 22 — incorrect scheduling order

Key rule: In priority scheduling, always sort processes by priority value first. Turnaround time = completion time − arrival time.

AI-generated — may contain errors