ITPEC FE Subject A April 2024 Question 13
An operating system adopts the priority scheduling algorithm for scheduling processes. There are five processes A, B, C, D and E arriving at the same time with estimated running times of 10, 6, 2, 4, and 8 milliseconds, respectively. Their priorities are 3, 5, 2, 1, and 4, where 1 is the highest priority. What is the mean process turnaround time?
Explanation
16 ms — run the processes from highest to lowest priority and average their completion times.
All five arrive at time 0, so each turnaround time equals its completion time. By priority (1 is highest) the order is D, C, A, E, B.
Mean turnaround: (4 + 6 + 16 + 24 + 30) ÷ 5 = 80 ÷ 5 = 16
Answer (b)
Why not others:
- •(a) 14 — shortest job first (C, D, B, E, A): (2 + 6 + 12 + 20 + 30) ÷ 5
- •(c) 20 — treats 5 as the highest priority (B, E, A, C, D): (6 + 14 + 24 + 26 + 30) ÷ 5
- •(d) 22 — longest job first (A, E, B, D, C): (10 + 18 + 24 + 28 + 30) ÷ 5
Key rule: When all processes arrive together, turnaround time is completion time and the mean depends only on the running order; the scheduling policy just picks that order.
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