ITPEC FE Subject A April 2026 Question 14
Which of the following is the CPU utilization in percentage rounded to a decimal number when three (3) jobs with 30% average I/O (Input/Output) wait time are running on a single CPU? Here, I/O wait time refers to the percentage of time a job is waiting for its I/O operation to complete.
Explanation
97% — the CPU is idle only when all three jobs are waiting for I/O at the same time.
Each job waits 30% of the time, independently of the others:
- •Probability all three wait at once: 0.33 = 0.027, i.e. 2.7% idle
- •CPU utilization: 1 − 0.027 = 0.973, about 97%
Answer (d)
Why not others:
- •(b) 34 — 0.73 = 0.343, the probability that all three jobs are computing at once; the CPU is busy when at least one job is ready, not all
- •(a) 10 — 1 − 3 × 0.3 = 0.1, adding the three wait fractions as if they never overlap and counting all of it as idle
- •(c) 90 — 3 × 0.3 = 0.9, the same sum of wait fractions read as utilization
Key rule: With n independent jobs each waiting a fraction p for I/O, CPU utilization is 1 − pn: the CPU idles only when every job waits at once.
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