ITPEC FE Subject A April 2026 Question 14
ITPEC FE Subject A April 2026 — Question 14 of 60
97% — the CPU is idle only when every job is waiting on I/O simultaneously.
With each job independently waiting 30% of the time, the chance that all three wait at once is:
- •Idle probability:
0.3³= 0.027, i.e. 2.7% - •CPU utilization:
1 − 0.027= 0.973 → 97%
Answer (d)
Why not others:
- (c) 90 — comes from combining the three 30% figures additively (30 × 3) instead of multiplying the independent probabilities
- (a) 10 and (b) 34 — follow neither the correct formula nor that additive mistake
Key rule: Utilization with n jobs each waiting a fraction p of the time is 1 − pⁿ. Multitasking helps because idle time requires all jobs to stall together, which becomes unlikely fast as n grows.
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