Consider a set of 5 processes whose arrival time, CPU time needed and the…

2009

Consider a set of 5 processes whose arrival time, CPU time needed and the priority are given below: 

Process       Arrival Time   CPU Time Needed     Priority
              (in ms)

P1              0             10                 5
P2              0             5                  2
P3              2             3                  1
P4              5             20                 4
P5              10            2                  3

(smaller the number, higher the priority) If the CPU scheduling policy is priority scheduling without preemption, the average waiting time will be

Answer: C. 10.8 ms — At T=0, P2 (priority 2) starts before P1 (priority 5). At T=5, P3 (priority 1) arrives and has higher priority than P1 or P4, so it runs until T=8. At T=8, P4…

  1. A.

    12.8 ms

  2. B.

    11.8 ms

  3. C.

    10.8 ms

  4. D.

    9.8 ms

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Show answer & explanation

Correct answer: C

At T=0, P2 (priority 2) starts before P1 (priority 5). At T=5, P3 (priority 1) arrives and has higher priority than P1 or P4, so it runs until T=8. At T=8, P4 (priority 4) has higher priority than P1, running until T=28. At T=28, P5 (priority 3) runs before P1, finishing at T=30. Finally, P1 runs from T=30 to 40. Waiting times: P1(30), P2(0), P3(3), P4(3), P5(18). Total = 54 ms. Average = 54/5 = 10.8 ms.

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