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GATE | Gate IT 2005 | Question 60

  • Difficulty Level : Medium
  • Last Updated : 28 Jun, 2021
We wish to schedule three processes P1, P2 and P3 on a uniprocessor system. The priorities, CPU time requirements and arrival times of the processes are as shown below.
 Process Priority CPU time required Arrival time
 P1 10(highest) 20 sec 00:00:05
 P2 9 10 sec 00:00:03
 P3 8 (lowest) 15 sec 00:00:00

We have a choice of preemptive or non-preemptive scheduling. In preemptive scheduling, a late-arriving higher priority process can preempt a currently running process with lower priority. In non-preemptive scheduling, a late-arriving higher priority process must wait for the currently executing process to complete before it can be scheduled on the processor.
What are the turnaround times (time from arrival till completion) of P2 using preemptive and non-preemptive scheduling respectively.
(A) 30 sec, 30 sec
(B) 30 sec, 10 sec
(C) 42 sec, 42 sec
(D) 30 sec, 42 sec

Answer: (D)

Explanation: For Non preemptive scheduling


0                                                       15                                                         35                                       45

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Turn Around Time= Completion Time – Arrival Time = 45 -3 = 42


For Preemptive scheduling


0                  1                    2                 3                  4                      5                  25                  33           45

Turn Around Time= Completion Time – Arrival Time = 33 – 3 = 30

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