GATE | Gate IT 2005 | Question 60

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
(hh:mm:ss)
 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

P3(AT=0) P1(AT=5) P2(AT=3)

0                                                       15                                                         35                                       45

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



 

For Preemptive scheduling

P3 P3 P3 P2 P2 P1 P2 P3

0                  1                    2                 3                  4                      5                  25                  33           45

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

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