Calculate the Average Waiting Time

CPU burst time points out the time, the process needs the CPU. The subsequent are the set of processes with their respective CPU burst time (in milliseconds).  

                  Processes                CPU-burst time

                  P1                                     10                      

                  P2                                       5

                  P3                                       5            

Calculate or find out the average waiting time if the process arrived in the subsequent order:

                (i) P1, P2 & P3                                 (ii) P2, P3 & P1

Ans:  consider FCFS scheduling   

Process                              Burst Time 

P1                                      10

P2                                       5

P3                                       5 

(i) Assume that the processes arrive in the order: P1 , P2 , P3  

The Gantt Chart for the schedule is as follow:

Waiting time for P1 = 0; P2 = 10; P3 = 15

Average waiting time:  (0 + 10 + 15)/3 = 8.33 unit of time

(ii)Assume that the processes arrive in the order P2, P3, P1.

The Gantt chart for the schedule is as follow: 

Waiting time for P1 = 10; P2 = 0; P3 = 5

Average waiting time:   (10 + 0 + 5)/3 = 5 unit of time.