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

Processes       CPU-burst time

P1                           10

P2                            5

P3                            5

Calculate  the  average  waiting  time  if the  process  arrived  in  the  following order:

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

By considering the FCFS scheduling

Process                                           Burst Time

P1                                                         10

P2                                                          5

P3                                                          5

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

The Gantt chart for the schedule given below:

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

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

(ii) Assume that the processes arrive in the sequence P2, P3, P1. the Gantt chart for the schedule given below:

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

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