We have already discussed FCFS Scheduling of processes with same arrival time. In this post, scenario when processes have different arrival times are discussed. Given n processes with their burst times and arrival times, the task is to find average waiting time and average turn around time using FCFS scheduling algorithm.
FIFO simply queues processes in the order they arrive in the ready queue. Here, the process that comes first will be executed first and next process will start only after the previous gets fully executed.
- Completion Time: Time at which process completes its execution.
- Turn Around Time: Time Difference between completion time and arrival time. Turn Around Time = Completion Time – Arrival Time
- Waiting Time(W.T): Time Difference between turn around time and burst time.
Waiting Time = Turn Around Time – Burst Time
Process Wait Time : Service Time - Arrival Time P0 0 - 0 = 0 P1 5 - 1 = 4 P2 8 - 2 = 6 P3 16 - 3 = 13 Average Wait Time: (0 + 4 + 6 + 13) / 4 = 5.75
Service Time : Service time means amount of time after which a process can start execution. It is summation of burst time of previous processes (Processes that came before)
Changes in code as compare to code of FCFS with same arrival time:
To find waiting time: Time taken by all processes before the current process to be started (i.e. burst time of all previous processes) – arrival time of current process
wait_time[i] = (bt + bt +…… bt[i-1] ) – arrival_time[i]
1- Input the processes along with their burst time(bt) and arrival time(at) 2- Find waiting time for all other processes i.e. for a given process i: wt[i] = (bt + bt +...... bt[i-1]) - at[i] 3- Now find turn around time = waiting_time + burst_time for all processes 4- Average waiting time = total_waiting_time / no_of_processes 5- Average turn around time = total_turn_around_time / no_of_processes
Processes Burst Time Arrival Time Waiting Time Turn-Around Time Completion Time 1 5 0 0 5 5 2 9 3 2 11 14 3 6 6 8 14 20 Average waiting time = 3.33333 Average turn around time = 10
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