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C Program for FCFS Scheduling
We are given with the n number of processes i.e. P1, P2, P3, ..., Pn and their corresponding burst times. The task is to find the average waiting time and average turnaround time using FCFS CPU Scheduling algorithm.
What is Waiting Time and Turnaround Time?
Turnaround Time is the time interval between the submission of a process and its completion.
Turnaround Time = completion of a process – submission of a process
Waiting Time is the difference between turnaround time and burst time
Waiting Time = turnaround time – burst time
What is FCFS Scheduling?
First Come, First Served (FCFS) also known as First In, First Out (FIFO) is the CPU scheduling algorithm in which the CPU is allocated to the processes in the order they are queued in the ready queue.
FCFS follows non-preemptive scheduling which mean once the CPU is allocated to a process it does not leave the CPU until the process will not get terminated or may get halted due to some I/O interrupt.
Example
Let’s say, there are four processes arriving in the sequence as P2, P3, P1 with their corresponding execution time as shown in the table below. Also, taking their arrival time to be 0.
| Process | Order of arrival | Execution time in msec |
|---|---|---|
| P1 | 3 | 15 |
| P2 | 1 | 3 |
| P3 | 2 | 3 |
Gantt chart showing the waiting time of processes P1, P2 and P3 in the system
As shown above,
The waiting time of process P2 is 0
The waiting time of process P3 is 3
The waiting time of process P1 is 6
Average time = (0 + 3 + 6) / 3 = 3 msec.
As we have taken arrival time to be 0 therefore turn around time and completion time will be same.
Example
Input-: processes = P1, P2, P3 Burst time = 5, 8, 12 Output-: Processes Burst Waiting Turn around 1 5 0 5 2 8 5 13 3 12 13 25 Average Waiting time = 6.000000 Average turn around time = 14.333333
Algorithm
Start Step 1-> In function int waitingtime(int proc[], int n, int burst_time[], int wait_time[])
Set wait_time[0] = 0
Loop For i = 1 and i < n and i++
Set wait_time[i] = burst_time[i-1] + wait_time[i-1]
End For
Step 2-> In function int turnaroundtime( int proc[], int n, int burst_time[], int wait_time[], int tat[])
Loop For i = 0 and i < n and i++
Set tat[i] = burst_time[i] + wait_time[i]
End For
Step 3-> In function int avgtime( int proc[], int n, int burst_time[])
Declare and initialize wait_time[n], tat[n], total_wt = 0, total_tat = 0;
Call waitingtime(proc, n, burst_time, wait_time)
Call turnaroundtime(proc, n, burst_time, wait_time, tat)
Loop For i=0 and i<n and i++
Set total_wt = total_wt + wait_time[i]
Set total_tat = total_tat + tat[i]
Print process number, burstime wait time and turnaround time
End For
Print "Average waiting time =i.e. total_wt / n
Print "Average turn around time = i.e. total_tat / n
Step 4-> In int main()
Declare the input int proc[] = { 1, 2, 3}
Declare and initialize n = sizeof proc / sizeof proc[0]
Declare and initialize burst_time[] = {10, 5, 8}
Call avgtime(proc, n, burst_time)
StopExample
#include <stdio.h> // Function to find the waiting time for all processes int waitingtime(int proc[], int n, int burst_time[], int wait_time[]) { // waiting time for first process is 0 wait_time[0] = 0; // calculating waiting time for (int i = 1; i < n ; i++ ) wait_time[i] = burst_time[i-1] + wait_time[i-1] ; return 0; } // Function to calculate turn around time int turnaroundtime( int proc[], int n, int burst_time[], int wait_time[], int tat[]) { // calculating turnaround time by adding // burst_time[i] + wait_time[i] int i; for ( i = 0; i < n ; i++) tat[i] = burst_time[i] + wait_time[i]; return 0; } //Function to calculate average time int avgtime( int proc[], int n, int burst_time[]) { int wait_time[n], tat[n], total_wt = 0, total_tat = 0; int i; //Function to find waiting time of all processes waitingtime(proc, n, burst_time, wait_time); //Function to find turn around time for all processes turnaroundtime(proc, n, burst_time, wait_time, tat); //Display processes along with all details printf("Processes Burst Waiting Turn around
"); // Calculate total waiting time and total turn // around time for ( i=0; i<n; i++) { total_wt = total_wt + wait_time[i]; total_tat = total_tat + tat[i]; printf(" %d\t %d\t\t %d \t%d
", i+1, burst_time[i], wait_time[i], tat[i]); } printf("Average waiting time = %f
", (float)total_wt / (float)n); printf("Average turn around time = %f
", (float)total_tat / (float)n); return 0; } // main function int main() { //process id's int proc[] = { 1, 2, 3}; int n = sizeof proc / sizeof proc[0]; //Burst time of all processes int burst_time[] = {5, 8, 12}; avgtime(proc, n, burst_time); return 0; }
Output
Processes Burst Waiting Turn around 1 5 0 5 2 8 5 13 3 12 13 25 Average Waiting time = 6.000000 Average turn around time = 14.333333
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