pre-emptive Priority scheduling algorithm in c code example

Example 1: c program to implement non preemptive priority scheduling algorithm

#include<stdio.h>
 
int main()
{
    int bt[20],p[20],wt[20],tat[20],pr[20],i,j,n,total=0,pos,temp,avg_wt,avg_tat;
    printf("Enter Total Number of Process:");
    scanf("%d",&n);
 
    printf("\nEnter Burst Time and Priority\n");
    for(i=0;i<n;i++)
    {
        printf("\nP[%d]\n",i+1);
        printf("Burst Time:");
        scanf("%d",&bt[i]);
        printf("Priority:");
        scanf("%d",&pr[i]);
        p[i]=i+1;           //contains process number
    }
 
    //sorting burst time, priority and process number in ascending order using selection sort
    for(i=0;i<n;i++)
    {
        pos=i;
        for(j=i+1;j<n;j++)
        {
            if(pr[j]<pr[pos])
                pos=j;
        }
 
        temp=pr[i];
        pr[i]=pr[pos];
        pr[pos]=temp;
 
        temp=bt[i];
        bt[i]=bt[pos];
        bt[pos]=temp;
 
        temp=p[i];
        p[i]=p[pos];
        p[pos]=temp;
    }
 
    wt[0]=0;	//waiting time for first process is zero
 
    //calculate waiting time
    for(i=1;i<n;i++)
    {
        wt[i]=0;
        for(j=0;j<i;j++)
            wt[i]+=bt[j];
 
        total+=wt[i];
    }
 
    avg_wt=total/n;      //average waiting time
    total=0;
 
    printf("\nProcess\t    Burst Time    \tWaiting Time\tTurnaround Time");
    for(i=0;i<n;i++)
    {
        tat[i]=bt[i]+wt[i];     //calculate turnaround time
        total+=tat[i];
        printf("\nP[%d]\t\t  %d\t\t    %d\t\t\t%d",p[i],bt[i],wt[i],tat[i]);
    }
 
    avg_tat=total/n;     //average turnaround time
    printf("\n\nAverage Waiting Time=%d",avg_wt);
    printf("\nAverage Turnaround Time=%d\n",avg_tat);
 
	return 0;
}

Example 2: preemptive priority scheduling implementation in c

#include <iostream>
#include <algorithm> 
#include <iomanip>
#include <string.h> 
using namespace std;

struct process {
    int pid;
    int arrival_time;
    int burst_time;
    int priority;
    int start_time;
    int completion_time;
    int turnaround_time;
    int waiting_time;
    int response_time;
};

int main() {

    int n;
    struct process p[100];
    float avg_turnaround_time;
    float avg_waiting_time;
    float avg_response_time;
    float cpu_utilisation;
    int total_turnaround_time = 0;
    int total_waiting_time = 0;
    int total_response_time = 0;
    int total_idle_time = 0;
    float throughput;
    int burst_remaining[100];
    int is_completed[100];
    memset(is_completed,0,sizeof(is_completed));

    cout << setprecision(2) << fixed;

    cout<<"Enter the number of processes: ";
    cin>>n;

    for(int i = 0; i < n; i++) {
        cout<<"Enter arrival time of process "<<i+1<<": ";
        cin>>p[i].arrival_time;
        cout<<"Enter burst time of process "<<i+1<<": ";
        cin>>p[i].burst_time;
        cout<<"Enter priority of the process "<<i+1<<": ";
        cin>>p[i].priority;
        p[i].pid = i+1;
        burst_remaining[i] = p[i].burst_time;
        cout<<endl;
    }

    int current_time = 0;
    int completed = 0;
    int prev = 0;

    while(completed != n) {
        int idx = -1;
        int mx = -1;
        for(int i = 0; i < n; i++) {
            if(p[i].arrival_time <= current_time && is_completed[i] == 0) {
                if(p[i].priority > mx) {
                    mx = p[i].priority;
                    idx = i;
                }
                if(p[i].priority == mx) {
                    if(p[i].arrival_time < p[idx].arrival_time) {
                        mx = p[i].priority;
                        idx = i;
                    }
                }
            }
        }

        if(idx != -1) {
            if(burst_remaining[idx] == p[idx].burst_time) {
                p[idx].start_time = current_time;
                total_idle_time += p[idx].start_time - prev;
            }
            burst_remaining[idx] -= 1;
            current_time++;
            prev = current_time;
            
            if(burst_remaining[idx] == 0) {
                p[idx].completion_time = current_time;
                p[idx].turnaround_time = p[idx].completion_time - p[idx].arrival_time;
                p[idx].waiting_time = p[idx].turnaround_time - p[idx].burst_time;
                p[idx].response_time = p[idx].start_time - p[idx].arrival_time;

                total_turnaround_time += p[idx].turnaround_time;
                total_waiting_time += p[idx].waiting_time;
                total_response_time += p[idx].response_time;

                is_completed[idx] = 1;
                completed++;
            }
        }
        else {
             current_time++;
        }  
    }

    int min_arrival_time = 10000000;
    int max_completion_time = -1;
    for(int i = 0; i < n; i++) {
        min_arrival_time = min(min_arrival_time,p[i].arrival_time);
        max_completion_time = max(max_completion_time,p[i].completion_time);
    }

    avg_turnaround_time = (float) total_turnaround_time / n;
    avg_waiting_time = (float) total_waiting_time / n;
    avg_response_time = (float) total_response_time / n;
    cpu_utilisation = ((max_completion_time - total_idle_time) / (float) max_completion_time )*100;
    throughput = float(n) / (max_completion_time - min_arrival_time);

    cout<<endl<<endl;

    cout<<"#P\t"<<"AT\t"<<"BT\t"<<"PRI\t"<<"ST\t"<<"CT\t"<<"TAT\t"<<"WT\t"<<"RT\t"<<"\n"<<endl;

    for(int i = 0; i < n; i++) {
        cout<<p[i].pid<<"\t"<<p[i].arrival_time<<"\t"<<p[i].burst_time<<"\t"<<p[i].priority<<"\t"<<p[i].start_time<<"\t"<<p[i].completion_time<<"\t"<<p[i].turnaround_time<<"\t"<<p[i].waiting_time<<"\t"<<p[i].response_time<<"\t"<<"\n"<<endl;
    }
    cout<<"Average Turnaround Time = "<<avg_turnaround_time<<endl;
    cout<<"Average Waiting Time = "<<avg_waiting_time<<endl;
    cout<<"Average Response Time = "<<avg_response_time<<endl;
    cout<<"CPU Utilization = "<<cpu_utilisation<<"%"<<endl;
    cout<<"Throughput = "<<throughput<<" process/unit time"<<endl;


}

/*

AT - Arrival Time of the process
BT - Burst time of the process
ST - Start time of the process
CT - Completion time of the process
TAT - Turnaround time of the process
WT - Waiting time of the process
RT - Response time of the process

Formulas used:

TAT = CT - AT
WT = TAT - BT
RT = ST - AT

*/

}