Priority Scheduling Program in C and C++

Here you will get the implementation of the priority scheduling algorithm in C and C++.

What is Priority Scheduling Algorithm?

In the priority scheduling algorithm, each process has a priority associated with it and as each process hits the queue, it is stored based on its priority so that processes with higher priority is dealt with first. It should be noted that equal priority processes are scheduled in FCFS order.

Also Read: FCFS Scheduling Program in C and C++

To prevent high priority processes from running indefinitely the scheduler may decrease the priority of the currently running process at each clock tick (i.e., at each clock interrupt). If this action causes its priority to drop below that of the next highest process, a process switch occurs. Alternatively, each process may be assigned a maximum time quantum that it is allowed to run. When this quantum is used up, the next highest priority process is given a chance to run.

Example:

Here is an example of a Priority Scheduling algorithm.

Process	Arrival Time	Burst Time	Priority
P1	0	5	2
P2	1	3	1
P3	2	6	4
P4	4	4	3
P5	6	2	5

In this example, there are 5 processes with their arrival time, burst time, and priority. The execution order, waiting time, and turnaround time for each process will be as given below.

Process	Burst Time	Waiting Time	Turnaround Time
P2	3	0	3
P1	5	3	8
P4	4	8	12
P3	6	12	18
P5	2	18	20

• Average Waiting Time = (0 + 3 + 8 + 12 + 18) / 5 = 8.2
• Average Turnaround Time = (3 + 8 + 12 + 18 + 20) / 5 = 12.2

Limitations

The problem occurs when the operating system gives a particular task a very low priority, so it sits in the queue for a larger amount of time, not being dealt with by the CPU. If this process is something the user needs, there could be a very long wait, this process is known as “Starvation” or “Infinite Blocking”.

Solution

Many operating systems use a technique called “aging”, in which a low priority process slowly gains priority over time as it sits in the queue. Even if, the priority of the process is low, there is a surety of its execution.

C Program for Priority Scheduling

#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;
}

C++ Program for Priority Scheduling

#include<iostream>
using namespace std;
int main()
{
    int bt[20],p[20],wt[20],tat[20],pr[20],i,j,n,total=0,pos,temp,avg_wt,avg_tat;
    cout<<"Enter Total Number of Process:";
    cin>>n;
    cout<<"\nEnter Burst Time and Priority\n";
    for(i=0;i<n;i++)
    {
        cout<<"\nP["<<i+1<<"]\n";
        cout<<"Burst Time:";
        cin>>bt[i];
        cout<<"Priority:";
        cin>>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;
    cout<<"\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];
        cout<<"\nP["<<p[i]<<"]\t\t  "<<bt[i]<<"\t\t    "<<wt[i]<<"\t\t\t"<<tat[i];
    }
    avg_tat=total/n;     //average turnaround time
    cout<<"\n\nAverage Waiting Time="<<avg_wt;
    cout<<"\nAverage Turnaround Time="<<avg_tat;
    return 0;
}

Output:

I hope you got the idea of how to implement a priority scheduling algorithm. Comment below for any queries related to priority scheduling.