Program for Priority Scheduling | Set 1

1.9

Priority scheduling is a non-preemptive algorithm and one of the most common scheduling algorithms in batch systems. Each process is assigned a priority. Process with the highest priority is to be executed first and so on.
Processes with the same priority are executed on first come first served basis. Priority can be decided based on memory requirements, time requirements or any other resource requirement.

Implementation :

1- First input the processes with their burst time 
   and priority.
2- Sort the processes, burst time and priority
   according to the priority.
3- Now simply apply FCFS algorithm.

prior
prior2

Note: A major problem with priority scheduling is indefinite blocking or starvation. A solution to the problem of indefinite blockage of the low-priority process is aging. Aging is a technique of gradually increasing the priority of processes that wait in the system for a long period of time.

// C++ program for implementation of FCFS
// scheduling
#include<bits/stdc++.h>
using namespace std;

struct Process
{
    int pid;  // Process ID
    int bt;   // CPU Burst time required
    int priority; // Priority of this process
};

// Function to sort the Process acc. to priority
bool comparison(Process a, Process b)
{
    return (a.priority > b.priority);
}

// Function to find the waiting time for all
// processes
void findWaitingTime(Process proc[], int n,
                     int wt[])
{
    // waiting time for first process is 0
    wt[0] = 0;

    // calculating waiting time
    for (int  i = 1; i < n ; i++ )
        wt[i] =  proc[i-1].bt + wt[i-1] ;
}

// Function to calculate turn around time
void findTurnAroundTime( Process proc[], int n,
                         int wt[], int tat[])
{
    // calculating turnaround time by adding
    // bt[i] + wt[i]
    for (int  i = 0; i < n ; i++)
        tat[i] = proc[i].bt + wt[i];
}

//Function to calculate average time
void findavgTime(Process proc[], int n)
{
    int wt[n], tat[n], total_wt = 0, total_tat = 0;

    //Function to find waiting time of all processes
    findWaitingTime(proc, n, wt);

    //Function to find turn around time for all processes
    findTurnAroundTime(proc, n, wt, tat);

    //Display processes along with all details
    cout << "\nProcesses  "<< " Burst time  "
         << " Waiting time  " << " Turn around time\n";

    // Calculate total waiting time and total turn
    // around time
    for (int  i=0; i<n; i++)
    {
        total_wt = total_wt + wt[i];
        total_tat = total_tat + tat[i];
        cout << "   " << proc[i].pid << "\t\t"
             << proc[i].bt << "\t    " << wt[i]
             << "\t\t  " << tat[i] <<endl;
    }

    cout << "\nAverage waiting time = "
         << (float)total_wt / (float)n;
    cout << "\nAverage turn around time = "
         << (float)total_tat / (float)n;
}

void priorityScheduling(Process proc[], int n)
{
    // Sort processes by priority
    sort(proc, proc + n, comparison);

    cout<< "Order in which processes gets executed \n";
    for (int  i = 0 ; i <  n; i++)
        cout << proc[i].pid <<" " ;

    findavgTime(proc, n);
}

// Driver code
int main()
{
    Process proc[] = {{1, 10, 2}, {2, 5, 0}, {3, 8, 1}};
    int n = sizeof proc / sizeof proc[0];
    priorityScheduling(proc, n);
    return 0;
}

Output:

Order in which processes gets executed 
1 3 2 
Processes  Burst time  Waiting time  Turn around time
 1		10	 0		 10
 3		8	 10		 18
 2		5	 18		 23

Average waiting time = 9.33333
Average turn around time = 17

In this post, the processes with arrival time 0 are discussed. In next set, we will be considering different arrival times to evaluate waiting times.

This article is contributed by Sahil Chhabra (akku). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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