Program for Priority CPU Scheduling | Set 1

Priority scheduling is 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. Also priority can be decided on the ratio of average I/O to average CPU burst time.
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.

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++

// 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;
}

Java

// Java program for implementation of FCFS
// scheduling
import java.util.*;

class Process
{
    int pid; // Process ID
    int bt; // CPU Burst time required
    int priority; // Priority of this process
    Process(int pid, int bt, int priority)
    {
        this.pid = pid;
        this.bt = bt;
        this.priority = priority;
    }
    public int prior() {
        return priority;
    }
}


public class GFG
{

// Function to find the waiting time for all
// processes
public 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
public 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
public void findavgTime(Process proc[], int n)
{
    int wt[] = new int[n], tat[] = new int[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
    System.out.print("\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];
        System.out.print(" " + proc[i].pid + "\t\t" + proc[i].bt + "\t " + wt[i] + "\t\t " + tat[i] + "\n");
    }

    System.out.print("\nAverage waiting time = "
            +(float)total_wt / (float)n);
    System.out.print("\nAverage turn around time = "+(float)total_tat / (float)n);
}

public void priorityScheduling(Process proc[], int n)
{

    // Sort processes by priority
    Arrays.sort(proc, new Comparator<Process>() {
        @Override
        public int compare(Process a, Process b) {
            return b.prior() - a.prior();
        }
    });
    System.out.print("Order in which processes gets executed \n");
    for (int i = 0 ; i < n; i++)
        System.out.print(proc[i].pid + " ") ;

    findavgTime(proc, n);
}

// Driver code
public static void main(String[] args)
{
    GFG ob=new GFG();
    int n = 3;
    Process proc[] = new Process[n];
    proc[0] = new Process(1, 10, 2);
    proc[1] = new Process(2, 5, 0);
    proc[2] = new Process(3, 8, 1);
    ob.priorityScheduling(proc, n);
}
}

// This code is contributed by rahulpatil07109.

Python3

# Python3 program for implementation of
# Priority Scheduling

# Function to find the waiting time
# for all processes
def findWaitingTime(processes, n, wt):
    wt[0] = 0

    # calculating waiting time
    for i in range(1, n):
        wt[i] = processes[i - 1][1] + wt[i - 1]

# Function to calculate turn around time
def findTurnAroundTime(processes, n, wt, tat):

    # Calculating turnaround time by
    # adding bt[i] + wt[i]
    for i in range(n):
        tat[i] = processes[i][1] + wt[i]

# Function to calculate average waiting
# and turn-around times.
def findavgTime(processes, n):
    wt = [0] * n
    tat = [0] * n

    # Function to find waiting time
    # of all processes
    findWaitingTime(processes, n, wt)

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

    # Display processes along with all details
    print("\nProcesses    Burst Time    Waiting",
          "Time    Turn-Around Time")
    total_wt = 0
    total_tat = 0
    for i in range(n):

        total_wt = total_wt + wt[i]
        total_tat = total_tat + tat[i]
        print(" ", processes[i][0], "\t\t",
                   processes[i][1], "\t\t",
                   wt[i], "\t\t", tat[i])

    print("\nAverage waiting time = %.5f "%(total_wt /n))
    print("Average turn around time = ", total_tat / n)

def priorityScheduling(proc, n):

    # Sort processes by priority
    proc = sorted(proc, key = lambda proc:proc[2],
                                  reverse = True);

    print("Order in which processes gets executed")
    for i in proc:
        print(i[0], end = " ")
    findavgTime(proc, n)

# Driver code
if __name__ =="__main__":

    # Process id's
    proc = [[1, 10, 1],
            [2, 5, 0],
            [3, 8, 1]]
    n = 3
    priorityScheduling(proc, n)

# This code is contributed
# Shubham Singh(SHUBHAMSINGH10)

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.
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