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. The process with the highest priority is to be executed first and so on. Processes with the same priority are executed on a 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)

Javascript

// JavaScript program for implementation of Priority Scheduling
class Process {
    constructor(pid, bt, priority) {
        this.pid = pid; // Process ID
        this.bt = bt; // CPU Burst time required
        this.priority = priority; // Priority of this process
    }

    prior() {
        return this.priority;
    }
}

class GFG {
    // Function to find the waiting time for all processes
    findWaitingTime(proc, n, wt) {
        // waiting time for first process is 0
        wt[0] = 0;
        // calculating waiting time
        for (let i = 1; i < n; i++) {
            wt[i] = proc[i - 1].bt + wt[i - 1];
        }
    }

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

    // Function to calculate average time
    findavgTime(proc, n) {
        let wt = new Array(n);
        let tat = new Array(n);
        let total_wt = 0;
        let total_tat = 0;
        // Function to find waiting time of all processes
        this.findWaitingTime(proc, n, wt);

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

        // Display processes along with all details
        console.log("Processes   Burst time   Waiting time   Turn around time");

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

        console.log(
            "Average waiting time = " + total_wt / n
        );
        console.log(
            "Average turn around time = " + total_tat / n
        );
    }

    priorityScheduling(proc, n) {
        // Sort processes by priority
        proc.sort((a, b) => b.prior() - a.prior());
        console.log("Order in which processes get executed:");
        for (let i = 0; i < n; i++) {
            console.log(proc[i].pid + " ");
        }

        this.findavgTime(proc, n);
    }
}

// Driver code
let ob = new GFG();
let n = 3;
let proc = [];
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 chinmaya121221

C#

using System;
using System.Collections.Generic;

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

public int Prior
{
    get { return priority; }
}
}

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];
    int[] tat = new int[n];
    int total_wt = 0;
    int 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
    Console.WriteLine("\nProcesses   Burst time   Waiting time   Turn around time");

    // 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];
        Console.WriteLine(" " + proc[i].pid + "\t\t" + proc[i].bt + "\t " + wt[i] + "\t\t " + tat[i]);
    }

    Console.WriteLine("\nAverage waiting time = " + (float)total_wt / (float)n);
    Console.WriteLine("Average turn around time = " + (float)total_tat / (float)n);
}

public void priorityScheduling(Process[] proc, int n)
{
    // Sort processes by priority
    Array.Sort(proc, new Comparison<Process>((a, b) => b.Prior.CompareTo(a.Prior)));
    Console.WriteLine("Order in which processes gets executed ");

    for (int i = 0; i < n; i++)
        Console.Write(proc[i].pid + " ");

    findavgTime(proc, n);
}

// Driver code
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);
}
}

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

Advantages:

• Priority-based scheduling ensures that high-priority processes are executed first, which can lead to faster completion of critical tasks.
• Priority scheduling is useful for real-time systems that require processes to meet strict timing constraints.
• Priority scheduling can reduce the average waiting time for processes that require a significant amount of CPU time.

Disadvantages:

• Priority inversion: Priority inversion occurs when a low-priority process holds a resource that a high-priority process requires. This can cause delays in the execution of high-priority processes.
• Starvation: If the system is heavily loaded with high-priority processes, low-priority processes may never get a chance to execute.
• Priority inversion avoidance techniques, such as priority inheritance or priority ceiling protocols, can introduce additional complexity to the system.
• Setting priorities can be a difficult task, especially when there are many processes with different priorities.

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