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.
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; } |
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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