Program for Round Robin scheduling | Set 1 - GeeksforGeeks

04 May, 2022
Round Robin is a CPU scheduling algorithm where each process is assigned a fixed time slot in a cyclic way. It is basically the preemptive version of First come First Serve CPU Scheduling algorithm. 
Round Robin CPU Algorithm generally focuses on Time Sharing technique. 
The period of time for which a process or job is allowed to run in a pre-emptive method is called time quantum. 
Each process or job present in the ready queue is assigned the CPU for that time quantum, if the execution of the process is completed during that time then the process will end else the process will go back to the waiting table and wait for the its next turn to complete the execution. 
It is simple, easy to implement, and starvation-free as all processes get fair share of CPU.
One of the most commonly used technique in CPU scheduling as a core.
It is preemptive as processes are assigned CPU only for a fixed slice of time at most.
The disadvantage of it is more overhead of context switching.
There is fairness since every process gets equal share of CPU.
The newly created process is added to end of ready queue.
A round-robin scheduler generally employs time-sharing, giving each job a time slot or quantum. 
While performing a round-robin scheduling,a particular time quantum is alloted to different jobs. 
Each process get a chance to reschedule after a particular quantum time in this scheduling. 
There is Larger waiting time and Response time.
There is Low throughput.
There is Context Switches.
Gantt chart seems to come too big (if quantum time is less for scheduling.For Example:1 ms for big scheduling.)
Time consuming scheduling for small quantums .
Example-1: Consider the following table of arrival time and burst time for five processes P1, P2, P3, and P4 and given Time Quantum = 2
The Round Robin CPU Scheduling Algorithm will work on the basis of steps as mentioned below:
At time = 0,
The execution begins with process P1, which has burst time 5. 
Here, every process executes for 2 seconds (Time Quantum Period). P2 and P3 are still in the waiting queue.
At time = 2,
The processes P2 and P3 arrives in the ready queue and P2 starts executing for TQ period
At time = 4,
The process P4 arrives in the ready queue, 
Then P3 executes for TQ period.
At time = 6,
Process P3 completes its execution
Process P1 starts executing for TQ period as it is next in the b.
At time = 8,
Process P4 starts executing, it will not execute for Time Quantum period as it has burst time = 1
Hence, it will execute for only 1ms. 
At time = 9,
Process P4 completes its execution
Process P2 starts executing for TQ period as it is next in the ready queue
At time = 11,
Process P2 completes its execution.
Process P1 starts executing, it will execute for 1ms only
At time = 12,
Process P1 completes its execution.
The overall execution of the processes will be as shown below:
Gantt chart will be as following below: 
Gantt chart for Round Robin Scheduling Algorithm
How to compute below times in Round Robin using a program? 
Completion Time: Time at which process completes its execution.
Turn Around Time: Time Difference between completion time and arrival time. Turn Around Time = Completion Time – Arrival Time
Waiting Time(W.T): Time Difference between turn around time and burst time. Waiting Time = Turn Around Time – Burst Time
Now, lets calculate average waiting time and turn around time:
Now, 
Average Turn around time = (12 + 10 + 4 + 5)/4 = 31/4 = 7.7
Average waiting time = (7 + 6 + 2 + 4)/4 = 19/4 = 4.7
Example 2: Consider the following table of arrival time and burst time for four processes P1, P2 and P3 and given Time Quantum = 2
Similarly, Gantt chart for this example:
Gantt chart for example 2
Now, lets calculate average waiting time and turn around time:
Total Turn Around Time = 59 msSo, Average Turn Around Time = 59/3 = 19.667 ms
And, Total Waiting Time = 36 msSo, Average Waiting Time = 36/3 = 12.00 ms 
Steps to find waiting times of all processes: 
Create an array rem_bt[] to keep track of remaining burst time of processes. This array is initially a copy of bt[] (burst times array)
Create another array wt[] to store waiting times of processes. Initialize this array as 0.
Initialize time : t = 0
Keep traversing the all processes while all processes are not done. Do following for i’th process if it i not done yet.If rem_bt[i] > quantumt = t + quantumrem_bt[i] -= quantum;Else // Last cycle for this processt = t + rem_bt[i];wt[i] = t – bt[i]rem_bt[i] = 0; // This process is over
If rem_bt[i] > quantumt = t + quantumrem_bt[i] -= quantum;
t = t + quantum
rem_bt[i] -= quantum;
Else // Last cycle for this processt = t + rem_bt[i];wt[i] = t – bt[i]rem_bt[i] = 0; // This process is over
t = t + rem_bt[i];
wt[i] = t – bt[i]
rem_bt[i] = 0; // This process is over
Once we have waiting times, we can compute turn around time tat[i] of a process as sum of waiting and burst times, i.e., wt[i] + bt[i]
Below is implementation of above steps. 
C++
Java
Python3
C#
Javascript
// C++ program for implementation of RR scheduling#include<iostream>using namespace std; // Function to find the waiting time for all// processesvoid findWaitingTime(int processes[], int n,            int bt[], int wt[], int quantum){    // Make a copy of burst times bt[] to store remaining    // burst times.    int rem_bt[n];    for (int i = 0 ; i < n ; i++)        rem_bt[i] = bt[i];     int t = 0; // Current time     // Keep traversing processes in round robin manner    // until all of them are not done.    while (1)    {        bool done = true;         // Traverse all processes one by one repeatedly        for (int i = 0 ; i < n; i++)        {            // If burst time of a process is greater than 0            // then only need to process further            if (rem_bt[i] > 0)            {                done = false; // There is a pending process                 if (rem_bt[i] > quantum)                {                    // Increase the value of t i.e. shows                    // how much time a process has been processed                    t += quantum;                     // Decrease the burst_time of current process                    // by quantum                    rem_bt[i] -= quantum;                }                 // If burst time is smaller than or equal to                // quantum. Last cycle for this process                else                {                    // Increase the value of t i.e. shows                    // how much time a process has been processed                    t = t + rem_bt[i];                     // Waiting time is current time minus time                    // used by this process                    wt[i] = t - bt[i];                     // As the process gets fully executed                    // make its remaining burst time = 0                    rem_bt[i] = 0;                }            }        }         // If all processes are done        if (done == true)        break;    }} // Function to calculate turn around timevoid findTurnAroundTime(int processes[], int n,                        int bt[], int wt[], int tat[]){    // calculating turnaround time by adding    // bt[i] + wt[i]    for (int i = 0; i < n ; i++)        tat[i] = bt[i] + wt[i];} // Function to calculate average timevoid findavgTime(int processes[], int n, int bt[],                                    int quantum){    int wt[n], tat[n], total_wt = 0, total_tat = 0;     // Function to find waiting time of all processes    findWaitingTime(processes, n, bt, wt, quantum);     // Function to find turn around time for all processes    findTurnAroundTime(processes, n, bt, wt, tat);     // Display processes along with all details    cout << "PN\t "<< " \tBT "        << "  WT " << " \tTAT\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 << " " << i+1 << "\t\t" << bt[i] <<"\t "            << wt[i] <<"\t\t " << tat[i] <<endl;    }     cout << "Average waiting time = "        << (float)total_wt / (float)n;    cout << "\nAverage turn around time = "        << (float)total_tat / (float)n;} // Driver codeint main(){    // process id's    int processes[] = { 1, 2, 3};    int n = sizeof processes / sizeof processes[0];     // Burst time of all processes    int burst_time[] = {10, 5, 8};     // Time quantum    int quantum = 2;    findavgTime(processes, n, burst_time, quantum);    return 0;}
// Java program for implementation of RR scheduling public class GFG{    // Method to find the waiting time for all    // processes    static void findWaitingTime(int processes[], int n,                 int bt[], int wt[], int quantum)    {        // Make a copy of burst times bt[] to store remaining        // burst times.        int rem_bt[] = new int[n];        for (int i = 0 ; i < n ; i++)            rem_bt[i] =  bt[i];              int t = 0; // Current time              // Keep traversing processes in round robin manner        // until all of them are not done.        while(true)        {            boolean done = true;                  // Traverse all processes one by one repeatedly            for (int i = 0 ; i < n; i++)            {                // If burst time of a process is greater than 0                // then only need to process further                if (rem_bt[i] > 0)                {                    done = false; // There is a pending process                          if (rem_bt[i] > quantum)                    {                        // Increase the value of t i.e. shows                        // how much time a process has been processed                        t += quantum;                              // Decrease the burst_time of current process                        // by quantum                        rem_bt[i] -= quantum;                    }                          // If burst time is smaller than or equal to                    // quantum. Last cycle for this process                    else                    {                        // Increase the value of t i.e. shows                        // how much time a process has been processed                        t = t + rem_bt[i];                              // Waiting time is current time minus time                        // used by this process                        wt[i] = t - bt[i];                              // As the process gets fully executed                        // make its remaining burst time = 0                        rem_bt[i] = 0;                    }                }            }                  // If all processes are done            if (done == true)              break;        }    }          // Method to calculate turn around time    static void findTurnAroundTime(int processes[], int n,                            int bt[], int wt[], int tat[])    {        // calculating turnaround time by adding        // bt[i] + wt[i]        for (int i = 0; i < n ; i++)            tat[i] = bt[i] + wt[i];    }          // Method to calculate average time    static void findavgTime(int processes[], int n, int bt[],                                         int quantum)    {        int wt[] = new int[n], tat[] = new int[n];        int total_wt = 0, total_tat = 0;              // Function to find waiting time of all processes        findWaitingTime(processes, n, bt, wt, quantum);              // Function to find turn around time for all processes        findTurnAroundTime(processes, n, bt, wt, tat);              // Display processes along with all details        System.out.println("PN " + " B " +                      " WT " + " TAT");              // 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.println(" " + (i+1) + "\t\t" + bt[i] +"\t " +                              wt[i] +"\t\t " + tat[i]);        }              System.out.println("Average waiting time = " +                          (float)total_wt / (float)n);        System.out.println("Average turn around time = " +                           (float)total_tat / (float)n);    }         // Driver Method    public static void main(String[] args)    {        // process id's        int processes[] = { 1, 2, 3};        int n = processes.length;              // Burst time of all processes        int burst_time[] = {10, 5, 8};              // Time quantum        int quantum = 2;        findavgTime(processes, n, burst_time, quantum);    }}
# Python3 program for implementation of# RR scheduling # Function to find the waiting time# for all processesdef findWaitingTime(processes, n, bt,                         wt, quantum):    rem_bt = [0] * n     # Copy the burst time into rt[]    for i in range(n):        rem_bt[i] = bt[i]    t = 0 # Current time     # Keep traversing processes in round    # robin manner until all of them are    # not done.    while(1):        done = True         # Traverse all processes one by        # one repeatedly        for i in range(n):                         # If burst time of a process is greater            # than 0 then only need to process further            if (rem_bt[i] > 0) :                done = False # There is a pending process                                 if (rem_bt[i] > quantum) :                                     # Increase the value of t i.e. shows                    # how much time a process has been processed                    t += quantum                     # Decrease the burst_time of current                    # process by quantum                    rem_bt[i] -= quantum                                 # If burst time is smaller than or equal                 # to quantum. Last cycle for this process                else:                                     # Increase the value of t i.e. shows                    # how much time a process has been processed                    t = t + rem_bt[i]                     # Waiting time is current time minus                    # time used by this process                    wt[i] = t - bt[i]                     # As the process gets fully executed                    # make its remaining burst time = 0                    rem_bt[i] = 0                         # If all processes are done        if (done == True):            break             # Function to calculate turn around timedef findTurnAroundTime(processes, n, bt, wt, tat):         # Calculating turnaround time    for i in range(n):        tat[i] = bt[i] + wt[i]  # Function to calculate average waiting# and turn-around times.def findavgTime(processes, n, bt, quantum):    wt = [0] * n    tat = [0] * n     # Function to find waiting time    # of all processes    findWaitingTime(processes, n, bt,                         wt, quantum)     # Function to find turn around time    # for all processes    findTurnAroundTime(processes, n, bt,                                wt, tat)     # Display processes along with all details    print("Processes    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(" ", i + 1, "\t\t", bt[i],              "\t\t", wt[i], "\t\t", tat[i])     print("\nAverage waiting time = %.5f "%(total_wt /n) )    print("Average turn around time = %.5f "% (total_tat / n))     # Driver codeif __name__ =="__main__":         # Process id's    proc = [1, 2, 3]    n = 3     # Burst time of all processes    burst_time = [10, 5, 8]     # Time quantum    quantum = 2;    findavgTime(proc, n, burst_time, quantum) # This code is contributed by# Shubham Singh(SHUBHAMSINGH10)
// C# program for implementation of RR// schedulingusing System; public class GFG {         // Method to find the waiting time    // for all processes    static void findWaitingTime(int []processes,             int n, int []bt, int []wt, int quantum)    {                 // Make a copy of burst times bt[] to        // store remaining burst times.        int []rem_bt = new int[n];                 for (int i = 0 ; i < n ; i++)            rem_bt[i] = bt[i];             int t = 0; // Current time             // Keep traversing processes in round        // robin manner until all of them are        // not done.        while(true)        {            bool done = true;                 // Traverse all processes one by            // one repeatedly            for (int i = 0 ; i < n; i++)            {                // If burst time of a process                // is greater than 0 then only                // need to process further                if (rem_bt[i] > 0)                {                                         // There is a pending process                    done = false;                         if (rem_bt[i] > quantum)                    {                        // Increase the value of t i.e.                        // shows how much time a process                        // has been processed                        t += quantum;                             // Decrease the burst_time of                        // current process by quantum                        rem_bt[i] -= quantum;                    }                         // If burst time is smaller than                    // or equal to quantum. Last cycle                    // for this process                    else                    {                                                 // Increase the value of t i.e.                        // shows how much time a process                        // has been processed                        t = t + rem_bt[i];                             // Waiting time is current                        // time minus time used by                        // this process                        wt[i] = t - bt[i];                             // As the process gets fully                        // executed make its remaining                        // burst time = 0                        rem_bt[i] = 0;                    }                }            }                 // If all processes are done            if (done == true)            break;        }    }         // Method to calculate turn around time    static void findTurnAroundTime(int []processes,               int n, int []bt, int []wt, int []tat)    {        // calculating turnaround time by adding        // bt[i] + wt[i]        for (int i = 0; i < n ; i++)            tat[i] = bt[i] + wt[i];    }         // Method to calculate average time    static void findavgTime(int []processes, int n,                             int []bt, int quantum)    {        int []wt = new int[n];        int []tat = new int[n];        int total_wt = 0, total_tat = 0;             // Function to find waiting time of        // all processes        findWaitingTime(processes, n, bt, wt, quantum);             // Function to find turn around time        // for all processes        findTurnAroundTime(processes, n, bt, wt, tat);             // Display processes along with        // all details        Console.WriteLine("Processes " + " 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(" " + (i+1) + "\t\t" + bt[i]                         + "\t " + wt[i] +"\t\t " + tat[i]);        }             Console.WriteLine("Average waiting time = " +                        (float)total_wt / (float)n);        Console.Write("Average turn around time = " +                        (float)total_tat / (float)n);    }         // Driver Method    public static void Main()    {        // process id's        int []processes = { 1, 2, 3};        int n = processes.Length;             // Burst time of all processes        int []burst_time = {10, 5, 8};             // Time quantum        int quantum = 2;        findavgTime(processes, n, burst_time, quantum);    }} // This code is contributed by nitin mittal.
<script>    // JavaScript program for implementation of RR scheduling     // Function to find the waiting time for all    // processes    const findWaitingTime = (processes, n, bt, wt, quantum) => {        // Make a copy of burst times bt[] to store remaining        // burst times.        let rem_bt = new Array(n).fill(0);        for (let i = 0; i < n; i++)            rem_bt[i] = bt[i];         let t = 0; // Current time         // Keep traversing processes in round robin manner        // until all of them are not done.        while (1) {            let done = true;             // Traverse all processes one by one repeatedly            for (let i = 0; i < n; i++) {                // If burst time of a process is greater than 0                // then only need to process further                if (rem_bt[i] > 0) {                    done = false; // There is a pending process                     if (rem_bt[i] > quantum) {                        // Increase the value of t i.e. shows                        // how much time a process has been processed                        t += quantum;                         // Decrease the burst_time of current process                        // by quantum                        rem_bt[i] -= quantum;                    }                     // If burst time is smaller than or equal to                    // quantum. Last cycle for this process                    else {                        // Increase the value of t i.e. shows                        // how much time a process has been processed                        t = t + rem_bt[i];                         // Waiting time is current time minus time                        // used by this process                        wt[i] = t - bt[i];                         // As the process gets fully executed                        // make its remaining burst time = 0                        rem_bt[i] = 0;                    }                }            }             // If all processes are done            if (done == true)                break;        }    }     // Function to calculate turn around time    const findTurnAroundTime = (processes, n, bt, wt, tat) => {        // calculating turnaround time by adding        // bt[i] + wt[i]        for (let i = 0; i < n; i++)            tat[i] = bt[i] + wt[i];    }     // Function to calculate average time    const findavgTime = (processes, n, bt, quantum) => {        let wt = new Array(n).fill(0), tat = new Array(n).fill(0);        let total_wt = 0, total_tat = 0;         // Function to find waiting time of all processes        findWaitingTime(processes, n, bt, wt, quantum);         // Function to find turn around time for all processes        findTurnAroundTime(processes, n, bt, wt, tat);         // Display processes along with all details        document.write(`Processes Burst time Waiting time Turn around time<br/>`);         // 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];             document.write(`${i + 1} ${bt[i]} ${wt[i]} ${tat[i]}<br/>`);        }         document.write(`Average waiting time = ${total_wt / n}`);        document.write(`<br/>Average turn around time = ${total_tat / n}`);    }     // Driver code    // process id's    processes = [1, 2, 3];    let n = processes.length;     // Burst time of all processes    let burst_time = [10, 5, 8];     // Time quantum    let quantum = 2;    findavgTime(processes, n, burst_time, quantum);     // This code is contributed by rakeshsahni </script>
PN          BT   WT      TAT
 1        10     13         23
 2        5     10         15
 3        8     13         21
Average waiting time = 12
Average turn around time = 19.6667
For detailed implementation of Preemptive Round Robin algorithm with different arrival times for all processes please refer: Program for Round Robin Scheduling with different arrival times. 
This article is contributed by Sahil Chhabra. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
nitin mittal
SHUBHAMSINGH10
AshishBansal
rakeshsahni
kashishkumar2
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