Round Robin scheduling algorithm

Round Robin is a CPU scheduling algorithm where each process is assigned a fixed time slot in a cyclic way.

• 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.

操作系统规定了每个process 运行最多的时间段，运行这段 就必须换另外一个process， 每个process 都有自己的完成时间（burst time）,起始时间 arrival time，

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

举例：

这里假设一个process只能连续执行1 unit。 按p1->p2->p3顺序执行。

C++

代码实现。

#include<iostream>
using namespace std;
 
// Function to find the waiting time for all
// processes
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[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 time
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];
}
 
// Function to calculate average time
void 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 << "Processes "<< " 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 << " " << 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 code
int 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;
}