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Round Robin Scheduling

Round Robin(RR) scheduling algorithm is mainly designed for time-sharing systems. This algorithm is similar to FCFS scheduling, but in Round Robin(RR) scheduling, preemption is added which enables the system to switch between processes.

  • A fixed time is allotted to each process, called a quantum, for execution.

  • Once a process is executed for the given time period that process is preempted and another process executes for the given time period.

  • Context switching is used to save states of preempted processes.

  • This algorithm is simple and easy to implement and the most important is thing is this algorithm is starvation-free as all processes get a fair share of CPU.

  • It is important to note here that the length of time quantum is generally from 10 to 100 milliseconds in length.

Some important characteristics of the Round Robin(RR) Algorithm are as follows:

  1. Round Robin Scheduling algorithm resides under the category of Preemptive Algorithms.

  2. This algorithm is one of the oldest, easiest, and fairest algorithm.

  3. This Algorithm is a real-time algorithm because it responds to the event within a specific time limit.

  4. In this algorithm, the time slice should be the minimum that is assigned to a specific task that needs to be processed. Though it may vary for different operating systems.

  5. This is a hybrid model and is clock-driven in nature.

  6. This is a widely used scheduling method in the traditional operating system.

Important terms

  1. Completion Time
    It is the time at which any process completes its execution.

  2. Turn Around Time
    This mainly indicates the time Difference between completion time and arrival time. The Formula to calculate the same is: Turn Around Time = Completion Time – Arrival Time

  3. Waiting Time(W.T):
    It Indicates the time Difference between turn around time and burst time.
    And is calculated as Waiting Time = Turn Around Time – Burst Time

Let us now cover an example for the same:

round robin scheduling

In the above diagram, arrival time is not mentioned so it is taken as 0 for all processes.

Note: If arrival time is not given for any problem statement then it is taken as 0 for all processes; if it is given then the problem can be solved accordingly.

Explanation

The value of time quantum in the above example is 5.Let us now calculate the Turn around time and waiting time for the above example :

Processes Burst Time

Turn Around Time

Turn Around Time = Completion Time – Arrival Time

Waiting Time

Waiting Time = Turn Around Time – Burst Time

P1 21 32-0=32 32-21=11
P2 3 8-0=8 8-3=5
P3 6 21-0=21 21-6=15
P4 2 15-0=15 15-2=13

Average waiting time is calculated by adding the waiting time of all processes and then dividing them by no.of processes.

average waiting time = waiting time of all processes/ no.of processes

average waiting time=11+5+15+13/4 = 44/4= 11ms

C++ Implementation For RR Scheduling

// Program implementation in C++ for Round Robin scheduling 
#include<iostream> 
using namespace std; 

//The Function to find the waiting time for all processes 
void fWaitingTime(int processes[], int n, 
			int bt[], int wt[], int quantum) 
{ 
	// Let us 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; // for Current time 

	// Let us keep traverse the processes in the round robin manner until all of them are not done.

	while (1) 
	{ 
		bool done = true; 

		//let us 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 there is a need to process further
			if (rem_bt[i] > 0) 
			{ 
				done = false; // indicates there is a pending process 

				if (rem_bt[i] > quantum) 
				{ 
					// By Increasing the value of t it shows how much time a process has been processed 
					t += quantum; 

					// Decreasing the burst_time of current process by the quantum
					rem_bt[i] -= quantum; 
				} 

				// If burst time is smaller than or equal to the quantum then it is Last cycle for this process 
				else
				{ 
					// Increase the value of t to show 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 thus remaining burst time becomes 0.
					
					rem_bt[i] = 0; 
				} 
			} 
		} 

		// If all the processes are done 
		if (done == true) 
		break; 
	} 
} 

// Function used to calculate the turn around time 
void fTurnAroundTime(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 the 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 
	fWaitingTime(processes, n, bt, wt, quantum); 

	// Function to find turn around time for all processes 
	fTurnAroundTime(processes, n, bt, wt, tat); 

	// Display processes along with all details 
	cout << "Processes "<< " Burst time "
		<< " Waiting time " << " Turn around time\n"; 

	// Calculate the 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; 
} 

//Given below is the Driver Code
int main() 
{ 
	// process id's 
	int processes[] = { 1, 2, 3,4}; 
	int x = sizeof processes / sizeof processes[0]; 

	// Burst time of all processes 
	int burst_time[] = {21, 13, 6,12}; 

	// Time quantum 
	int quantum = 2; 
	findavgTime(processes, x, burst_time, quantum); 
	return 0; 
} 

Output

The output of the above code is as follows:

round robin algo output

Advantages of Round Robin Scheduling Algorithm

Some advantages of the Round Robin scheduling algorithm are as follows:

  • While performing this scheduling algorithm, a particular time quantum is allocated to different jobs.

  • In terms of average response time, this algorithm gives the best performance.

  • With the help of this algorithm, all the jobs get a fair allocation of CPU.

  • In this algorithm, there are no issues of starvation or convoy effect.

  • This algorithm deals with all processes without any priority.

  • This algorithm is cyclic in nature.

  • In this, the newly created process is added to the end of the ready queue.

  • Also, in this, a round-robin scheduler generally employs time-sharing which means providing each job a time slot or quantum.

  • In this scheduling algorithm, each process gets a chance to reschedule after a particular quantum time.

Disadvantages of Round Robin Scheduling Algorithm

Some disadvantages of the Round Robin scheduling algorithm are as follows:

  • This algorithm spends more time on context switches.

  • For small quantum, it is time-consuming scheduling.

  • This algorithm offers a larger waiting time and response time.

  • In this, there is low throughput.

  • If time quantum is less for scheduling then its Gantt chart seems to be too big.

Some Points to Remember

1.Decreasing value of Time quantum

With the decreasing value of time quantum

  • The number of context switches increases.

  • The Response Time decreases

  • Chances of starvation decrease in this case.

For the smaller value of time quantum, it becomes better in terms of response time.

2.Increasing value of Time quantum

With the increasing value of time quantum

  • The number of context switch decreases

  • The Response Time increases

  • Chances of starvation increases in this case.

For the higher value of time quantum, it becomes better in terms of the number of the context switches.

3. If the value of time quantum is increasing then Round Robin Scheduling tends to become FCFS Scheduling.

4.In this case, when the value of time quantum tends to infinity then the Round Robin Scheduling becomes FCFS Scheduling.

5. Thus the performance of Round Robin scheduling mainly depends on the value of the time quantum.

6.And the value of the time quantum should be such that it is neither too big nor too small.