Round Robin (RR) CPU Scheduling Algorithm

 Round Robin Scheduling (RR):

The time sharing systems uses the Round Robin algorithm. The use of a small time quantum allows Round Robin to provide good response time. It is a preemptive algorithm.

 

CPU selects the process from the ready queue. To implement RR scheduling, ready queue is maintained as a FIFO queue. New processes are added to the tail of the ready queue. The CPU scheduler picks the first process from the ready queue, set a timer to interrupt after one time quantum and dispatches the process.

 

With RR algorithm, the principal design issue is the length of the time quantum or time slice to be used. If the time quantum is very short, then short processes will move through the system relatively quickly. It increases the processing overhead involved in handling the clock interrupt and performing the scheduling and dispatch function. So, a very short time quantum should be avoided.

 

Consider the set of processes with the burst time in milliseconds. All the processes arrive at time 0. The Gantt chart, waiting time and so on are as follows:

Process	Burst time
P1	3
P2	6
P3	4
P4	2

Waiting time:

Process	Waiting time
P1	0 + 6 = 6
P2	2 + 5 + 2 = 9
P3	4 + 5 =9
P4	6 = 6

Turnaround time:

 It is the sum of burst time + waiting time of each process.

 

Process	Turnaround time
P1	3 + 6 = 9
P2	6 + 9 = 15
P3	4 + 9 = 13
P4	2 + 6 = 8