Operating System Tutorial

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Round-Robin Scheduling Algorithm in OS

Round-Robin is a Preemptive Scheduling Algorithm and is the most commonly used algorithm in CPU Scheduling.In this, each Process is assigned with a fix time interval to execute, which is called quantum. In the round-robin, the Process gets preempted if it is executed in the given time period, and the rest of the Processes executes accordingly. Round robin is a starvation free CPU scheduling algorithm as all the Processes get fair sharing of CPU. Round-Robin is a Preemptive version of the first-come-first-serve scheduling. Time-sharing is the objective of this algorithm. In the round-robin algorithm, each Process is cyclically executed.

Advantages of the Round-Robin Scheduling Algorithm

The advantages of the round-robin scheduling algorithm are:

  1. Round-Robin is independent of starvation or convoy effect.
  2. In Round-Robin, every process gets a fair allocation of CPU.
  3. If the number of processes present in the running queue is known, then it becomes easy to estimate the worst-case response time of the process.
  4. Handles all the processes without priority.
  5. Round-Robin is easy to implement.

Disadvantages of Round-Robin Scheduling Algorithm

The disadvantages of the round-robin scheduling algorithm are:

  1. Average Waiting Time is Higher.
  2. It depends on the length of the time slice.
  3. If the time quantum is more than the response time is also more.
  4. If the time quantum is less than the context switching overhead is more.

Example of Round-Robin Scheduling Algorithm

In the following explained example, we have 4 processes with process ID P1, P2, P3, and P4. The arrival time and burst time of the proceses are given in the following table. (The Quantum time is 6).

       Process ID      Arriving Time      Burst Time  Completion Time     Turnaround Time       Waiting Time
            P1                0              8           25            25             17
            P2                1              5             11            10              5
            P3                2             10            29            27             17
            P4                3              11            34            31             20
Round-Robin Scheduling Algorithm

Steps for Gantt Chart

Step 1. In the above Gantt chart, firstly the Process P1 starts its execution, which has burst time = 8. But each process is executing only for 6 seconds because quantum time is 6. The process P2, P3, and P4 are in the waiting queue.

Step 2. At time=6, the process P1 is added to the end of the queue, and the process P2 starts its execution.

Step 3. At time=11, the process P2, which has the burst time=5, completes its execution in the given time of quantum.

Step 4. After P2, the process P3 starts its execution, which has burst time=10, but again it is also executed for 6 seconds due to the limit of quantum time. At time=17, the process P3 is added to the end of the queue.

Step 5. After P3, the next process P4 starts its execution, the burst time of P4 is 11, but it also executes for only 6 seconds.

Step 6. After P4, the process P1 again starts its execution because P1 Process left its execution due to less quantum time. So, process P1 again starts its execution to complete its execution.

Step 7. At time=29, P3 completes its execution, and P4 starts its execution again. The burst time of P4 is 11, and it is also executed for 6 intervals. At time=34, the process P4 completes its execution.

The waiting time and Turnaround time are calculated with the help of the following formula.

       Waiting Time = Turnaround time – Burst Time

       Turnaround Time = Completion time – Arrival time 

Process Turnaround Time

P1= 25-0 =0

P2= 11-1=10

P3= 29-2 = 27

P4= 34-3=31

Average Turnaround Time= 0+10+27+31/4

                                              = 68/4


Process Waiting Time

P1= 25-8=17




Average Waiting Time=17+5+17+20/4