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Round-Robin Scheduling in operating system-computer world

 Round-Robin Scheduling in operating system-computer world

Round-Robin Scheduling 

The round-robin(R-R Scheduling) Scheduling algorithm is designed for time sharing operating system. Round-Robin is similar to FCFS Scheduling, but preemption is added to enable the system to switch between processes. In Round-Robin Scheduling a small unit of time is used which is called a time quantum or time slice. A time quantum is generally from 10 to 100 millisecond. The CPU Scheduler goes around the ready queue, allocate the CPU to every process for a time interval of up to 1 time quantum. In this scheduling new processes are added to the tail of the ready queue. The CPU scheduler picks the first process from ready queue, set a timer after one time slice and dispatch the process. One of two things will happens. When a process have CPU burst of less the 1 time quantum. In this case the process itself release the CPU. If the CPU burst time of the currently running process is longer then one time quantum, the will go off . A context will executed, and the process will be put at the tail of the ready queue. The CPU scheduler will the select the next process in the ready queue. The average waiting time under the R-R scheduling is long.  The performance of R-R scheduling is depends on the size the time quantum. In R-R scheduling algorithm, the operating system defines a time quantum. All the process will executed in the cyclic way each process will get the CPU for a small amount of time and then get back to the ready queue to wait for its next turn. The new process is added to the ready queue. Context switching is used to save the processes in PCB. Consider the following set of processes that arrive time 0, with the length of the CPU burst given in milliseconds.

Process                                    Burst Time

 P1                                                             24

 P3                                                              3

 P3                                                              3

In this case we use time quantum of 4 milliseconds, the process p1 gets the first 4 millisecond. Since it require another 20 milliseconds, it is permitted after the first time quantum, and the CPU is given to the next process ,process p2. Process p2 does not need 4 milliseconds, so it quits before it's time quantum expires. The CPU is given to the next process , process p3 once each process received 1 time quantum, the CPU return to the process p1. 

Let's calculate the average waiting time for this schedule. P1 wait for 6 millisecond (10-4), P2 wait for 4 millisecond, p3 wait for 7 millisecond. The average waiting time is 17/3 = 5.66 millisecond.



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