Round Robin Scheduling 342b3h

Round Robin Scheduling     

Round Robin is the preemptive process scheduling algorithm. Each process is provided a fix time to execute, it is called a quantum. Once a process is executed for a given time period, it is preempted and other process executes for a given time period. Context switching is used to save states of preempted processes. This method is quite same as the FCFS but the difference is the in this case the processor will not process the whole job (process) at a time. Instead, it will complete an amount of job (quantum) at a turn and then will go to the next process and so on. When all job has got a turn, it will again start from the first job and work for a quantum of time/cycle on each job and proceed.

Wait time of each process is as follows − Process P0 P1 P2 P3

Wait Time : Service Time - Arrival Time (0 - 0) + (12 - 3) = 9 (3 - 1) = 2 (6 - 2) + (14 - 9) + (20 - 17) = 12 (9 - 3) + (17 - 12) = 11

Average Wait Time: (9+2+12+11) / 4 = 8.5 Round robin scheduling is a preemptive version of first-come, first-served scheduling. Processes are dispatched in a first-in-first-out sequence but each process is allowed to run for only a limited amount of time. This time interval is known as a time-slice or quantum. If a process does not complete or get blocked because of an I/O operation within the time slice, the time slice expires and the process is preempted. This preempted process is placed at the back of the run queue where it must wait for all the processes that were already in the queue to cycle through the U. If a process gets blocked due to an I/O operation before its time slice expires, it is, of course, enters a blocked because of that I/O operation. Once that operation completes, it is placed on the end of the run queue and waits its turn. Advantage OF Round robin

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A big advantage of round robin scheduling over non-preemptive schedulers is that it dramatically improves average response times. Round robin scheduling is fair in that every process gets an equal share of the U. It is easy to implement

Disadvantage OF Round robin 

Giving every process an equal share of the U is not always a good idea. For instance, highly interactive processes will get scheduled no more frequently than U-bound processes.