Round Robin

  

Round Robin

·         Round robin Scheduling is similar to FCFS but preemption is addede to switch between processes.

·         In RR scheduling, processes are dispatched in FIFO but given a small amount of CPU time. This small amount of CPU time this small amount of time is known as time quantum or time slice. A time quantum is generally from 10 to 100 milliseconds

·         If a process does not complete before its time slice expires, the CPU is time slice and is given to the next waiting process in ready queue.

·         The preempted process in then places at the  tail of the ready queue.

·         If a process is completed before its time slice expires, the process itself release the CPU. The scheduler then proceeds to the next process in ready queue.

·         Whenever any new process arrives in the system it is added at the tail of the ready queue.

·         Round robin scheduling is effective in timesharing environment in which the system need to guarantee reasonable response time for interactive users.

·         The performance of round robin scheduling algorithm depends upon severalfactors these are:-

1. Size Of Time Quantum Or Slice

·         If the size of time is considerably larger then this algorithm becomes same as FCFS algorithm and thus performance degrades.

·         If the size of time slice is very small than the number of context switches increses and CPU will be busy in switches itself from one process to another rather than performing calculations

·         Thus, the time quantum should not be very and also should not be too small to achieve good system performance

2. Context Switches Overhead

·         Context switches is performed frequently in round robin scheduling as compared to FCFS or SJF scheduling

·         Context switch in round robimn scheduling should be planned in such as way that it takes very less time.

·         In other words, the ratio between the context switching time and the calculation or execution time should be low.

Example of Round-robin Scheduling

Consider this following three processes

Process Queue	Burst time
P1	4
P2	3
P3	5

Step 1) The execution begins with process P1, which has burst time 4. Here, every process executes for 2 seconds. P2 and P3 are still in the waiting queue.

Step 2) At time =2, P1 is added to the end of the Queue and P2 starts executing

Step 3) At time=4 , P2 is preempted and add at the end of the queue. P3 starts executing.

Step 4) At time=6 , P3 is preempted and add at the end of the queue. P1 starts executing.

Step 5) At time=8 , P1 has a burst time of 4. It has completed execution. P2 starts execution

Step 6) P2 has a burst time of 3. It has already executed for 2 interval. At time=9, P2 completes execution. Then, P3 starts execution till it completes.

Step 7) Let's calculate the average waiting time for above example.

Wait time 
P1= 0+ 4= 4
P2= 2+4= 6
P3= 4+3= 7

Advantage of Round-robin Scheduling

Here, are pros/benefits of Round-robin scheduling method:

• It doesn't face the issues of starvation or convoy effect.
• All the jobs get a fair allocation of CPU.
• It deals with all process without any priority
• If you know the total number of processes on the run queue, then you can also assume the worst-case response time for the same process.
• This scheduling method does not depend upon burst time. That's why it is easily implementable on the system.
• Once a process is executed for a specific set of the period, the process is preempted, and another process executes for that given time period.
• Allows OS to use the Context switching method to save states of preempted processes.
• It gives the best performance in terms of average response time.

Disadvantages of Round-robin Scheduling

Here, are drawbacks/cons of using Round-robin scheduling:

• If slicing time of OS is low, the processor output will be reduced.
• This method spends more time on context switching
• Its performance heavily depends on time quantum.
• Priorities cannot be set for the processes.
• Round-robin scheduling doesn't give special priority to more important tasks.
• Decreases comprehension
• Lower time quantum results in higher the context switching overhead in the system.
• Finding a correct time quantum is a quite difficult task in this system.