In a multilevel queue-scheduling algorithm, procedures are permanently assigned come a queue ~ above entry come the system. Processes execute not move between queues. This setup has actually the advantage of low scheduling overhead, however the disadvantage of gift inflexible.

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Multilevel feedback queue scheduling, however, enables a procedure to move in between queues. The idea is to separate processes with different CPU-burst characteristics. If a procedure uses too much CPU time, it will be moved to a lower-priority queue. Similarly, a procedure that waits too lengthy in a lower-priority queue may be moved to a higher-priority queue. This type of aging prevents starvation.

In general, a multilevel feedback queue scheduler is defined by the following parameters:

The number of queues.

The scheduling algorithm for each queue.

The technique used to recognize when to upgrade a process to a higher-priority queue.

The method used to identify when to demote a procedure to a lower-priority queue.

The technique used to recognize which queue a process will get in when that procedure needs service.

The definition of a multilevel feedback queue scheduler makes it the most basic CPU-scheduling algorithm. It deserve to be configured to complement a specific system under design. Unfortunately, it also requires some way of choosing values for every the parameters to specify the finest scheduler. Return a multilevel feedback queue is the most basic scheme, the is also the most complex.

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An example of a multilevel feedback queue can be viewed in the over figure.

Explanation:

First that all, mean that queue 1 and 2 follow round robin with time quantum 8 and 16 respectively and also queue 3 adheres to FCFS. One of the implementations the Multilevel Feedback Queue Scheduling is together follows:

If any process starts executing then firstly it enters queue 1.

In queue 1, the process executes for 8 unit and if it completes in this 8 systems or it gives CPU because that I/O operation in these 8 systems unit 보다 the priority the this procedure does not change, and if because that some reasons it again come in the prepared queue 보다 it again starts its execution in the Queue 1.

If a procedure that is in queue 1 walk not finish in 8 units then that priority it s okay reduced and it gets shifted come queue 2.

Above clues 2 and 3 are also true for processes in queue 2 but the time quantum is 16 units. Generally, if any process does not finish in a offered time quantum climate it gets shifted to the reduced priority queue.

After the in the last queue, all procedures are booked in one FCFS manner.

It is important to keep in mind that a process that is in a lower priority queue can only execute only once the higher priority queues room empty.

Any running procedure in the reduced priority queue have the right to be interrupted by a procedure arriving in the greater priority queue.

Also, the over implementation might differ because that the example in which the last queue will certainly follow Round-robin Scheduling.

In the over Implementation, there is a problem and also that is; Any process that is in the lower priority queue needs to suffer starvation as result of some brief processes that room taking every the CPU time.

And the systems to this problem is :There is a equipment that is to boost the priority of every the procedure after constant intervals then ar all the procedures in the greatest priority queue.

The need for Multilevel Feedback Queue Scheduling(MFQS)

Following room some points to know the need for such complicated scheduling:

This scheduling is more flexible than Multilevel queue scheduling.

This algorithm help in reduce the an answer time.

In order come optimize the turnaround time, the SJF algorithm is needed which basically requires the running time of processes in order to schedule them. As we know that the to run time of procedures is not known in advance. Also, this scheduling mainly runs a procedure for a time quantum and after that, that can readjust the priority of the process if the process is long. For this reason this scheduling algorithm mostly learns indigenous the past habits of the processes and also then it deserve to predict the future actions of the processes. In this way, MFQS tries to operation a much shorter process very first which in return leads to optimize the come back time.

Advantages the MFQS

This is a functional Scheduling Algorithm

This scheduling algorithm permits different procedures to move between different queues.

In this algorithm, A process that waits too long in a reduced priority queue might be relocated to a greater priority queue which helps in preventing starvation.

Disadvantages of MFQS

This algorithm is as well complex.

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As procedures are moving roughly different waiting line which leader to the manufacturing of an ext CPU overheads.

In order to choose the ideal scheduler this algorithm calls for some other means to choose the values