In a multilevel queue-scheduling algorithm, processes are permanently assigned to a queue on entry come the system. Processes carry out not move in between queues. This setup has the advantage of low scheduling overhead, yet the disadvantage of being inflexible.

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Multilevel feedback queue scheduling, however, enables a procedure to move between queues. The idea is come separate procedures with various CPU-burst characteristics. If a procedure uses too lot CPU time, it will be relocated to a lower-priority queue. Similarly, a process that waits too long in a lower-priority queue might be moved to a higher-priority queue. This form of aging avoids starvation.

In general, a multilevel feedback queue scheduler is characterized by the adhering to parameters:

The variety of queues.

The scheduling algorithm for each queue.

The an approach used to recognize when to update a process to a higher-priority queue.

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

The technique used to determine which queue a procedure will enter when that procedure needs service.

The meaning of a multilevel feedback queue scheduler makes it the most general CPU-scheduling algorithm. It can be configured to complement a certain system under design. Unfortunately, it additionally requires some method of picking values for every the parameters to specify the ideal scheduler. Although a multilevel feedback queue is the most general scheme, that is additionally the most complex.


An instance of a multilevel feedback queue have the right to be checked out in the over figure.


First the all, intend that queues 1 and 2 follow ring robin v time quantum 8 and also 16 respectively and also queue 3 follows FCFS. Among the implementations that Multilevel Feedback Queue Scheduling is together follows:

If any procedure starts executing climate firstly the enters queue 1.

In queue 1, the procedure executes for 8 unit and if the completes in these 8 units or it gives CPU for I/O operation in these 8 systems unit than the priority of this process does no change, and if for some reasons it again comes in the all set queue 보다 it again beginning its execution in the Queue 1.

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

Above clues 2 and also 3 are additionally true for processes in queue 2 yet the time quantum is 16 units. Generally, if any process does not complete in a given time quantum then it gets shifted come the lower priority queue.

After that in the critical queue, all processes are scheduled in one FCFS manner.

It is vital to note that a process that is in a reduced priority queue deserve to only execute only when the greater priority queues space empty.

Any running process in the lower priority queue deserve to be interrupted by a procedure arriving in the higher priority queue.

Also, the above implementation might differ for the example in i beg your pardon the last queue will follow Round-robin Scheduling.

In the over Implementation, over there is a problem and that is; Any process that is in the lower priority queue has to suffer starvation due to some quick processes that space taking every the CPU time.

And the equipment to this trouble is :There is a systems that is to an increase the priority of all the procedure after regular intervals then place all the processes in the highest possible priority queue.

The need for Multilevel Feedback Queue Scheduling(MFQS)

Following room some clues to know the require for such complex scheduling:

This scheduling is more flexible 보다 Multilevel queue scheduling.

This algorithm help in to reduce the solution time.

In order to optimize the turnaround time, the SJF algorithm is needed which basically requires the to run time of processes in order to schedule them. Together we recognize that the to run time of procedures is not known in advance. Also, this scheduling largely runs a process for a time quantum and also after that, that can change the priority the the procedure if the process is long. For this reason this scheduling algorithm mostly learns indigenous the past actions of the processes and also then it can predict the future behavior of the processes. In this way, MFQS make the efforts to run a shorter process first which in return leader to optimize the turnaround time.

Advantages of MFQS

This is a functional Scheduling Algorithm

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

In this algorithm, A process that waits too lengthy in a lower priority queue may be moved to a higher priority queue which helps in avoiding starvation.

Disadvantages of MFQS

This algorithm is too complex.

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As processes are moving approximately different waiting line which leads to the production of more CPU overheads.

In order to select the ideal scheduler this algorithm needs some other way to choose the values