Multilevel feedback queues





Multilevel feedback Queue scheduling
It is an enhancement of multilevel queue scheduling where process can move between the queues. In approach, the ready queue is partitioned into multiple queues of different priorities. The system use to assign processes to queue based on their CPU burst characteristic. If a process consumes too much CPU time, it is placed into a lower priority queue. It favors I/O bound jobs to get good input/output device utilization. A technique called aging promotes lower priority processes to the next higher priority queue after a suitable interval of time.

In figure, the process queue is displayed from top to bottom in order of decreasing priority. The top priority queue has the smallest CPU-time quantum. After a process from the top queue exhausts its time quantum on the CPU, it is placed on the next lower queue. The process is next serviced when it reaches on the top queue if the top queue is empty.

multilevel feedback queue scheduling

multilevel feedback queue scheduling

Advantages
A process that waits too long in a lower priority queue may be moved to a higher priority queue.

Disadvantage
Moving the process around queue produce more CPU overhead.



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