Explain Multilevel Queue Scheduling and Multilevel feedback queue scheduling. - Question & Answer Book

Answer: -

·         When processes can be readily categorized, then multiple separate queues can be established, each implementing whatever scheduling algorithm is most appropriate for that type of job, and/or with different parametric adjustments.

·         Scheduling must also be done between queues, that is scheduling one queue to get time relative to other queues. Two common options are strict priority (no job in a lower priority queue runs until all higher priority queues are empty) and round-robin (each queue gets a time slice in turn, possibly of different sizes.)

·         Note that under this algorithm jobs cannot switch from queue to queue – Once they are assigned a queue, that is their queue until they finish.

Multilevel queue scheduling

Multilevel Feedback-Queue Scheduling

·         Multilevel feedback queue scheduling is similar to the ordinary multilevel queue scheduling described above, except jobs may be moved from one queue to another for a variety of reasons:

Ø  If the characteristics of a job change between CPU-intensive and I/O-intensive, then it may be appropriate to switch a job from one queue to another.

Ø  Aging can also be incorporated so that a job that has waited for a long time can get bumped up into a higher priority queue for a while.

·         Multilevel feedback queue scheduling is the most flexible because it can be turned for any situation. But it is also the most complex to implement because of all the adjustable parameters. Some of the parameters which define one of these systems include:

Ø  The number of queues.

Ø  The scheduling algorithm for each queue.

Ø  The methods used to upgrade or demote processes from one queue to another. (Which may be different.)

Ø  The method used to determine which queue a process enters initially.