Multilevel Feedback Queue Scheduling in Operating Systems | OS Tutorials

In an operating system, the multilevel queue scheduling algorithm assigns processes permanently upon entry. Hence, it is an inflexible approach. In contrast, the multilevel feedback queue scheduling algorithm allows a process to move between queues. It is also known as Multilevel Adaptive Scheduling.

Multilevel Feedback Queue Scheduling

The idea is to separate processes based on the characteristics of their CPU burst times. If a process uses too much CPU time, it will be moved to the lower-priority queue. This scheme leaves I/O bound and interactive processes in the higher-priority queue. A process that waits too long in a lower-priority queue may be moved to a higher-priority queue. Hence, it prevents starvation.

Figure 1 illustrates the feedback scheduling mechanism by showing the path that a process will follow through the various queues. The approach is known as multilevel feedback.

• Thus, we can say that in this approach, the operating system allocates the processor to a process, and when the process blocks or is preempted, feed it back into one of several priority queues.
• In a Multilevel feedback queue, a process is given just one chance to complete at a given queue level before it is forced down to a lower-level queue.
• This scheduling algorithm gives highest priority to any process with a CPU burst of 8 milliseconds or less.
• In general, a multilevel feedback queue scheduler is defined by the following parameters:
  • The number of queues.
  • The Scheduling algorithm for each queue.
• The Method used to determine when to upgrade a process to a higher priority queue.
• The Method used to determine when to denote a process to a lower priority queue.
• The Method used to determine which queue a process will enter when that process needs service.