Question

It is a consensus in operating systems design to assign high priorities or allocate more CPU time to interactive processes (which are I/O-bound). Explain how this is accomplished by multilevel queue scheduling and the Linux CFS (Completely Fair Scheduler).

Answer 1

actually, the macOS and Windows schedulers can be regarded as examples of multilevel feedback queue schedulers. The algorithm meets the following design requirements for multimode systems:

*Give more preference to short jobs.
*Give more preference to I/O bound processes.
*Separate process into categories based on their need for the processor.

The multilevel queue scheduling algorithm is permanently assigned to a queue, and multilevel feedback queue scheduling allows a process to move between queues. here, This movement is ease by the characteristic of the CPU burst of the process. If a procedure uses too much CPU time, it will be moved to a lower priority queue. and This leaves I/O-bound and interactive procedures in the higher priority queues. also, a procedure that waits too long in a lower-priority queue may be moved to a higher priority queue. This form of aging also helps to stop starvation of certain lower priority processes.

*Advantages is...
here, The procedures are permanently assigned to the queue, so it has the advantage of low scheduling overhead.

*Disadvantages is...
here, Some procedures may starve for CPU if some higher priority queues are never becoming empty.
and also It is inflexible in nature.

The Completely Fair Scheduler (CFS) is a process scheduler.CFS ensures that the CPU is allocated equitably.
here, If the CPU access time provided to many tasks isn’t balanced, CFS gives the shortchanged tasks the time they need to execute.CFS keeps track of the balance between tasks by maintaining CPU access times in the virtual runtime. The smaller a task’s virtual runtime, the greater its recognized CPU need.CFS uses sleeper fairness to make sure even tasks that aren’t currently running will still receive their fair share of CPU when needed.and CFS doesn’t directly use priorities.

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