Multilevel Queue (MLQ) Scheduling Algorithm

Introduction

Multilevel Queue Scheduling, or MLQ, is used when one ready queue is not enough for all process categories. Instead of treating every process the same way, the scheduler separates them into fixed queues.

This is useful because system tasks, interactive tasks, and batch tasks often need different scheduling behavior. Some work needs fast response, while some background work can wait longer.

Core Idea of MLQ Scheduling

Different process classes may have different importance and response needs. MLQ helps the operating system organize these classes instead of mixing all ready processes into one queue.

Common queue categories include System tasks, Interactive tasks and Batch tasks. Each process is assigned to one queue when it enters the system.

Each queue can have its own priority, scheduling algorithm, and CPU share. For example, a system queue may have strict priority, while an interactive queue may use Round Robin and a batch queue may use FCFS.

MLQ Scheduling Example

Consider three fixed queues:

Inside each queue, FCFS is used. Higher queues can preempt lower queues, and processes do not move between queues.

Given processes:

The compact scheduling flow is:

• At time 0, only P1 is available, so P1 starts in Q2.

• At time 1, P2 arrives in Q1 and preempts P1.

• P3 arrives in Q2, but it cannot preempt P2.

• P4 arrives in Q1, but FCFS inside Q1 keeps P2 running.

• At time 5, P2 finishes and P4 starts.

• At time 7, P5 arrives in Q0 and preempts P4.

• After P5, P4 resumes, then P1, and finally P3.

Metric Calculation

The main formulas are:

• CT: time when the process finishes

• TAT: CT - AT

• WT: TAT - BT

• RT: first CPU start time - AT

Average TAT = 12.0, average WT = 6.4, and average RT = 3.8.

Strengths and Weaknesses of MLQ

If a process is placed in the wrong queue, the scheduler does not automatically move it to a better queue in the basic MLQ model. This is one major difference between MLQ and Multilevel Feedback Queue scheduling.

Summary

Multilevel Queue Scheduling divides the ready queue into multiple fixed queues for process classes such as system, interactive, and batch work. The scheduler first chooses the highest non-empty queue, then applies the scheduling policy inside that queue.

MLQ is simple and useful when process categories are clear, but it is rigid because processes do not move between queues. Its main concerns are fixed classification, queue priority design, and starvation prevention for lower-priority queues.