Multilevel Queues and Feedback Queues

In real-world operating systems, different types of processes require different kinds of scheduling. For example, system processes, user applications, and background tasks all have varying needs for CPU time. To address this, operating systems use advanced scheduling techniques like Multilevel Queues and Multilevel Feedback Queues. These approaches help manage diverse workloads efficiently.

Multilevel Queue Scheduling

Multilevel Queue Scheduling divides processes into separate queues based on their type or priority. Each queue can have its own scheduling algorithm and is assigned a fixed priority relative to the other queues.

Key Characteristics:

• Fixed classification: Once a process is assigned to a queue (e.g., system, interactive, batch), it stays there.
• No movement between queues.
• Priority-based queue execution: Higher priority queues are served first.

Example Setup:

• Queue 1: System processes (highest priority) → Scheduled using Round Robin
• Queue 2: Interactive processes → Scheduled using FCFS
• Queue 3: Batch jobs (lowest priority) → Scheduled using SJF

If all queues are ready, the CPU first checks Queue 1. Only when it’s empty does it move to the next.

Pros:

• Simple and organized
• Ideal for systems where process types are predictable

Cons:

• Rigid: A low-priority process might starve if high-priority queues are always full
• No adaptability: Process behavior can’t change its assigned queue

Multilevel Feedback Queue Scheduling

Multilevel Feedback Queues build on the idea of multilevel queues but add flexibility. Processes are allowed to move between queues based on their behavior and execution history.

Key Characteristics:

• Multiple queues with different priorities
• Dynamic movement: A process can move up or down based on how much CPU time it uses
• Aging is built-in: Prevents starvation by gradually moving long-waiting processes to higher-priority queues

How it Works:

• A new process starts in the highest-priority queue.
• If it uses too much CPU time, it’s moved to a lower-priority queue.
• If it waits too long, it may be promoted back to a higher-priority queue.

This system rewards short, interactive jobs and gradually deprioritizes long-running or CPU-intensive tasks.

Pros:

• Highly flexible and fair
• Better at balancing responsiveness and throughput
• Helps avoid starvation and improves user experience

Cons:

• More complex to implement
• Requires tuning of multiple parameters (number of queues, time quantum, promotion/demotion rules)

Practical Scenario

Think of a customer service center:

• Multilevel Queue is like separating calls into support levels (technical, billing, general), each with its own response pattern.

• Feedback Queue is like adjusting the support level based on how long a customer has been on hold or the complexity of their issue.

Summary

Both Multilevel Queues and Feedback Queues offer structured ways to manage processes with varying needs. While multilevel queues provide a strict and static hierarchy, feedback queues offer a dynamic and fair method of scheduling based on how a process behaves over time. Modern operating systems often use feedback-based methods to ensure both efficiency and responsiveness.