Multi Level Queue Scheduling - TarisMajor/5143-OpSystems GitHub Wiki

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Multilevel Queue Scheduling is a CPU scheduling algorithm that segregates processes into different queues based on their priority or type, with each queue having its own scheduling policy. This approach is particularly useful for systems that handle a diverse set of processes with varying priorities and requirements.

Key Characteristics of Multilevel Queue Scheduling

  1. Multiple Queues: Processes are divided into several queues, each representing a different priority level or type of process (e.g., interactive, batch, system, etc.).
  2. Separate Scheduling Policies: Each queue can have its own scheduling algorithm, such as Round Robin, First-Come, First-Served (FCFS), or Priority Scheduling, tailored to the specific needs of the processes in that queue.
  3. Fixed Priority Scheduling: Queues are assigned fixed priorities. Higher priority queues are serviced before lower priority queues.

Advantages of Multilevel Queue Scheduling

  1. Flexibility: The ability to use different scheduling algorithms for different types of processes provides flexibility and allows for optimized scheduling.
  2. Prioritization: High-priority processes can be executed promptly, ensuring critical tasks are handled efficiently.
  3. Specialized Handling: Different types of processes can be managed according to their specific needs, improving overall system performance.

Disadvantages of Multilevel Queue Scheduling

  1. Complexity: Managing multiple queues and ensuring proper scheduling within and between queues can be complex and require sophisticated algorithms.
  2. Starvation: Lower priority queues may experience starvation if higher priority queues consistently have processes to execute.
  3. Inflexible Queue Boundaries: Fixed priority levels can be inflexible, making it difficult to adjust the scheduling dynamically based on changing system conditions.

Use Cases for Multilevel Queue Scheduling

  1. Operating Systems: Commonly used in operating systems that manage a diverse set of processes with varying priorities, such as desktop and server operating systems.
  2. Real-Time Systems: Suitable for real-time systems where different types of processes (e.g., real-time tasks, user interactions, background tasks) need to be managed separately.
  3. Interactive Systems: Effective in interactive systems where user processes and background tasks can be handled in different queues with appropriate scheduling policies.

Example of Multilevel Queue Scheduling

Consider a system with three queues:

  1. Queue 1 (High Priority): Interactive processes, using Round Robin scheduling.
  2. Queue 2 (Medium Priority): Batch processes, using Shortest Job Next (SJN) scheduling.
  3. Queue 3 (Low Priority): Background processes, using First-Come, First-Served (FCFS) scheduling.

The scheduling sequence would be:

  1. High Priority Queue: All interactive processes in Queue 1 are serviced first using Round Robin.
  2. Medium Priority Queue: Once Queue 1 is empty, batch processes in Queue 2 are serviced using SJN.
  3. Low Priority Queue: Finally, background processes in Queue 3 are serviced using FCFS.

In case new interactive processes arrive while batch or background processes are running, the scheduler preempts the current processes and resumes servicing the high-priority interactive processes.

Sources for Further Reading