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

1_uj_KY1XlD_vE0ZYhLSJ8jQ

Multilevel Feedback Queue Scheduling is an advanced CPU scheduling algorithm designed to adapt dynamically to the behavior of processes. Unlike simple multilevel queue scheduling, where processes are permanently assigned to a queue based on their priority, multilevel feedback queue scheduling allows processes to move between queues based on their execution history and needs.

Key Characteristics of Multilevel Feedback Queue Scheduling

  1. Multiple Queues: The system maintains multiple queues, each with its own scheduling algorithm and priority level.
  2. Dynamic Movement: Processes can move between queues based on their behavior and system needs. For example, a process that uses too much CPU time may be moved to a lower-priority queue.
  3. Aging Mechanism: To prevent starvation, the algorithm can include an aging mechanism that gradually increases the priority of processes waiting too long in lower-priority queues.

Advantages of Multilevel Feedback Queue Scheduling

  1. Dynamic Adaptation: The ability to move processes between queues based on their behavior allows the system to adapt dynamically to changing conditions and workloads.
  2. Improved Performance: By adjusting priorities and moving processes between queues, the algorithm can optimize CPU utilization and improve overall system performance.
  3. Fairness: The inclusion of aging mechanisms ensures that low-priority processes do not starve and eventually receive CPU time.

Disadvantages of Multilevel Feedback Queue Scheduling

  1. Complexity: The algorithm is complex to implement and manage due to the dynamic nature of queue adjustments and the need to track process behavior.
  2. Overhead: Continuously monitoring and adjusting process priorities can introduce significant overhead, impacting system performance.
  3. Tuning Requirements: The effectiveness of the algorithm depends on properly tuning the parameters, such as time slices and aging intervals, which can be challenging.

Use Cases for Multilevel Feedback Queue Scheduling

  1. General-Purpose Operating Systems: Ideal for general-purpose operating systems that need to handle a wide variety of processes with different priorities and requirements.
  2. Interactive Systems: Suitable for interactive systems where responsiveness and fairness are critical.
  3. High-Performance Computing: Effective in high-performance computing environments that require efficient CPU utilization and dynamic adjustment to varying workloads.

Example of Multilevel Feedback Queue Scheduling

Consider a system with three queues:

  1. Queue 1 (Highest Priority): Interactive processes, scheduled with a short time slice using Round Robin.
  2. Queue 2 (Medium Priority): Batch processes, scheduled with a longer time slice using Round Robin.
  3. Queue 3 (Lowest Priority): Background processes, scheduled with First-Come, First-Served (FCFS).

Processes start in the highest priority queue. If a process uses its entire time slice without completing, it is moved to a lower priority queue. Conversely, if a process waits too long in a lower-priority queue, it may be moved up to a higher priority queue to prevent starvation.

For example:

  • Process P1 (short interactive process) starts in Queue 1 and completes quickly.
  • Process P2 (longer batch process) starts in Queue 1 but uses its entire time slice, moving to Queue 2 for further execution.
  • Process P3 (background process) starts in Queue 3 but moves to a higher-priority queue if it waits too long.

Sources for Further Reading