Summary of "CPU Scheduling: Multilevel Queue & Feedback Scheduling | L 11 | Operating System | GATE 2022"

Summary of the Video:

"CPU Scheduling: Multilevel Queue & Feedback Scheduling | L 11 | Operating System | GATE 2022"

Main Ideas and Concepts:

Introduction to CPU Scheduling Challenges:
- Different types of processes (system, foreground, background) have distinct scheduling needs.
- Traditional single scheduling algorithms (like Round Robin, Priority Scheduling) face problems handling diverse process types efficiently.
- To address these issues, Multilevel Queue Scheduling and Multilevel Feedback Queue Scheduling were introduced.
Multilevel Queue Scheduling:
- Processes are divided into different queues based on their type or priority (e.g., system processes, interactive processes, background processes).
- Each queue can have its own scheduling algorithm.
- Queues have fixed priorities; higher priority queues are served before lower priority queues.
- Time allocation is divided among queues (e.g., 30% for system processes, 50% for user processes, 20% for background processes).
- Advantages:
  - Different process types get appropriate scheduling.
  - Improves efficiency by segregating processes.
- Disadvantages:
  - Starvation can occur for lower priority queues.
  - Inflexible as processes cannot move between queues.
  - Difficult to handle dynamically changing process priorities.
Multilevel Feedback Queue Scheduling:
- An enhancement over Multilevel Queue Scheduling.
- Processes can move between queues based on their behavior and execution history.
- Uses Round Robin scheduling within each queue with different time quantums.
- If a process does not finish in a given quantum, it is moved to a lower-priority queue (degraded).
- If a process waits too long in a lower queue, it can be promoted to a higher-priority queue (to prevent starvation).
- This dynamic adjustment helps balance responsiveness and fairness.
- Advantages:
  - More flexible and prevents starvation.
  - Adapts to process behavior dynamically.
- Disadvantages:
  - More complex to implement.
  - Still possible to have starvation if not properly managed.
Scheduling Algorithm Details and Examples:
- Explanation of how processes are scheduled in multilevel queues with priorities.
- Example problem walkthrough involving multiple queues with different priorities and quantum times.
- Emphasis on preemption: higher priority processes can preempt lower priority ones.
- Use of Round Robin within queues and Priority Scheduling among queues.
Common Issues and Solutions:
- Starvation is a critical problem, especially in fixed Priority Scheduling.
- Multilevel feedback queues help mitigate starvation by allowing processes to move between queues.
- Real-time responsiveness is improved by prioritizing system and interactive processes.
Practical Considerations:
- Exact process execution times are often unknown, making precise scheduling challenging.
- Multilevel feedback queue is more practical than fixed Multilevel Queue Scheduling.
- Trade-offs between complexity and fairness need to be considered.
Exam Preparation Tips:
- Focus on understanding problem-solving techniques related to CPU Scheduling.
- Practice solving scheduling problems (especially multilevel queue and feedback queue) for competitive exams like GATE.
- Use systematic approaches and understand the behavior of different algorithms.
- Utilize resources like Unacademy Plus for guided learning and practice.

Methodology / Instructions for Multilevel Queue Scheduling:

Step 1: Classify processes into different queues based on their type or priority.
Step 2: Assign each queue a scheduling algorithm (e.g., Round Robin for user processes, Priority Scheduling for system processes).
Step 3: Allocate fixed CPU time slices to each queue.
Step 4: Execute all processes in the highest priority queue first before moving to the next.
Step 5: Preempt lower priority queues if a higher priority process arrives.
Step 6: Continue until all processes complete.

Methodology / Instructions for Multilevel Feedback Queue Scheduling:

Step 1: Assign new processes to the highest priority queue initially.
Step 2: Schedule processes in each queue using Round Robin with a defined quantum.
Step 3: If a process does not complete in its quantum, move it to the next lower priority queue.
Step 4: If a process waits too long in a lower priority queue, promote it to a higher priority queue to prevent starvation.
Step 5: Repeat scheduling and movement until all processes finish.
Step 6: Adjust quantum times and priorities dynamically based on process behavior.

Advantages and Disadvantages:

Aspect Multilevel Queue Scheduling Multilevel Feedback Queue Scheduling Flexibility Low (fixed queues, no movement between queues) High (processes can move between queues) Starvation Possible for lower priority queues Reduced by

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