Priority Scheduling - TarisMajor/5143-OpSystems GitHub Wiki

Priority-Scheduling

Priority scheduling is a CPU scheduling algorithm that allocates the CPU based on the priority of processes. Each process is assigned a priority level, and the CPU is allocated to the process with the highest priority. Priority scheduling can be either preemptive or non-preemptive.

Key Characteristics of Priority Scheduling

  1. Priority Levels: Each process is assigned a priority value. The process with the highest priority is selected for execution next.
  2. Preemptive or Non-Preemptive: In preemptive priority scheduling, a running process can be interrupted if a higher-priority process arrives. In non-preemptive priority scheduling, the CPU is allocated to the highest-priority process that is ready to execute, and it runs to completion or until it voluntarily yields the CPU.
  3. Dynamic vs. Static Priority: Priorities can be static (fixed at process creation) or dynamic (changing during execution based on criteria such as aging or resource usage).

Advantages of Priority Scheduling

  1. Efficient Handling of Critical Processes: High-priority processes can be executed promptly, ensuring critical tasks are handled efficiently.
  2. Flexibility: The system can dynamically adjust priorities based on process requirements and system conditions, providing a flexible scheduling mechanism.
  3. Improved System Performance: By prioritizing important processes, the system can achieve better performance and meet critical deadlines.

Disadvantages of Priority Scheduling

  1. Starvation: Low-priority processes may experience indefinite delays (starvation) if higher-priority processes continually arrive.
  2. Complexity: Managing and adjusting priorities dynamically can be complex and requires sophisticated algorithms.
  3. Fairness Issues: Ensuring that all processes get a fair share of CPU time can be challenging, especially in systems with a mix of high and low-priority tasks.

Use Cases for Priority Scheduling

  1. Real-Time Systems: Ideal for real-time systems where meeting deadlines and handling critical tasks are essential.
  2. Interactive Systems: Useful in interactive systems where user tasks can be prioritized to ensure a responsive experience.
  3. Embedded Systems: Suitable for embedded systems with specific tasks that require different priority levels for efficient operation.

Example of Priority Scheduling

Consider five processes with the following burst times and priorities (lower number indicates higher priority):

  • Process P1: Burst Time = 10 units, Priority = 3
  • Process P2: Burst Time = 1 unit, Priority = 1
  • Process P3: Burst Time = 2 units, Priority = 4
  • Process P4: Burst Time = 1 unit, Priority = 5
  • Process P5: Burst Time = 5 units, Priority = 2

The priority scheduling sequence would be:

  1. Process P2 (Priority = 1)
  2. Process P5 (Priority = 2)
  3. Process P1 (Priority = 3)
  4. Process P3 (Priority = 4)
  5. Process P4 (Priority = 5)

Preemptive vs. Non-Preemptive Priority Scheduling

Feature Preemptive Priority Scheduling Non-Preemptive Priority Scheduling
Interruptions Can interrupt running processes No interruptions, processes run to completion
Responsiveness Higher, can handle urgent tasks promptly Lower, tasks must wait for current process to finish
Implementation Complexity More complex due to preemption logic Simpler, no need to handle preemption
Risk of Starvation High for low-priority processes Moderate, based on process mix
Fairness Can be challenging to ensure Easier to manage without preemption

Sources for Further Reading