Difference Between Priority Inversion and Priority Inheritance

Real-time operating systems also use scheduling methods where certain operations are given higher precedence over others so that they can be executed on time. But this type of prioritization leads to some problems, for instance Priority Inversion, which is the state in which a high-priority task is blocked by low-priority ones. From this article, we shall look at Priority Inversion and the effects associated with it before looking at how Priority Inheritance is a solution to the problem. It enables you to control the time you allocate to different tasks to avoid overloading the system or running it late at night.

What is Priority Inversion?

Priority Inversion is a condition that occurs in a priority-based scheduling system. It is a situation where an activity of higher priority (H) has to wait while a work of lower priority (L) executes due to its ongoing work on the critical section. The above scenario is made worse if there is a medium-priority task (M) that interrupts L while it is with the critical section. In this case, though task H has a higher priority, its execution is delayed by M and L Since priorities of a task depend on priority matrices and priority vectors, such reciprocity can derail the right chronological order of a task and potentially harm the system.

Advantages

• Enhanced Awareness: Priority Inversion detection allows for recognition of the problems with scheduling and thus improves the system and its strategies.
• Improved Task Management: The understanding of Priority Inversion allows the application of solutions to control task priorities properly.
• System Optimization: The understanding of Priority Inversion can help to bring correction and avoid delays impacting the priority client’s service and system in general.
• Prevention of Deadlocks: Solving Priority Inversion decreases the likelihood of a deadlock and problems with resource sharing in a complicated environment.
• Better Real-Time Performance: In the context of real-time systems, Priority Inversion factors need to be clarified and handled, and that is how Priority Inversion must be treated.

Disadvantages

• Increased Complexity: Solving Priority Inversion can increase the level of complications concerning the system design and the subsequent implementation.
• Potential Overheads: The solutions for Priority Inversion, like priority inheritance lead to extra charges in the scheduling stage.
• Difficult Debugging: Priority Inversion may make the jobs of debugger and tracer more complex.
• Possible Performance Penalties: Priority Inversion can be solved, but that comes at the cost of performance penalties needed to handle priorities in a system.
• Limited Applicability: Extensions of solving Priority Inversion might not always provide a solution for all kinds of systems or even scheduling algorithms.

What is Priority Inheritance?

Priority Inheritance is the technique that is intended to solve the issues related to the Priority Inversion. In Priority Inheritance, the priority of task L is raised in order to match that of higher priority task H once L has acquired a critical section and H is waiting for it; this is to prevent other mid priority tasks to preempt task L, which could delay execution of task H once it releases the critical section. The task L comes out the critical section with the return of the priority to its original level. This approach assist in the conservation of the efficiency level of task scheduling from the impact of Priority Inversion.

Advantages

• Mitigates Priority Inversion: Priority Inheritance solves the problem of Priority Inversion where high priority tasks are not held back or delayed for a long time.
• Improves Task Scheduling: As this increases the value related to the tasks possessing critical sections, it improves the effectiveness of the scheduling for a time being.
• Reduces Waiting Time: Organizes the system queues better and reduces waiting time for more important jobs making the system more predictable.
• Increases System Responsiveness: Enhances overall system performance by reducing interruption by non-critical operations on the computer.
• Facilitates Real-Time Processing: Improves the response rates of real-time systems as such systems manage priorities and avoid procrastination.

Disadvantages

• Increased Complexity: Adding Priority Inheritance into the scheduling mechanism increases the level of complexity into the scheduling mechanism and management.
• Potential for Nested Locks: This can result in reduced worst case execution of systems that use nested locks since priority inheritance needs to be performed efficiently.
• Overhead Costs: Fluctuation and restoration of task schedules create additional demands to the system that can compromise its efficiency.
• Implementation Challenges: Priority Inheritance can pose many difficulties and might be implemented incorrectly rather easily and with mistakes in large systems.
• Limited Scope: Although Priority Inheritance can solve Priority Inversion, it may not tackle all the related scheduling problems or efficiently function in all situations.

Both of these Concepts come under Priority Scheduling in Operating System. But are they same?

In one line, Priority Inversion is a problem while Priority Inheritance is a solution. Priority Inversion means that the priority of tasks gets inverted and Priority Inheritance means that the priority of tasks gets inherited. Both of these phenomena happen in priority scheduling. Basically, in Priority Inversion, the higher priority task (H) ends up waiting for the middle priority task (M) when H is sharing a critical section with the lower priority task (L) and L is already in the critical section. Effectively, H waiting for M results in inverted priority i.e. Priority Inversion. One of the solutions to this problem is Priority Inheritance. In Priority Inheritance, when L is in the critical section, L inherits the priority of H at the time when H starts pending for the critical section. By doing so, M doesn’t interrupt L and H doesn’t wait for M to finish. Please note that inheriting of priority is done temporarily i.e. L goes back to its old priority when L comes out of the critical section.

Conclusion

In this article, Priority Inversion and Priority Inheritance have both been described in detail. Priority Inversion is a scheduling problem, where tasks of higher priorities are prevented from execution by tasks of lower priorities, while Priority Inheritance is the approach wherein the tasks holding critical sections are temporarily given higher priority to solve this problem. With the help of these concepts, the necessary ideas about scheduling of tasks and the further improvement of the system will also be received.