Consider a system using preemptive priority scheduling with multiple real-time and user-level processes. Priority inheritance pro…

Question

Consider a system using preemptive priority scheduling with multiple real-time and user-level processes. Priority inheritance protocol is implemented.

I. Priority inversion can occur when a low-priority process holds a resource that a high-priority process needs.

II. Under priority inheritance, the low-priority process temporarily inherits the higher priority to avoid blocking.

III. Priority inversion can be completely avoided if all processes are scheduled in a round-robin fashion.

IV. Starvation may still occur for low-priority processes even with priority inheritance.

Which of the statements are correct?

1. A. I and II only
2. B. II and III only
3. C. II and IV only
4. D. I, II and IV only (Correct answer)

Correct Answer

Option D — D

Detailed Solution & Explanation

The correct answer is Option D.

Key Points

• I is correct: priority inversion occurs when a low-priority process holds a resource that a high-priority process needs, so the high-priority one is effectively blocked by the low-priority one.
• II is correct: under the priority inheritance protocol, the low-priority process temporarily inherits the higher priority so it can finish and release the resource quickly.
• IV is correct: starvation of low-priority processes can still occur even with priority inheritance.
• III is wrong: round-robin does NOT completely avoid priority inversion — the problem is about resource holding, not time-slicing.

Exam Tip

• the famous real-world case of priority inversion was the 1997 Mars Pathfinder mission, fixed by enabling priority inheritance.

Additional Information

• Priority inversion occurs when a high-priority task waits on a resource held by a low-priority one, which is itself preempted by a medium-priority task. The classic real-world case is the Mars Pathfinder rover's repeated resets in 1997.
• The standard remedies are priority inheritance (the holder temporarily takes the waiter's priority) and priority ceiling protocols.
• Starvation is the related hazard, where a low-priority process never runs at all; ageing — raising priority the longer a process waits — is the usual fix.