What is priority inversion?

Question

I\'ve heard the phrase \'priority inversion\' in reference to development of operating systems.

What exactly is priority inversion?

What is the problem it\'s

Answer 1

Priority inversion is a problem, not a solution. The typical example is a low priority process acquiring a resource that a high priority process needs, and then being preempted by a medium priority process, so the high priority process is blocked on the resource while the medium priority one finishes (effectively being executed with a lower priority).

A rather famous example was the problem experienced by the Mars Pathfinder rover: http://www.cs.duke.edu/~carla/mars.html, it's a pretty interesting read.

Answer 2

Priority inversion is where a lower priority process gets ahold of a resource that a higher priority process needs, preventing the higher priority process from proceeding till the resource is freed.

eg: FileA needs to be accessed by Proc1 and Proc2. Proc 1 has a higher priority than Proc2, but Proc2 manages to open FileA first.

Normally Proc1 would run maybe 10 times as often as Proc2, but won't be able to do anything because Proc2 is holding the file.

So what ends up happening is that Proc1 blocks until Proc2 finishes with FileA, essentially their priorities are 'inverted' while Proc2 holds FileA's handle.

As far as 'Solving a problem' goes, priority inversion is a problem in itself if it keeps happening. The worst case (most operating systems won't let this happen though) is if Proc2 wasn't allowed to run until Proc1 had. This would cause the system to lock as Proc1 would keep getting assigned CPU time, and Proc2 will never get CPU time, so the file will never be released.

Answer 3

It is the problem rather than the solution.

It describes the situation that when low-priority threads obtain locks during their work, high-priority threads will have to wait for them to finish (which might take especially long since they are low-priority). The inversion here is that the high-priority thread cannot continue until the low-priority thread does, so in effect it also has low priority now.

A common solution is to have the low-priority threads temporarily inherit the high priority of everyone who is waiting on locks they hold.

Answer 4

Priority inversion occurs as such: Given processes H, M and L where the names stand for high, medium and low priorities, only H and L share a common resource.

Say, L acquires the resource first and starts running. Since H also needs that resource, it enters the waiting queue. M doesn't share the resource and can start to run, hence it does. When L is interrupted by any means, M takes the running state since it has higher priority and it is running on the instant that interrupt happens. Although H has higher priority than M, since it is on the waiting queue, it cannot acquire the resource, implying a lower priority than even M. After M finishes, L will again take over CPU causing H to wait the whole time.

Answer 5

Priority Inversion can be avoided if the blocked high priority thread transfers its high priority to the low priority thread that is holding onto the resource.

Answer 6

Consider a system with two processes,H with high priority and L with low priority. The scheduling rules are such that H is run whenever it is in ready state because of its high priority. At a certain moment, with L in its critical region, H becomes ready to run (e.g., an I/O operation completes). H now begins busy waiting, but since L is never scheduled while H is running, L never gets the chance to leave the critical section. So H loops forever.

This situation is called Priority Inversion. Because higher priority process is waiting on lower priority process.