Priority Inversion - Example of A Priority Inversion

Example of A Priority Inversion

Consider there is a task L, with low priority. This task requires resource R. Now, there is another task H, with high priority. This task also requires resource R. Consider H starts after L has acquired resource R. Now H has to wait until L relinquishes resource R.

Everything works as expected up to this point, but problems arise when a new task M (which does not use R) starts with medium priority during this time. Since R is still in use (by L), H cannot run. Since M is the highest priority unblocked task, it will be scheduled before L. Since L has been preempted by M, L cannot relinquish R. So M will run till it is finished, then L will run - at least up to a point where it can relinquish R - and then H will run. Thus, in the scenario above, a task with medium priority ran before a task with high priority, effectively giving us a priority inversion.

In some cases, priority inversion can occur without causing immediate harm—the delayed execution of the high priority task goes unnoticed, and eventually the low priority task releases the shared resource. However, there are also many situations in which priority inversion can cause serious problems. If the high priority task is left starved of the resources, it might lead to a system malfunction or the triggering of pre-defined corrective measures, such as a watch dog timer resetting the entire system. The trouble experienced by the Mars lander "Mars Pathfinder" is a classic example of problems caused by priority inversion in realtime systems.

Priority inversion can also reduce the perceived performance of the system. Low priority tasks usually have a low priority because it is not important for them to finish promptly (for example, they might be a batch job or another non-interactive activity). Similarly, a high priority task has a high priority because it is more likely to be subject to strict time constraints—it may be providing data to an interactive user, or acting subject to realtime response guarantees. Because priority inversion results in the execution of the low priority task blocking the high priority task, it can lead to reduced system responsiveness, or even the violation of response time guarantees.

A similar problem called deadline interchange can occur within earliest deadline first scheduling (EDF).