priority inheritance

<parallel> A technique for avoiding priority inversion by temporarily raising the prioriry of all processes that want to access a shared resource to the highest priority level of any of them. Priority inversion occurs where a low priority process, L is holding a resource required by a high priority process, H, but L is not running because a medium priority process, M is running. Under priority inheritance, L temporarily inherits H's priority, allowing L to run and release the resource H is waiting for.

For example, an ambulance (H) is stuck behind a lorry (L) waiting at a junction (the shared resource) for a gap in a line of cars (M) using the junction. Applying priority inheritance, the cars give way to the lorry as they would to the ambulance, thus allowing the lorry and then the ambulance to use the junction.

Last updated: 2005-02-11

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priority interrupt

<jargon> Any stimulus compelling enough to yank one right out of hack mode. Classically used to describe being dragged away by an SO for immediate sex, but may also refer to more mundane interruptions such as a fire alarm going off in the near vicinity. Also called an NMI (non-maskable interrupt), especially in PC-land.

[Jargon File]

Last updated: 2005-02-13

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priority inversion

<parallel> The state of a concurrent system where a high priority task is waiting for a low priority task which is waiting for a medium priority task. The system may become unstable and crash under these circumstances.

In an operating system that uses multiple tasks, each task (or context) may be given a priority. These priorities help the scheduler decide which task to run next. Consider tasks, L, M, and H, with priorities Low, Medium, and High. M is running and H is blocked waiting for some resource that is held by L. So long as any task with a priority higher than L is runable, it will prevent task L, and thus task H, from running.

Priority inversion is generally considered either as a high-level design failure or an implementation issue to be taken into account depending on who is talking. Most operating systems have methods in place to prevent or take inversion into account. Priority inheritance is one method.

The most public instance of priority inversion is the repeated 'fail-safe' rebooting of the Mars Pathfinder. base station ('Sagan Memorial Station').

Last updated: 2003-06-04

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priority queue

<programming> A data structure with three operations: insert a new item, return the highest priority item, and remove the highest priority item. The obvious way to represent priority queues is by maintaining a sorted list but this can make the insert operation very slow. Greater efficiency can be achieved by using heaps.

Last updated: 1996-03-12

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priority scheduling

<operating system> Processes scheduling in which the scheduler selects tasks to run based on their priority as opposed to, say, a simple round-robin.

Priorities may be static or dynamic. Static priorities are assigned at the time of creation, while dynamic priorities are based on the processes' behaviour while in the system. For example, the scheduler may favour I/O-intensive tasks so that expensive requests can be issued as early as possible.

A danger of priority scheduling is starvation, in which processes with lower priorities are not given the opportunity to run. In order to avoid starvation, in preemptive scheduling, the priority of a process is gradually reduced while it is running. Eventually, the priority of the running process will no longer be the highest, and the next process will start running. This method is called aging.

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