Priority inheritance

Priority inheritance

In real-time computing, priority inheritance is a method for eliminating priority inversion problems. Using this programming method, a process scheduling algorithm will increase the priority of a process to the maximum priority of any process waiting for any resource on which the process has a resource lock.

The basic idea of the priority inheritance protocol is that when a job blocks one or more high priority jobs, it ignores its original priority assignment and executes its critical section at the highest priority level of all the jobs it blocks. After executing its critical section, the job returns to its original priority level.

Example

Consider three jobs:

Job Name Priority
A High
B Medium
C Low

Suppose A is blocked by C for some shared resource. The priority inheritance protocol requires that C executes its critical section at the (high) priority of A. As a result, B will be unable to preempt C and will be blocked. That is, the higher priority job B must wait for the critical section of the lower priority job C to be executed, because C now inherits the priority of A. When C exits its critical section, it regains its original (low) priority and awakens A (which was blocked by C). A, having high priority, immediately preempts C and runs to completion. This enables B and C to resume in succession and run to completion.

Problems

The basic priority inheritance protocol has two problems:

  1. It does not prevent a deadlock from happening in a program with circular lock dependencies.
  2. A chain of blocking may be formed; blocking duration can be substantial, though bounded.

See also

References

External links

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