Книга: Real-Time Concepts for Embedded Systems
5.3.3 Blocked State
5.3.3 Blocked State
The possibility of blocked states is extremely important in real-time systems because without blocked states, lower priority tasks could not run. If higher priority tasks are not designed to block, CPU starvation can result.
CPU starvation occurs when higher priority tasks use all of the CPU execution time and lower priority tasks do not get to run.
A task can only move to the blocked state by making a blocking call, requesting that some blocking condition be met. A blocked task remains blocked until the blocking condition is met. (It probably ought to be called the un blocking condition, but blocking is the terminology in common use among real-time programmers.) Examples of how blocking conditions are met include the following:
· a semaphore token (described later) for which a task is waiting is released,
· a message, on which the task is waiting, arrives in a message queue, or
· a time delay imposed on the task expires.
When a task becomes unblocked, the task might move from the blocked state to the ready state if it is not the highest priority task. The task is then put into the task-ready list at the appropriate priority-based location, as described earlier.
However, if the unblocked task is the highest priority task, the task moves directly to the running state (without going through the ready state) and preempts the currently running task. The preempted task is then moved to the ready state and put into the appropriate priority-based location in the task-ready list.
Оглавление книги
- 5.3 Task States and Scheduling
- 5.3.1 Ready State
- 5.3.2 Running State
- 7.3 Message Queue States
- Chapter 7. The state machine
- User-land states
- State match
- State NEW packets but no SYN bit set
- Конкурентный бенчмаркинг в GE Real Estate
- Свойство WindowsState
- Conditional Statements
- Special Statements: for, while, and Others