- The task dispatching model specifies preemptive scheduling, based on
conceptual priority-ordered ready queues.
- A task runs (that is, it becomes a running task) only when it is ready
(see 9.2) and the execution resources required by that task are available.
Processors are allocated to tasks based on each task's active priority.
- It is implementation defined whether, on a multiprocessor, a task that is
waiting for access to a protected object keeps its processor busy.
- Task dispatching is the process by which one ready task is selected for
execution on a processor. This selection is done at certain points during
the execution of a task called task dispatching points. A task reaches a
task dispatching point whenever it becomes blocked, and whenever it becomes
ready. In addition, the completion of an accept_statement (see 9.5.2), and
task termination are task dispatching points for the executing task. Other
task dispatching points are defined throughout this Annex.
- Task dispatching policies are specified in terms of conceptual ready
queues, task states, and task preemption. A ready queue is an ordered list
of ready tasks. The first position in a queue is called the head of the
queue, and the last position is called the tail of the queue. A task is
ready if it is in a ready queue, or if it is running. Each processor has one
ready queue for each priority value. At any instant, each ready queue of a
processor contains exactly the set of tasks of that priority that are ready
for execution on that processor, but are not running on any processor; that
is, those tasks that are ready, are not running on any processor, and can be
executed using that processor and other available resources. A task can be
on the ready queues of more than one processor.
- Each processor also has one running task, which is the task currently
being executed by that processor. Whenever a task running on a processor
reaches a task dispatching point, one task is selected to run on that
processor. The task selected is the one at the head of the highest priority
nonempty ready queue; this task is then removed from all ready queues to
which it belongs.
- A preemptible resource is a resource that while allocated to one task can
be allocated (temporarily) to another instead. Processors are preemptible
resources. Access to a protected object (see 9.5.1) is a nonpreemptible
resource. When a higher-priority task is dispatched to the processor, and
the previously running task is placed on the appropriate ready queue, the
latter task is said to be preempted.
- A new running task is also selected whenever there is a nonempty ready
queue with a higher priority than the priority of the running task, or when
the task dispatching policy requires a running task to go back to a ready
queue. These are also task dispatching points.
- An implementation is allowed to define additional resources as execution
resources, and to define the corresponding allocation policies for them.
Such resources may have an implementation defined effect on task dispatching
- An implementation may place implementation-defined restrictions on tasks
whose active priority is in the Interrupt_Priority range.
(7) Section 9 specifies under which circumstances a task becomes ready.
The ready state is affected by the rules for task activation and
termination, delay statements, and entry calls. When a task is not
ready, it is said to be blocked.
(8) An example of a possible implementation-defined execution resource
is a page of physical memory, which needs to be loaded with a particular
page of virtual memory before a task can continue execution.
(9) The ready queues are purely conceptual; there is no requirement that
such lists physically exist in an implementation.
(10) While a task is running, it is not on any ready queue. Any time
the task that is running on a processor is added to a ready queue, a new
running task is selected for that processor.
(11) In a multiprocessor system, a task can be on the ready queues of
more than one processor. At the extreme, if several processors share
the same set of ready tasks, the contents of their ready queues is
identical, and so they can be viewed as sharing one ready queue, and can
be implemented that way. Thus, the dispatching model covers
multiprocessors where dispatching is implemented using a single ready
queue, as well as those with separate dispatching domains.
(12) The priority of a task is determined by rules specified in this
subclause, and under D.1, ``Task Priorities'', D.3, ``Priority Ceiling
Locking'', and D.5, ``Dynamic Priorities''.
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