D.2.1 The Task Dispatching Model
The task dispatching model specifies task scheduling,
based on conceptual priority-ordered ready queues.
The following language-defined
library package exists:
Dispatching_Policy_Error : exception
Dispatching serves as the parent of other language-defined
library units concerned with task dispatching.
A task can become a running task
only if it
is ready (see 9
) 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
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 when
it terminates. Other task dispatching points are defined throughout this
Annex for specific policies.
are specified in terms of conceptual ready
and task states. 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
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
, which is the task currently being executed by that processor.
Whenever a task running on a processor reaches a task dispatching point
it goes back to one or more ready queues; a task (possibly the same task)
is then 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.
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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
For optimization purposes, an implementation may
alter the points at which task dispatching occurs, in an implementation-defined
manner. However, a delay_statement
always corresponds to at least one task dispatching point.
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.
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.
13 The setting of a task's base priority
as a result of a call to Set_Priority does not always take effect immediately
when Set_Priority is called. The effect of setting the task's base priority
is deferred while the affected task performs a protected action.