9 Tasks and Synchronization
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The execution of an Ada program consists of the execution
of one or more
tasks.
Each
task represents a separable activity that proceeds independently and
concurrently between the points where it
interacts with other
tasks. A single task, when within the context of a parallel construct,
can represent multiple
logical threads of control which can proceed
in parallel; in other contexts, each task represents one logical thread
of control.
To be honest: The execution of an Ada
program consists of the execution of one or more partitions (see
10.2),
each of which in turn consists of the execution of an environment task
and zero or more subtasks.
Term entry: logical thread of control
— activity within the execution of a program that can proceed in
parallel with other activities of the same task, or of separate tasks
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The various forms of task interaction are described in this clause, and
include:
the activation and termination of a task;
a call on a protected subprogram
of a
protected object, providing exclusive read-write access,
or concurrent read-only access to shared data;
a call on an entry, either of another task, allowing
for synchronous communication with that task, or of a protected object,
allowing for asynchronous communication with one or more other tasks
using that same protected object;
a timed operation, including a simple delay statement,
a timed entry call or accept, or a timed asynchronous select statement
(see next item);
an asynchronous transfer of control as part of
an asynchronous select statement, where a task stops what it is doing
and begins execution at a different point in response to the completion
of an entry call or the expiration of a delay;
an abort statement, allowing one task to cause
the termination of another task.
In addition, tasks can communicate indirectly by
reading and updating (unprotected) shared variables, presuming the access
is properly synchronized through some other kind of task interaction.
Static Semantics
The properties of a task are
defined by a corresponding task declaration and
task_body,
which together define a program unit called a
task unit.
Dynamic Semantics
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Over time, tasks proceed through various
states.
A
task is initially
inactive; upon activation, and prior to its
termination it is either
blocked (as part of some task
interaction) or
ready to run.
While ready,
a task competes for the available
execution resources that it
requires to run. In the context of a parallel construct, a single task
can utilize multiple processing resources simultaneously.
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The means for selecting which
of the ready tasks to run, given the currently available execution resources,
is determined by the
task dispatching policy in effect, which
is generally implementation defined, but may be controlled by aspects,
pragmas, and operations defined in the Real-Time Annex (see
D.2
and
D.5).
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Concurrent task execution can be implemented on multicomputers, multiprocessors,
or with interleaved execution on a single physical processor. On the
other hand, whenever an implementation can determine that the required
semantic effects can be achieved when parts of the execution of a single
logical thread of control are performed by different physical processors
acting in parallel, it can choose to perform them in this way.
Wording Changes from Ada 83
The introduction has been rewritten.
We use the term "concurrent" rather
than "parallel" when talking about logically independent execution
of threads of control. The term "parallel" is reserved for
referring to the situation where multiple physical processors run simultaneously.
Wording Changes from Ada 2012
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Rewrote the introduction to emphasize that threads of control can be
generated by parallel constructs as well as tasks.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
