D.16 Multiprocessor Implementation
{
AI05-0171-1}
{
AI05-0299-1}
This subclause allows implementations on multiprocessor platforms to
be configured.
Static Semantics
{
AI05-0171-1}
The following language-defined library package exists:
{
AI12-0241-1}
{
AI12-0302-1}
package System.Multiprocessors
with Preelaborate, Nonblocking, Global =>
in out synchronized is type CPU_Range
is range 0 ..
implementation-defined;
Not_A_Specific_CPU :
constant CPU_Range := 0;
subtype CPU
is CPU_Range
range 1 .. CPU_Range'Last;
Implementation defined: The value of
CPU_Range'Last in System.Multiprocessors.
function Number_Of_CPUs
return CPU;
end System.Multiprocessors;
{
AI05-0171-1}
A call of Number_Of_CPUs returns the number of processors available to
the program. Within a given partition, each call on Number_Of_CPUs will
return the same value.
CPU
The aspect CPU is an
expression,
which shall be of type System.Multiprocessors.CPU_Range.
Aspect Description for CPU: Processor
on which a given task, or calling task for a protected operation, should
run.
Legality Rules
Dynamic Semantics
{
AI05-0171-1}
{
AI05-0229-1}
The CPU aspect has no effect if it is specified for a subprogram other
than the main subprogram; the CPU value is not associated with any task.
{
AI05-0171-1}
{
AI05-0229-1}
The CPU value is associated with the environment task if the CPU aspect
is specified for the main subprogram. If the CPU aspect is not specified
for the main subprogram it is implementation defined on which processor
the environment task executes.
Implementation defined: The processor
on which the environment task executes in the absence of a value for
the aspect CPU.
{
AI05-0171-1}
{
AI05-0264-1}
{
AI12-0281-1}
For a task, the CPU value determines the processor on which the task
will activate and execute; the task is said to be assigned to that processor.
If the CPU value is Not_A_Specific_CPU, then the task is not assigned
to a processor. A task without a CPU aspect specified will activate and
execute on the same processor as its activating task if the activating
task is assigned a processor. If the CPU value is not in the range of
System.Multiprocessors.CPU_Range or is greater than Number_Of_CPUs the
task is defined to have failed, and it becomes a completed task (see
9.2).
{
AI12-0281-1}
For a protected type, the CPU value determines the processor on which
calling tasks will execute; the protected object is said to be assigned
to that processor. If the CPU value is Not_A_Specific_CPU, then the protected
object is not assigned to a processor. A call to a protected object that
is assigned to a processor from a task that is not assigned a processor
or is assigned a different processor raises Program_Error.
Discussion: {
AI12-0005-1}
When a protected object is assigned to a CPU, only tasks also assigned
to that CPU can call it. In contrast, a protected object that is not
assigned to a specific CPU can be called by any task on any processor
(subject, of course, to visibility and ceiling priority restrictions).
As noted below, when the tasks and protected object are necessarily on
the same CPU, a simpler implementation can be used.
Implementation Advice
{
AI12-0281-1}
{
AI12-0323-1}
Starting a protected action on a protected object statically assigned
to a processor should be implemented without busy-waiting.
Reason: Busy-waiting is a form of lock
and can be a source of deadlock. Busy-waiting is typically needed for
starting protected actions on multiprocessors, but if all tasks calling
a protected object execute on the same CPU, this locking isn't needed
and the source of deadlock and associated overhead can be eliminated.
Implementation Advice: Starting a protected
action on a protected object statically assigned to a processor should
not use busy-waiting.
Extensions to Ada 2005
Extensions to Ada 2012
{
AI12-0281-1}
Aspect CPU can now be applied to protected types,
in order to avoid the overhead and deadlock potential of multiprocessor
execution.
Wording Changes from Ada 2012
{
AI12-0081-1}
Corrigendum: Clarified when the CPU aspect expression is evaluated.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
