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D.13 The Ravenscar Profile

{AI95-00249-01} {AI05-0246-1} {AI05-0299-1} [This subclause defines the Ravenscar profile.
Paragraphs 2 and 3 were moved to 13.12, “Pragma Restrictions and Pragma Profile”. 


{AI95-00249-01} {AI05-0246-1}

Legality Rules

{AI95-00249-01} {AI05-0246-1} The profile_identifier Ravenscar is a usage profile (see 13.12). For usage profile Ravenscar, there shall be no profile_pragma_argument_associations.

Static Semantics

{AI95-00249-01} {AI05-0246-1} The usage profile Ravenscar is equivalent to the following set of pragmas:
{AI95-00249-01} {AI95-00297-01} {AI95-00394-01} {AI05-0171-1} {AI05-0246-1} {AI12-0055-1} {AI12-0073-1} pragma Task_Dispatching_Policy (FIFO_Within_Priorities);
pragma Locking_Policy (Ceiling_Locking);
pragma Detect_Blocking;
pragma Restrictions (
              Max_Entry_Queue_Length => 1,
              Max_Protected_Entries => 1,
              Max_Task_Entries => 0,
              No_Dependence => Ada.Asynchronous_Task_Control,
              No_Dependence => Ada.Calendar,
              No_Dependence => Ada.Execution_Time.Group_Budgets,
              No_Dependence => Ada.Execution_Time.Timers,
              No_Dependence => Ada.Synchronous_Barriers,

              No_Dependence => Ada.Task_Attributes,
              No_Dependence => System.Multiprocessors.Dispatching_Domains);
Discussion: The Ravenscar profile is named for the location of the meeting that defined its initial version. The name is now in widespread use, so we stick with existing practice, rather than using a more descriptive name. 

Post-Compilation Rules

{AI95-00249-01} {AI05-0246-1}
Paragraph 7 was deleted.

Implementation Requirements

This paragraph was deleted.{AI05-0171-1} {AI05-0229-1} {AI12-0055-1} A task shall only be on the ready queues of one processor, and the processor to which a task belongs shall be defined statically. Whenever a task running on a processor reaches a task dispatching point, it goes back to the ready queues of the same processor. A task with a CPU value of Not_A_Specific_CPU will execute on an implementation defined processor. [A task without a CPU aspect will activate and execute on the same processor as its activating task.] 
Proof: The processor of a task without a CPU aspect is defined in D.16.

Implementation Advice

{AI05-0171-1} On a multiprocessor system, an implementation should support a fully partitioned approach. Each processor should have separate and disjoint ready queues.
Implementation Advice: On a multiprocessor system, each processor should have a separate and disjoint ready queue.
42  {AI95-00249-01} {AI05-0246-1} The effect of the Max_Entry_Queue_Length => 1 restriction applies only to protected entry queues due to the accompanying restriction of Max_Task_Entries => 0.
43  {AI12-0055-1} When the Ravenscar profile is in effect (via the effect of the No_Dynamic_CPU_Assignment restriction), all of the tasks in the partition will execute on a single CPU unless the programmer explicitly uses aspect CPU to specify the CPU assignments for tasks. The use of multiple CPUs requires care, as many guarantees of single CPU scheduling no longer apply.
44  {AI12-0055-1} It is not recommended to specify the CPU of a task to be Not_A_Specific_CPU when the Ravenscar profile is in effect. How a partition executes strongly depends on the assignment of tasks to CPUs.

Extensions to Ada 95

{AI95-00249-01} {AI05-0246-1} The Ravenscar profile is new; it was moved here by Ada 2012. 

Wording Changes from Ada 2005

{AI05-0171-1} How Ravenscar behaves on a multiprocessor system is now defined. 

Incompatibilities With Ada 2012

{AI05-0073-1} Corrigendum: The Ravenscar profile no longer allows the use of package Synchronous_Barriers, as this package violates the fundamental Ravenscar requirement that each waiting point can only block (and release) a single task. This is incompatible with the published Ada 2012 standard, but it is unlikely that any existing Ravenscar runtime ever usefully supported barriers.
{AI05-0055-1} Corrigendum:The Ravenscar profile (via the effect of the new restriction No_Dynamic_CPU_Assignment) no longer allows setting the CPU aspect of a task to a non-static value. While this was allowed, an implementation would have had to come up with a creative interpretation of the Ada 2012 requirement to define the association of tasks to processors statically. As such, the new check is more likely to catch bugs than break a working program. 

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