D.13 The Ravenscar and Jorvik ProfilesThe Ravenscar Profile
Legality Rules
Static Semantics
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AI95-00249-01}
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AI95-00297-01}
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AI95-00394-01}
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AI05-0171-1}
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AI05-0246-1}
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AI12-0055-1}
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pragma Task_Dispatching_Policy (FIFO_Within_Priorities);
pragma Locking_Policy (Ceiling_Locking);
pragma Detect_Blocking;
pragma Restrictions (
No_Abort_Statements,
No_Dynamic_Attachment
,
No_Dynamic_CPU_Assignment,
No_Dynamic_Priorities,
No_Implicit_Heap_Allocations,
No_Local_Protected_Objects,
No_Local_Timing_Events,
No_Protected_Type_Allocators,
No_Relative_Delay,
No_Requeue_Statements,
No_Select_Statements,
No_Specific_Termination_Handlers,
No_Task_Allocators,
No_Task_Hierarchy,
No_Task_Termination,
Simple_Barriers,
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.
The Jorvik profile is
named for the city of York, UK, near where the Ravenscar profile was
created. "Jorvik" (pronounced "Yorvick") was the
Viking name for York.
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The usage profile Jorvik is equivalent to the following
set of pragmas:{
AI12-0291-1}
pragma Task_Dispatching_Policy (FIFO_Within_Priorities);
pragma Locking_Policy (Ceiling_Locking);
pragma Detect_Blocking;
pragma Restrictions (
No_Abort_Statements,
No_Dynamic_Attachment,
No_Dynamic_CPU_Assignment,
No_Dynamic_Priorities,
No_Local_Protected_Objects,
No_Local_Timing_Events,
No_Protected_Type_Allocators,
No_Requeue_Statements,
No_Select_Statements,
No_Specific_Termination_Handlers,
No_Task_Allocators,
No_Task_Hierarchy,
No_Task_Termination,
Pure_Barriers,
Max_Task_Entries => 0,
No_Dependence => Ada.Asynchronous_Task_Control,
No_Dependence => Ada.Execution_Time.Group_Budgets,
No_Dependence => Ada.Execution_Time.Timers,
No_Dependence => Ada.Task_Attributes,
No_Dependence => System.Multiprocessors.Dispatching_Domains);Discussion:
The Jorvik profile removes a number of restrictions from the Ravenscar
profile to allow additional applications to benefit from predictability
and low overhead. Specifically, the following restrictions are removed:
No_Implicit_Heap_Allocations
No_Relative_Delay
Max_Entry_Queue_Length => 1
Max_Protected_Entries => 1
No_Dependence => Ada.Calendar
No_Dependence => Ada.Synchronous_Barriers
Jorvik also replaces restriction
Simple_Barriers with Pure_Barriers (a weaker requirement than Simple_Barriers).
Paragraph 7 and 8 were was deleted.
Implementation Requirements
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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
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On a multiprocessor system, an implementation should support a fully
partitioned approach
if one of these profiles is
specified. 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.
NOTE 1 {
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For the Ravenscar profile, the The
effect of the
restriction Max_Entry_Queue_Length
=> 1
restriction applies only to protected
entry queues due to the accompanying restriction
of
Max_Task_Entries => 0.
The restriction
Max_Entry_Queue_Length is not applied by the Jorvik profile.NOTE 2 {
AI12-0055-1}
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When the Ravenscar or
Jorvik 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.NOTE 3 {
AI12-0055-1}
{
AI12-0291-1}
It is not recommended to specify the CPU of a task
to be Not_A_Specific_CPU when the Ravenscar or
Jorvik profile is in effect. How a partition
executes strongly depends on the assignment of tasks to CPUs.
NOTE 4 {
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Any unit that meets the requirements of the Ravenscar
profile also meets the requirements of the Jorvik profile. Extensions to Ada 95
Wording Changes from Ada 2005
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How Ravenscar behaves on a multiprocessor system is now defined.
Incompatibilities With Ada 2012
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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. Extensions to Ada 2012
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
