D.13 The Ravenscar and Jorvik Profiles
Legality Rules
Static Semantics
{
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 (
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.
{
AI12-0291-1}
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 deleted.
Implementation Advice
{
AI05-0171-1}
{
AI12-0291-1}
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 {
AI95-00249-01}
{
AI05-0246-1}
{
AI12-0291-1}
For the Ravenscar profile, the effect of the restriction Max_Entry_Queue_Length
=> 1 applies only to protected entry queues due to the accompanying
restriction Max_Task_Entries => 0. The restriction Max_Entry_Queue_Length
is not applied by the Jorvik profile.
NOTE 2 {
AI12-0055-1}
{
AI12-0291-1}
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 {
AI12-0291-1}
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
{
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.
Extensions to Ada 2012
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
