D.16.1 Multiprocessor Dispatching Domains

This subclause allows implementations on multiprocessor platforms to be partitioned into distinct dispatching domains during program startup.

The following language-defined library package exists: 

A dispatching domain represents a set of processors on which a task may execute. Each processor is contained within exactly one dispatching domain. An object of type Dispatching_Domain identifies a dispatching domain. System_Dispatching_Domain identifies a domain that contains the processor or processors on which the environment task executes. At program start-up all processors are contained within this domain.

For a task type (including the anonymous type of a single_task_declaration), the following language-defined representation aspect may be specified:

The Dispatching_Domain aspect shall not be specified for a task interface.

The expression specified for the Dispatching_Domain aspect of a task type is evaluated each time an object of the task type is created (see 9.1). If the identified dispatching domain is empty, then Dispatching_Domain_Error is raised; otherwise the newly created task is assigned to the domain identified by the value of the expression.

If a task is not explicitly assigned to any domain, it is assigned to that of the activating task. A task always executes on some CPU in its domain.

If both the dispatching domain and CPU are specified for a task, and the CPU value is not contained within the set of processors for the domain (and is not Not_A_Specific_CPU), the activation of the task is defined to have failed, and it becomes a completed task (see 9.2).

The function Create with First and Last parameters creates and returns a dispatching domain containing all the processors in the range First .. Last. The function Create with a Set parameter creates and returns a dispatching domain containing the processors for which Set(I) is True. These processors are removed from System_Dispatching_Domain. A call of Create will raise Dispatching_Domain_Error if any designated processor is not currently in System_Dispatching_Domain, or if the system cannot support a distinct domain over the processors identified, or if a processor has a task assigned to it, or if the allocation would leave System_Dispatching_Domain empty. A call of Create will raise Dispatching_Domain_Error if the calling task is not the environment task, or if Create is called after the call to the main subprogram.

The function Get_First_CPU returns the first CPU in Domain, or CPU'First if Domain is empty; Get_Last_CPU returns the last CPU in Domain, or CPU_Range'First if Domain is empty. The function Get_CPU_Set(D) returns an array whose low bound is Get_First_CPU(D), whose high bound is Get_Last_CPU(D), with True values in the Set corresponding to the CPUs that are in the given Domain.

The function Get_Dispatching_Domain returns the dispatching domain on which the task is assigned.

A call of the procedure Assign_Task assigns task T to the CPU within the dispatching domain Domain. Task T can now execute only on CPU, unless CPU designates Not_A_Specific_CPU in which case it can execute on any processor within Domain. The exception Dispatching_Domain_Error is propagated if Domain is empty, T is already assigned to a dispatching domain other than System_Dispatching_Domain, or if CPU is not one of the processors of Domain (and is not Not_A_Specific_CPU). A call of Assign_Task is a task dispatching point for task T unless T is inside of a protected action, in which case the effect on task T is delayed until its next task dispatching point. If T is the Current_Task the effect is immediate if T is not inside a protected action, otherwise the effect is as soon as practical. Assigning a task already assigned to System_Dispatching_Domain to that domain has no effect.

A call of procedure Set_CPU assigns task T to the CPU. Task T can now execute only on CPU, unless CPU designates Not_A_Specific_CPU, in which case it can execute on any processor within its dispatching domain. The exception Dispatching_Domain_Error is propagated if CPU is not one of the processors of the dispatching domain on which T is assigned (and is not Not_A_Specific_CPU). A call of Set_CPU is a task dispatching point for task T unless T is inside of a protected action, in which case the effect on task T is delayed until its next task dispatching point. If T is the Current_Task the effect is immediate if T is not inside a protected action, otherwise the effect is as soon as practical.

The function Get_CPU returns the processor assigned to task T, or Not_A_Specific_CPU if the task is not assigned to a processor.

A call of Delay_Until_And_Set_CPU delays the calling task for the designated time and then assigns the task to the specified processor when the delay expires. The exception Dispatching_Domain_Error is propagated if P is not one of the processors of the calling task's dispatching domain (and is not Not_A_Specific_CPU).

The implementation shall perform the operations Assign_Task, Set_CPU, Get_CPU and Delay_Until_And_Set_CPU atomically with respect to any of these operations on the same dispatching_domain, processor or task.

  Any task that belongs to the system dispatching domain can execute on any CPU within that domain, unless the assignment of the task has been specified.

Each dispatching domain should have separate and disjoint ready queues.

The implementation shall document the processor(s) on which the clock interrupt is handled and hence where delay queue and ready queue manipulations occur. For any Interrupt_Id whose handler can execute on more than one processor the implementation shall also document this set of processors.

An implementation may limit the number of dispatching domains that can be created and raise Dispatching_Domain_Error if an attempt is made to exceed this number.

The implementation may defer the effect of a Set_CPU or an Assign_Task operation until the specified task leaves an ongoing parallel construct.