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D.4 Entry Queuing Policies

{8652/0074} {AI95-00068-01} {AI05-0299-1} [ This subclause specifies a mechanism for a user to choose an entry queuing policy. It also defines two such policies. Other policies are implementation defined.] 
Implementation defined: Implementation-defined queuing policies.


The form of a pragma Queuing_Policy is as follows: 
  pragma Queuing_Policy(policy_identifier);

Legality Rules

The policy_identifier shall be either FIFO_Queuing, Priority_Queuing or an implementation-defined identifier.

Post-Compilation Rules

A Queuing_Policy pragma is a configuration pragma.

Dynamic Semantics

[A queuing policy governs the order in which tasks are queued for entry service, and the order in which different entry queues are considered for service.] The queuing policy is specified by a Queuing_Policy pragma. 
Ramification: The queuing policy includes entry queuing order, the choice among open alternatives of a selective_accept, and the choice among queued entry calls of a protected object when more than one entry_barrier condition is True. 
{AI95-00355-01} Two queuing policies, FIFO_Queuing and Priority_Queuing, are language defined. If no Queuing_Policy pragma applies to any of the program units comprising the partition, the queuing policy for that partition is FIFO_Queuing. The rules for this policy are specified in 9.5.3 and 9.7.1.
The Priority_Queuing policy is defined as follows:
The calls to an entry [(including a member of an entry family)] are queued in an order consistent with the priorities of the calls. The priority of an entry call is initialized from the active priority of the calling task at the time the call is made, but can change later. Within the same priority, the order is consistent with the calling (or requeuing, or priority setting) time (that is, a FIFO order).
{8652/0075} {AI95-00205-01} After a call is first queued, changes to the active priority of a task do not affect the priority of the call, unless the base priority of the task is set while the task is blocked on an entry call.
When the base priority of a task is set (see D.5), if the task is blocked on an entry call, and the call is queued, the priority of the call is updated to the new active priority of the calling task. This causes the call to be removed from and then reinserted in the queue at the new active priority. 
Reason: A task is blocked on an entry call if the entry call is simple, conditional, or timed. If the call came from the triggering_statement of an asynchronous_select, or a requeue thereof, then the task is not blocked on that call; such calls do not have their priority updated. Thus, there can exist many queued calls from a given task (caused by many nested ATC's), but a task can be blocked on only one call at a time.
A previous version of Ada 9X required queue reordering in the asynchronous_select case as well. If the call corresponds to a “synchronous” entry call, then the task is blocked while queued, and it makes good sense to move it up in the queue if its priority is raised.
However, if the entry call is “asynchronous,” that is, it is due to an asynchronous_select whose triggering_statement is an entry call, then the task is not waiting for this entry call, so the placement of the entry call on the queue is irrelevant to the rate at which the task proceeds.
Furthermore, when an entry is used for asynchronous_selects, it is almost certain to be a “broadcast” entry or have only one caller at a time. For example, if the entry is used to notify tasks of a mode switch, then all tasks on the entry queue would be signaled when the mode changes. Similarly, if it is indicating some interrupting event such as a control-C, all tasks sensitive to the interrupt will want to be informed that the event occurred. Hence, the order on such a queue is essentially irrelevant.
Given the above, it seems an unnecessary semantic and implementation complexity to specify that asynchronous queued calls are moved in response to dynamic priority changes. Furthermore, it is somewhat inconsistent, since the call was originally queued based on the active priority of the task, but dynamic priority changes are changing the base priority of the task, and only indirectly the active priority. We say explicitly that asynchronous queued calls are not affected by normal changes in active priority during the execution of an abortable_part. Saying that, if a change in the base priority affects the active priority, then we do want the calls reordered, would be inconsistent. It would also require the implementation to maintain a readily accessible list of all queued calls which would not otherwise be necessary.
Several rules were removed or simplified when we changed the rules so that calls due to asynchronous_selects are never moved due to intervening changes in active priority, be they due to protected actions, some other priority inheritance, or changes in the base priority. 
When more than one condition of an entry_barrier of a protected object becomes True, and more than one of the respective queues is nonempty, the call with the highest priority is selected. If more than one such call has the same priority, the call that is queued on the entry whose declaration is first in textual order in the protected_definition is selected. For members of the same entry family, the one with the lower family index is selected.
If the expiration time of two or more open delay_alternatives is the same and no other accept_alternatives are open, the sequence_of_statements of the delay_alternative that is first in textual order in the selective_accept is executed.
When more than one alternative of a selective_accept is open and has queued calls, an alternative whose queue has the highest-priority call at its head is selected. If two or more open alternatives have equal-priority queued calls, then a call on the entry in the accept_alternative that is first in textual order in the selective_accept is selected.

Implementation Permissions

 {AI95-00256-01} Implementations are allowed to define other queuing policies, but need not support more than one queuing policy per partition. 
Discussion: {8652/0116} {AI95-00069-01} {AI95-00256-01} This rule is really redundant, as 10.1.5 allows an implementation to limit the use of configuration pragmas to an empty environment. In that case, there would be no way to have multiple policies in a partition. 
   {AI95-00188-02} Implementations are allowed to defer the reordering of entry queues following a change of base priority of a task blocked on the entry call if it is not practical to reorder the queue immediately. 
Reason: Priority change is immediate, but the effect of the change on entry queues can be deferred. That is necessary in order to implement priority changes on top of a non-Ada kernel. 
Discussion: The reordering should occur as soon as the blocked task can itself perform the reinsertion into the entry queue. 

Implementation Advice

The implementation should use names that end with “_Queuing” for implementation-defined queuing policies.
Implementation Advice: Names that end with “_Queuing” should be used for implementation-defined queuing policies.

Wording Changes from Ada 95

{8652/0074} {AI95-00068-01} Corrigendum: Corrected the number of queuing policies defined.
{8652/0075} {AI95-00205-01} Corrigendum: Corrected so that a call of Set_Priority in an abortable part does not change the priority of the triggering entry call.
{AI95-00188-02} Added a permission to defer queue reordering when the base priority of a task is changed. This is a counterpart to stronger requirements on the implementation of priority change.
{AI95-00256-01} Clarified that an implementation need support only one queuing policy (of any kind, language-defined or otherwise) per partition.
{AI95-00355-01} Fixed wording to make clear that pragma never appears inside of a unit; rather it “applies to” the unit. 

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