D.4 Entry Queuing Policies
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8652/0074}
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AI95-00068-01}
{
AI05-0299-1}
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AI12-0324-1}
[
This subclause specifies a mechanism for a user
to choose an entry
queuing policy. It also defines three such
policies. Other policies are implementation defined.]
Implementation defined: Implementation-defined
queuing policies.
Syntax
The form of a
pragma
Queuing_Policy is as follows:
Legality Rules
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.
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AI95-00355-01}
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AI12-0163-1}
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AI12-0183-1}
Three queuing policies, FIFO_Queuing, Ordered_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
the FIFO_Queuing policy are specified in
9.5.3
and
9.7.1.
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AI12-0163-1}
The Ordered_FIFO_Queuing policy is defined as follows:
Calls are selected on a given entry queue in order
of arrival.
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 that arrived first is selected.
When more than one alternative of a
selective_accept
is open and has queued calls, the alternative whose queue has the call
that arrived first is selected.
Implementation Note: A possible implementation
for this policy would be to assign a sequence number to each queued entry
call, where the sequence number is incremented globally across all queues
associated with the protected object or
selective_accept.
Reason: It would have been marginally
easier to use textual order for the case when multiple queues are selectable.
But textual order can lead to unfair queue servicing, since the queues
in earlier textual order will end up starving the later ones if calls
arrive fast enough. Applying arrival first as the selector provides fairness
for this policy.
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).
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8652/0075}
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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.
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
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AI95-00256-01}
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AI12-0444-1}
Implementations are allowed to define other queuing policies, but are
not required to support specifying more than one queuing policy per partition.
Discussion: {
8652/0116}
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AI95-00069-01}
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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.
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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.
Static Semantics
Max_Entry_Queue_Length
The type of aspect Max_Entry_Queue_Length is Integer.
Aspect Description for Max_Entry_Queue_Length:
The maximum entry queue length for a task type, protected type, or
entry.
If directly specified, the aspect_definition shall be a static expression
no less than -1. If not specified, the aspect has value -1 (representing
no additional restriction on queue length).
Legality Rules
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AI12-0164-1}
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AI12-0388-1}
If the Max_Entry_Queue_Length aspect for a type has a nonnegative value,
the Max_Entry_Queue_Length aspect for every individual entry of that
type shall not be greater than the value of the aspect for the type.
The Max_Entry_Queue_Length aspect of a type is nonoverridable (see
13.1.1).
Ramification: Aspect Max_Entry_Queue_Length
can specify less than the partition-wide or type-wide default, but it
can't expand the length of a queue.
Post-Compilation Rules
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AI12-0164-1}
If a restriction Max_Entry_Queue_Length applies to a partition, any value
specified for the Max_Entry_Queue_Length aspect specified for the declaration
of a type or entry in the partition shall not be greater than the value
of the restriction.
Ramification: 13.12(6) says that the
restriction value has to be static, so this is statically checkable.
But the restriction does not have to be in the same compilation as the
aspect, so the check cannot, in general, be done until link time.
Dynamic Semantics
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AI12-0164-1}
If a nonconfirming value is specified for Max_Entry_Queue_Length for
a type, and an entry call or requeue would cause the queue for any entry
of the type to become longer than the specified value, then Program_Error
is raised at the point of the call or requeue.
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AI12-0164-1}
If a nonconfirming value is specified for Max_Entry_Queue_Length for
an entry, and an entry call or requeue would cause the queue for an entry
to become longer than the specified value, then Program_Error is raised
at the point of the call or requeue.
Wording Changes from Ada 95
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8652/0075}
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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.
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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.
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AI95-00256-01}
Clarified that an implementation need support only one queuing policy
(of any kind, language-defined or otherwise) per partition.
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AI95-00355-01}
Fixed wording to make clear that
pragma
never appears inside of a unit; rather it “applies to” the
unit.
Extensions to Ada 2012
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AI12-0163-1}
Defined the new queuing policy Ordered_FIFO_Queuing.
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AI12-0164-1}
Defined the new aspect Max_Entry_Queue_Length.
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe
