D.4 Entry Queuing Policies
This clause specifies a mechanism for a user to
choose an entry queuing policy
. It also defines two one
such policies y
Other policies are implementation defined.]
Implementation defined: Implementation-defined
The form of a pragma
Queuing_Policy is as follows:
shall be either FIFO_Queuing, Priority_Queuing or an implementation-defined
pragma is a configuration pragma.
[A queuing policy
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.
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
Two queuing policies, FIFO_Queuing and Priority_Queuing, are language
defined. If no Queuing_Policy pragma applies to appears
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
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
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).
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
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.
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
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_select
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_select
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
Implementations are allowed to define other queuing policies, but need
not support more than one queuing such
policy per partition.
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. In any case, the wording here really ought to be "...more than one
queuing policy per partition.", since this part of the rule applies
to all queuing policies, not just implementation-defined ones.
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.
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.
reordering should occur as soon as the blocked task can itself perform
the reinsertion into the entry queue.
The implementation should use names that end with
“_Queuing” for implementation-defined queuing policies.
Names that end with “_Queuing”
should be used for implementation-defined queuing policies.
Wording Changes from Ada 95
Corrigendum: Corrected so that a call of
Set_Priority in an abortable part does not change the priority of the
triggering entry call.
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.
Clarified that an implementation need support only
one queuing policy (of any kind, language-defined or otherwise) per partition.
Fixed wording to make clear that pragma
never appears inside of a unit; rather it “applies to” the
Ada 2005 and 2012 Editions sponsored in part by Ada-Europe