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<H1> D.4 Entry Queuing Policies</H1>
<DIV Class="Paranum"><FONT SIZE=-2>1/1</FONT></DIV>
<DIV Class="Normal"> {<I><A HREF="defect1.html#8652/0074">8652/0074</A></I>}
[<A NAME="I6821"></A> This clause specifies a mechanism for a user to
choose an entry <I>queuing policy</I>. It also defines <U>two</U><S>one</S>
such polic<U>ies</U><S>y</S>. Other policies are implementation defined.]
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>1.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation-defined
queuing policies.</FONT></DIV>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented" Style="margin-bottom: 0.2em">The form of
a <FONT FACE="Arial, Helvetica">pragma</FONT> Queuing_Policy is as follows:
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6822"></A>Queuing_Policy(<I>policy_</I><A NAME="I6823"></A><FONT FACE="Arial, Helvetica">identifier</FONT>);
</DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> The <I>policy_</I><A NAME="I6824"></A><FONT FACE="Arial, Helvetica">identifier</FONT>
shall be either FIFO_Queuing, Priority_Queuing or an implementation-defined
<A NAME="I6825"></A><FONT FACE="Arial, Helvetica">identifier</FONT>.</DIV>
<H4 ALIGN=CENTER>Post-Compilation Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6826"></A><A NAME="I6827"></A>A Queuing_Policy
pragma is a configuration pragma.</DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6828"></A>[A <I>queuing policy</I> 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. </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>6.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>The queuing
policy includes entry queuing order, the choice among open alternatives
of a <FONT FACE="Arial, Helvetica">selective_accept</FONT>, and the choice
among queued entry calls of a protected object when more than one <FONT FACE="Arial, Helvetica">entry_barrier</FONT>
<FONT FACE="Arial, Helvetica">condition</FONT> is True. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal"> Two queuing policies, FIFO_Queuing and Priority_Queuing,
are language defined. If no Queuing_Policy pragma appears in 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 <A HREF="AA-9-5-3.html">9.5.3</A>
and <A HREF="AA-9-7-1.html">9.7.1</A>.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The Priority_Queuing
policy is defined as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC><A NAME="I6829"></A>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 <I>priority of an entry call</I> 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).</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10/1</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>{<I><A HREF="defect1.html#8652/0075">8652/0075</A></I>}
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<U> while the task is blocked on an entry call</U>.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When the base priority of a task is set (see <A HREF="AA-D-5.html">D.5</A>),
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. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Reason: </B>A task is blocked
on an entry call if the entry call is simple, conditional, or timed.
If the call came from the <FONT FACE="Arial, Helvetica">triggering_statement</FONT>
of an <FONT FACE="Arial, Helvetica">asynchronous_select</FONT>, 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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.b</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>A previous version of Ada 9X required
queue reordering in the <FONT FACE="Arial, Helvetica">asynchronous_select</FONT>
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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.c</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>However, if the entry call is
``asynchronous,'' that is, it is due to an <FONT FACE="Arial, Helvetica">asynchronous_select</FONT>
whose <FONT FACE="Arial, Helvetica">triggering_statement</FONT> 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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.d</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Furthermore, when an entry is
used for <FONT FACE="Arial, Helvetica">asynchronous_select</FONT>s, 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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.e</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>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 <FONT FACE="Arial, Helvetica">abortable_part</FONT>.
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.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>11.f</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1>Several rules were removed or
simplified when we changed the rules so that calls due to <FONT FACE="Arial, Helvetica">asynchronous_select</FONT>s
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. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When more than one <FONT FACE="Arial, Helvetica">condition</FONT>
of an <FONT FACE="Arial, Helvetica">entry_barrier</FONT> 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 <FONT FACE="Arial, Helvetica">protected_definition</FONT>
is selected. For members of the same entry family, the one with the lower
family index is selected.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>If the expiration time of two or more open <FONT FACE="Arial, Helvetica">delay_alternative</FONT>s
is the same and no other <FONT FACE="Arial, Helvetica">accept_alternative</FONT>s
are open, the <FONT FACE="Arial, Helvetica">sequence_of_statements</FONT>
of the <FONT FACE="Arial, Helvetica">delay_alternative</FONT> that is
first in textual order in the <FONT FACE="Arial, Helvetica">selective_accept</FONT>
is executed.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When more than one alternative of a <FONT FACE="Arial, Helvetica">selective_accept</FONT>
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 <FONT FACE="Arial, Helvetica">accept_alternative</FONT>
that is first in textual order in the <FONT FACE="Arial, Helvetica">selective_accept</FONT>
is selected.</LI></UL>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<DIV Class="Normal"> Implementations are allowed to define other queuing
policies, but need not support more than one such policy per partition.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15.a.1/1</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Discussion: </B>{<I><A HREF="defect2.html#8652/0116">8652/0116</A></I>}
<U>This rule is really redundant, as <A HREF="AA-10-1-5.html">10.1.5</A>
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.</U> </FONT></DIV>
<H4 ALIGN=CENTER>Implementation Advice</H4>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<DIV Class="Normal"> The implementation should use names that end
with ``_Queuing'' for implementation-defined queuing policies.</DIV>
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