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<H1> D.2.2 The Standard Task Dispatching Policy</H1>
<H4 ALIGN=CENTER>Syntax</H4>
<DIV Class="Paranum"><FONT SIZE=-2>1</FONT></DIV>
<DIV Class="SyntaxIndented" Style="margin-bottom: 0.2em">The form of
a <FONT FACE="Arial, Helvetica">pragma</FONT> Task_Dispatching_Policy
is as follows: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>2</FONT></DIV>
<DIV Class="SyntaxIndented"> <B>pragma</B> <A NAME="I6795"></A>Task_Dispatching_Policy(<I>policy_</I><A NAME="I6796"></A><FONT FACE="Arial, Helvetica">identifier</FONT>
); </DIV>
<H4 ALIGN=CENTER>Legality Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>3</FONT></DIV>
<DIV Class="Normal"> The <I>policy_</I><A NAME="I6797"></A><FONT FACE="Arial, Helvetica">identifier</FONT>
shall either be FIFO_Within_Priorities or an implementation-defined <A NAME="I6798"></A><FONT FACE="Arial, Helvetica">identifier</FONT>.
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>3.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation-defined
<I>policy_</I><A NAME="I6799"></A><FONT FACE="Arial, Helvetica">identifier</FONT>s
allowed in a <FONT FACE="Arial, Helvetica">pragma</FONT> Task_Dispatching_Policy.</FONT></DIV>
<H4 ALIGN=CENTER>Post-Compilation Rules</H4>
<DIV Class="Paranum"><FONT SIZE=-2>4</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6800"></A><A NAME="I6801"></A>A Task_Dispatching_Policy
pragma is a configuration pragma.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>5</FONT></DIV>
<DIV Class="Normal"> If the FIFO_Within_Priorities policy is specified
for a partition, then the Ceiling_Locking policy (see <A HREF="AA-D-3.html">D.3</A>)
shall also be specified for the partition. </DIV>
<H4 ALIGN=CENTER>Dynamic Semantics</H4>
<DIV Class="Paranum"><FONT SIZE=-2>6</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6802"></A>[A <I>task dispatching policy</I>
specifies the details of task dispatching that are not covered by the
basic task dispatching model. These rules govern when tasks are inserted
into and deleted from the ready queues, and whether a task is inserted
at the head or the tail of the queue for its active priority.] The task
dispatching policy is specified by a Task_Dispatching_Policy configuration
pragma. <A NAME="I6803"></A>If no such pragma appears in any of the program
units comprising a partition, the task dispatching policy for that partition
is unspecified.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>7</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> The language defines
only one task dispatching policy, FIFO_Within_Priorities; when this policy
is in effect, modifications to the ready queues occur only as follows:
</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>8</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When a blocked task becomes ready, it is added at the tail
of the ready queue for its active priority.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>9</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When the active priority of a ready task that is not running
changes, or the setting of its base priority takes effect, the task is
removed from the ready queue for its old active priority and is added
at the tail of the ready queue for its new active priority, except in
the case where the active priority is lowered due to the loss of inherited
priority, in which case the task is added at the head of the ready queue
for its new active priority.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>10</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When the setting of the base priority of a running task
takes effect, the task is added to the tail of the ready queue for its
active priority.</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>When a task executes a <FONT FACE="Arial, Helvetica">delay_statement</FONT>
that does not result in blocking, it is added to the tail of the ready
queue for its active priority. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>11.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Ramification: </B>If the delay
does result in blocking, the task moves to the ``delay queue'', not to
the ready queue. </FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>12</FONT></DIV>
<DIV Class="Normal"> <A NAME="I6804"></A><A NAME="I6805"></A>Each
of the events specified above is a task dispatching point (see <A HREF="AA-D-2-1.html">D.2.1</A>).</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>13</FONT></DIV>
<DIV Class="Normal"> In addition, when a task is preempted, it is
added at the head of the ready queue for its active priority.</DIV>
<H4 ALIGN=CENTER>Documentation Requirements</H4>
<DIV Class="Paranum"><FONT SIZE=-2>14</FONT></DIV>
<DIV Class="Normal" Style="margin-bottom: 0.4em"> <A NAME="I6806"></A><I>Priority
inversion</I> is the duration for which a task remains at the head of
the highest priority ready queue while the processor executes a lower
priority task. The implementation shall document: </DIV>
<DIV Class="Paranum"><FONT SIZE=-2>15</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>The maximum priority inversion a user task can experience
due to activity of the implementation (on behalf of lower priority tasks),
and</LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>16</FONT></DIV>
<UL Class="Bulleted"><LI TYPE=DISC>whether execution of a task can be preempted by the implementation
processing of delay expirations for lower priority tasks, and if so,
for how long. </LI></UL>
<DIV Class="Paranum"><FONT SIZE=-2>16.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation-defined
aspects of priority inversion.</FONT></DIV>
<H4 ALIGN=CENTER>Implementation Permissions</H4>
<DIV Class="Paranum"><FONT SIZE=-2>17</FONT></DIV>
<DIV Class="Normal"> Implementations are allowed to define other task
dispatching policies, but need not support more than one such policy
per partition.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18</FONT></DIV>
<DIV Class="Normal"> [For optimization purposes,] an implementation
may alter the points at which task dispatching occurs, in an implementation
defined manner. However, a <FONT FACE="Arial, Helvetica">delay_statement</FONT>
always corresponds to at least one task dispatching point.</DIV>
<DIV Class="Paranum"><FONT SIZE=-2>18.a</FONT></DIV>
<DIV Class="Annotations"><FONT SIZE=-1><B>Implementation defined: </B>Implementation
defined task dispatching.</FONT></DIV>
<DIV Class="NotesHeader"><FONT SIZE=-1>NOTES</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>19</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>13 If the active priority
of a running task is lowered due to loss of inherited priority (as it
is on completion of a protected operation) and there is a ready task
of the same active priority that is not running, the running task continues
to run (provided that there is no higher priority task).</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>20</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>14 The setting of a task's
base priority as a result of a call to Set_Priority does not always take
effect immediately when Set_Priority is called. The effect of setting
the task's base priority is deferred while the affected task performs
a protected action.</FONT></DIV>
<DIV Class="Paranum"><FONT SIZE=-2>21</FONT></DIV>
<DIV Class="Notes"><FONT SIZE=-1>15 Setting the base priority
of a ready task causes the task to move to the end of the queue for its
active priority, regardless of whether the active priority of the task
actually changes.</FONT></DIV>
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