xsh_chap02_08.html

IEEE 1003.1-2003, Single Unix Specification v3

<li>
<p>When the running thread with assigned priority equal to <i>sched_priority</i> becomes a blocked thread, the execution time
consumed is subtracted from the available execution capacity, and a replenishment operation is scheduled, as described in 6 and 7.
If the available execution capacity would become negative by this operation, it shall be set to zero.</p>
</li>

<li>
<p>When the running thread with assigned priority equal to <i>sched_priority</i> reaches the limit imposed on its execution time,
it becomes the tail of the thread list for <i>sched_ss_low_priority</i>, the execution time consumed is subtracted from the
available execution capacity (which becomes zero), and a replenishment operation is scheduled, as described in 6 and 7.</p>
</li>

<li>
<p>Each time a replenishment operation is scheduled, the amount of execution capacity to be replenished, <i>replenish_amount</i>,
is set equal to the execution time consumed by the thread since the <i>activation_time</i>. The replenishment is scheduled to occur
at <i>activation_time</i> plus <i>sched_ss_repl_period</i>. If the scheduled time obtained is before the current time, the
replenishment operation is carried out immediately. Several replenishment operations may be pending at the same time, each of which
will be serviced at its respective scheduled time. With the above rules, the number of replenishment operations simultaneously
pending for a given thread that is scheduled under the sporadic server policy shall not be greater than
<i>sched_ss_max_repl</i>.</p>
</li>

<li>
<p>A replenishment operation consists of adding the corresponding <i>replenish_amount</i> to the available execution capacity at
the scheduled time. If, as a consequence of this operation, the execution capacity would become larger than
<i>sched_ss_initial_budget</i>, it shall be rounded down to a value equal to <i>sched_ss_initial_budget</i>. Additionally, if the
thread was runnable or running, and had assigned priority equal to <i>sched_ss_low_priority</i>, then it becomes the tail of the
thread list for <i>sched_priority</i>.</p>
</li>
</ol>

<p>Execution time is defined in <a href="xsh_chap02_02.html#tag_02_02_02"><i>The Name Space</i></a> .</p>

<p>For this policy, changing the value of a CPU-time clock via <a href="../functions/clock_settime.html"><i>clock_settime</i>()</a>
shall have no effect on its behavior.</p>

<p>For this policy, valid priorities shall be within the range returned by the <a href=
"../functions/sched_get_priority_min.html"><i>sched_get_priority_min</i>()</a> and <a href=
"../functions/sched_get_priority_max.html"><i>sched_get_priority_max</i>()</a> functions when SCHED_SPORADIC is provided as the
parameter. Conforming implementations shall provide a priority range of at least 32 distinct priorities for this policy.</p>

<h5><a name="tag_02_08_04_05"></a>SCHED_OTHER</h5>

<p>Conforming implementations shall include one scheduling policy identified as SCHED_OTHER (which may execute identically with
either the FIFO or round robin scheduling policy). The effect of scheduling threads with the SCHED_OTHER policy in a system in
which other threads are executing under SCHED_FIFO, SCHED_RR, <sup>[<a href="javascript:open_code('SS')">SS</a>]</sup> <img src=
"../images/opt-start.gif" alt="[Option Start]" border="0"> &nbsp;or SCHED_SPORADIC <img src="../images/opt-end.gif" alt=
"[Option End]" border="0"> is implementation-defined.</p>

<p>This policy is defined to allow strictly conforming applications to be able to indicate in a portable manner that they no longer
need a realtime scheduling policy.</p>

<p>For threads executing under this policy, the implementation shall use only priorities within the range returned by the <a href=
"../functions/sched_get_priority_max.html"><i>sched_get_priority_max</i>()</a> and <a href=
"../functions/sched_get_priority_min.html"><i>sched_get_priority_min</i>()</a> functions when SCHED_OTHER is provided as the
parameter.</p>

<h4><a name="tag_02_08_05"></a>Clocks and Timers</h4>

<p><sup>[<a href="javascript:open_code('TMR')">TMR</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
The functionality described in this section is dependent on support of the Timers option (and the rest of this section is not
further shaded for this option). <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>

<p>The <a href="../basedefs/time.h.html"><i>&lt;time.h&gt;</i></a> header defines the types and manifest constants used by the
timing facility.</p>

<h5><a name="tag_02_08_05_01"></a>Time Value Specification Structures</h5>

<p>Many of the timing facility functions accept or return time value specifications. A time value structure <b>timespec</b>
specifies a single time value and includes at least the following members:</p>

<center>
<table border="1" cellpadding="3" align="center">
<tr valign="top">
<th align="center">
<p class="tent"><b>Member Type</b></p>
</th>
<th align="center">
<p class="tent"><b>Member Name</b></p>
</th>
<th align="center">
<p class="tent"><b>Description</b></p>
</th>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><b>time_t</b></p>
</td>
<td align="left">
<p class="tent"><i>tv_sec</i></p>
</td>
<td align="left">
<p class="tent">Seconds.</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><b>long</b></p>
</td>
<td align="left">
<p class="tent"><i>tv_nsec</i></p>
</td>
<td align="left">
<p class="tent">Nanoseconds.</p>
</td>
</tr>
</table>
</center>

<p>The <i>tv_nsec</i> member is only valid if greater than or equal to zero, and less than the number of nanoseconds in a second
(1000 million). The time interval described by this structure is (<i>tv_sec</i> * 10<sup><small>9</small></sup> + <i>tv_nsec</i>)
nanoseconds.</p>

<p>A time value structure <b>itimerspec</b> specifies an initial timer value and a repetition interval for use by the per-process
timer functions. This structure includes at least the following members:</p>

<center>
<table border="1" cellpadding="3" align="center">
<tr valign="top">
<th align="center">
<p class="tent"><b>Member Type</b></p>
</th>
<th align="center">
<p class="tent"><b>Member Name</b></p>
</th>
<th align="center">
<p class="tent"><b>Description</b></p>
</th>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><b>struct timespec</b></p>
</td>
<td align="left">
<p class="tent"><i>it_interval</i></p>
</td>
<td align="left">
<p class="tent">Timer period.</p>
</td>
</tr>

<tr valign="top">
<td align="left">
<p class="tent"><b>struct timespec</b></p>
</td>
<td align="left">
<p class="tent"><i>it_value</i></p>
</td>
<td align="left">
<p class="tent">Timer expiration.</p>
</td>
</tr>
</table>
</center>

<p>If the value described by <i>it_value</i> is non-zero, it indicates the time to or time of the next timer expiration (for
relative and absolute timer values, respectively). If the value described by <i>it_value</i> is zero, the timer shall be
disarmed.</p>

<p>If the value described by <i>it_interval</i> is non-zero, it specifies an interval which shall be used in reloading the timer
when it expires; that is, a periodic timer is specified. If the value described by <i>it_interval</i> is zero, the timer is
disarmed after its next expiration; that is, a one-shot timer is specified.</p>

<h5><a name="tag_02_08_05_02"></a>Timer Event Notification Control Block</h5>

<p><sup>[<a href="javascript:open_code('RTS')">RTS</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
Per-process timers may be created that notify the process of timer expirations by queuing a realtime extended signal. The
<b>sigevent</b> structure, defined in the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/signal.h.html"><i>&lt;signal.h&gt;</i></a>, is used in creating such a timer. The <b>sigevent</b> structure contains
the signal number and an application-specific data value which shall be used when notifying the calling process of timer expiration
events. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>

<h5><a name="tag_02_08_05_03"></a>Manifest Constants</h5>

<p>The following constants are defined in the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/time.h.html"><i>&lt;time.h&gt;</i></a>:</p>

<dl compact>
<dt>CLOCK_REALTIME</dt>

<dd>The identifier for the system-wide realtime clock.</dd>

<dt>TIMER_ABSTIME</dt>

<dd>Flag indicating time is absolute with respect to the clock associated with a timer.</dd>

<dt>CLOCK_MONOTONIC</dt>

<dd><sup>[<a href="javascript:open_code('MON')">MON</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
The identifier for the system-wide monotonic clock, which is defined as a clock whose value cannot be set via <a href=
"../functions/clock_settime.html"><i>clock_settime</i>()</a> and which cannot have backward clock jumps. The maximum possible clock
jump is implementation-defined. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
</dl>

<p>The maximum allowable resolution for CLOCK_REALTIME and <sup>[<a href="javascript:open_code('MON')">MON</a>]</sup> <img src=
"../images/opt-start.gif" alt="[Option Start]" border="0"> &nbsp;CLOCK_MONOTONIC <img src="../images/opt-end.gif" alt=
"[Option End]" border="0"> clocks and all time services based on these clocks is represented by {_POSIX_CLOCKRES_MIN} and shall be
defined as 20 ms (1/50 of a second). Implementations may support smaller values of resolution for these clocks to provide finer
granularity time bases. The actual resolution supported by an implementation for a specific clock is obtained using the <a href=
"../functions/clock_getres.html"><i>clock_getres</i>()</a> function. If the actual resolution supported for a time service based on
one of these clocks differs from the resolution supported for that clock, the implementation shall document this difference.</p>

<p>The minimum allowable maximum value for CLOCK_REALTIME and <sup>[<a href="javascript:open_code('MON')">MON</a>]</sup> <img src=
"../images/opt-start.gif" alt="[Option Start]" border="0"> &nbsp;CLOCK_MONOTONIC <img src="../images/opt-end.gif" alt=
"[Option End]" border="0"> clocks and all absolute time services based on them is the same as that defined by the ISO&nbsp;C
standard for the <b>time_t</b> type. If the maximum value supported by a time service based on one of these clocks differs from the
maximum value supported by that clock, the implementation shall document this difference.</p>

<h5><a name="tag_02_08_05_04"></a>Execution Time Monitoring</h5>

<p><sup>[<a href="javascript:open_code('CPT')">CPT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
If _POSIX_CPUTIME is defined, process CPU-time clocks shall be supported in addition to the clocks described in <a href=
"#tag_02_08_05_03">Manifest Constants</a> . <img src="../images/opt-end.gif" alt="[Option End]" border="0"></p>

<p><sup>[<a href="javascript:open_code('TCT')">TCT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
If _POSIX_THREAD_CPUTIME is defined, thread CPU-time clocks shall be supported. <img src="../images/opt-end.gif" alt="[Option End]"
border="0"></p>

<p><sup>[<a href="javascript:open_code('CPT')">CPT|TCT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border=
"0"> CPU-time clocks measure execution or CPU time, which is defined in the Base Definitions volume of
IEEE&nbsp;Std&nbsp;1003.1-2001, <a href="../basedefs/xbd_chap03.html#tag_03_117">Section 3.117, CPU Time (Execution Time)</a>. The
mechanism used to measure execution time is described in the Base Definitions volume of IEEE&nbsp;Std&nbsp;1003.1-2001, <a href=
"../basedefs/xbd_chap04.html#tag_04_09">Section 4.9, Measurement of Execution Time</a>. <img src="../images/opt-end.gif" alt=
"[Option End]" border="0"></p>

<p><sup>[<a href="javascript:open_code('CPT')">CPT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
If _POSIX_CPUTIME is defined, the following constant of the type <b>clockid_t</b> is defined in <a href=
"../basedefs/time.h.html"><i>&lt;time.h&gt;</i></a>:</p>

<dl compact>
<dt>CLOCK_PROCESS_CPUTIME_ID</dt>

<dd><br>
When this value of the type <b>clockid_t</b> is used in a <a href="../functions/clock.html"><i>clock</i>()</a> or <a href=
"../functions/timer_create.html"><i>timer*</i>()</a> function call, it is interpreted as the identifier of the CPU-time clock associated
with the process making the function call.</dd>
</dl>

<img src="../images/opt-end.gif" alt="[Option End]" border="0"> 

<p><sup>[<a href="javascript:open_code('TCT')">TCT</a>]</sup> <img src="../images/opt-start.gif" alt="[Option Start]" border="0">
If _POSIX_THREAD_CPUTIME is defined, the following constant of the type <b>clockid_t</b> is defined in <a href=
"../basedefs/time.h.html"><i>&lt;time.h&gt;</i></a>:</p>

<dl compact>
<dt>CLOCK_THREAD_CPUTIME_ID</dt>

<dd><br>
When this value of the type <b>clockid_t</b> is used in a <a href="../functions/clock.html"><i>clock</i>()</a> or <a href=
"../functions/timer_create.html"><i>timer*</i>()</a> function call, it is interpreted as the identifier of the CPU-time clock associated
with the thread making the function call. <img src="../images/opt-end.gif" alt="[Option End]" border="0"></dd>
</dl>



<hr size="2" noshade>
<center><font size="2"><!--footer start-->
UNIX &reg; is a registered Trademark of The Open Group.<br>
POSIX &reg; is a registered Trademark of The IEEE.<br>
[ <a href="../mindex.html">Main Index</a> | <a href="../basedefs/contents.html">XBD</a> | <a href=
"../utilities/contents.html">XCU</a> | <a href="../functions/contents.html">XSH</a> | <a href="../xrat/contents.html">XRAT</a>
]</font></center>

<!--footer end-->
<hr size="2" noshade>
</body>
</html>