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<h1 class="topictitle1">Partitioners</h1>
<div>
<div class="section"><h2 class="sectiontitle">Summary</h2>
<p>A partitioner specifies how a loop template should partition its work
among threads.
</p>
</div>
<div class="section"><h2 class="sectiontitle"> Description</h2>
<p>The default behavior of the loop templates<samp class="codeph"> parallel_for</samp>,
<samp class="codeph">parallel_reduce</samp>, and <samp class="codeph">parallel_scan</samp> tries to
recursively split a range into enough parts to keep processors busy, not necessarily
splitting as finely as possible. An optional partitioner parameter enables other
behaviors to be specified, as shown in the table below. The first column of the table shows
how the formal parameter is declared in the loop templates. An<samp class="codeph">
affinity_partitioner</samp> is passed by non-const reference because it is
updated to remember where loop iterations run. </p>
<p>
<div class="tablenoborder"><table cellpadding="4" summary="" width="100%" frame="hsides" border="1" rules="all"><caption><span class="tablecap">Partitioners</span></caption>
<thead align="left">
<tr>
<th class="cellrowborder" valign="top" id="d9145e71">
<p> Partitioner
</p>
</th>
<th class="row-nocellborder" valign="top" id="d9145e77">
<p> Loop Behavior
</p>
</th>
</tr>
</thead>
<tbody>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d9145e71 ">
<p><samp class="codeph">const auto_partitioner&
(default)</samp>
</p>
<div class="Note"><h3 class="NoteTipHead">
Note</h3><p>In Intel® Threading Building Blocks (Intel® TBB) 2.1, <samp class="codeph">simple_partitioner</samp> was the default. Intel®
TBB 2.2 changed the default to
<samp class="codeph">auto_partitioner</samp> to simplify common
usage of the loop templates. To get the old default,
compile with the preprocessor symbol
<samp class="codeph">TBB_DEPRECATED=1</samp>. </p>
</div>
</td>
<td class="row-nocellborder" valign="top" headers="d9145e77 ">
<p>Performs sufficient splitting to balance load, not necessarily splitting
as finely as <samp class="codeph">Range::is_divisible</samp> permits.
When used with classes such as
<samp class="codeph">blocked_range</samp>, the selection of an
appropriate grainsize is less important, and often
acceptable performance can be achieved with the default
grain size of 1. </p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d9145e71 ">
<p><samp class="codeph">affinity_partitioner&</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d9145e77 ">
<p> Similar to <samp class="codeph">auto_partitioner</samp>, but improves cache
affinity by its choice of mapping subranges to worker
threads. It can improve performance significantly when a
loop is re-executed over the same data set, and the data set
fits in cache. </p>
</td>
</tr>
<tr valign="top">
<td class="cellrowborder" valign="top" headers="d9145e71 ">
<p><samp class="codeph">const simple_partitioner&
</samp>
</p>
</td>
<td class="row-nocellborder" valign="top" headers="d9145e77 ">
<p>Recursively splits a range until it is no longer divisible. The
<samp class="codeph">Range::is_divisible</samp> function is wholly
responsible for deciding when recursive splitting halts.
When used with classes such as
<samp class="codeph">blocked_range</samp>, the selection of an
appropriate grainsize is critical to enabling concurrency
while limiting overheads (see the discussion in the blocked_range Template Class section). </p>
</td>
</tr>
</tbody>
</table>
</div>
</p>
</div>
</div>
<div class="familylinks">
<div class="parentlink"><strong>Parent topic:</strong> <a href="../../reference/algorithms.htm">Algorithms</a></div>
</div>
<div class="See Also">
<ul class="ullinks">
<li class="ulchildlink"><a href="../../reference/algorithms/partitioners/auto_partitioner_cls.htm">auto_partitioner Class</a><br>
</li>
<li class="ulchildlink"><a href="../../reference/algorithms/partitioners/affinity_partitioner.htm">affinity_partitioner</a><br>
</li>
<li class="ulchildlink"><a href="../../reference/algorithms/partitioners/simple_partitioner_cls.htm">simple_partitioner Class</a><br>
</li>
</ul>
<h2>See Also</h2>
<div class="linklist">
<div><a href="parallel_for_func.htm#parallel_for_func">parallel_for Template Function
</a></div>
<div><a href="parallel_reduce_func.htm#parallel_reduce_func">parallel_reduce Template Function
</a></div>
<div><a href="parallel_scan_func.htm#parallel_scan_func">parallel_scan Template Function
</a></div></div>
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