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<div class="title">settings.h File Reference</div> </div>
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<tr class="memitem:"><td class="memItemLeft" align="right" valign="top">struct  </td><td class="memItemRight" valign="bottom"><a class="el" href="a09635.html">__gnu_parallel::_Settings</a></td></tr>
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<tr class="memitem:ad816e8f52b933c9300598977cada7d18"><td class="memItemLeft" align="right" valign="top">#define </td><td class="memItemRight" valign="bottom"><a class="el" href="a00962.html#ad816e8f52b933c9300598977cada7d18">_GLIBCXX_PARALLEL_CONDITION</a>(__c)</td></tr>
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<a name="details" id="details"></a><h2 class="groupheader">Detailed Description</h2>
<div class="textblock"><p>Runtime settings and tuning parameters, heuristics to decide whether to use parallelized algorithms. This file is a GNU parallel extension to the Standard C++ Library. </p>
<h1><a class="anchor" id="parallelization_decision"></a>
parallelization_decision</h1>
<p>The decision whether to run an algorithm in parallel.</p>
<p>There are several ways the user can switch on and __off the parallel execution of an algorithm, both at compile- and run-time.</p>
<p>Only sequential execution can be forced at compile-time. This reduces code size and protects code parts that have non-thread-safe side effects.</p>
<p>Ultimately, forcing parallel execution at compile-time makes sense. Often, the sequential algorithm implementation is used as a subroutine, so no reduction in code size can be achieved. Also, the machine the program is run on might have only one processor core, so to avoid overhead, the algorithm is executed sequentially.</p>
<p>To force sequential execution of an algorithm ultimately at compile-time, the user must add the tag gnu_parallel::sequential_tag() to the end of the parameter list, e. g.</p>
<div class="fragment"><div class="line"><a class="code" href="a01485.html#gae3c0067ae83db1824367ded659a960b2">std::sort</a>(__v.begin(), __v.end(), <a class="code" href="a09639.html">__gnu_parallel::sequential_tag</a>());</div></div><!-- fragment --><p>This is compatible with all overloaded algorithm variants. No additional code will be instantiated, at all. The same holds for most algorithm calls with iterators not providing random access.</p>
<p>If the algorithm call is not forced to be executed sequentially at compile-time, the decision is made at run-time. The global variable __gnu_parallel::_Settings::algorithm_strategy is checked. _It is a tristate variable corresponding to:</p>
<p>a. force_sequential, meaning the sequential algorithm is executed. b. force_parallel, meaning the parallel algorithm is executed. c. heuristic</p>
<p>For heuristic, the parallel algorithm implementation is called only if the input size is sufficiently large. For most algorithms, the input size is the (combined) length of the input sequence(__s). The threshold can be set by the user, individually for each algorithm. The according variables are called gnu_parallel::_Settings::[algorithm]_minimal_n .</p>
<p>For some of the algorithms, there are even more tuning options, e. g. the ability to choose from multiple algorithm variants. See below for details. </p>
<p class="definition">Definition in file <a class="el" href="a00962_source.html">settings.h</a>.</p>
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<h2 class="memtitle"><span class="permalink"><a href="#ad816e8f52b933c9300598977cada7d18">◆ </a></span>_GLIBCXX_PARALLEL_CONDITION</h2>
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<td class="memname">#define _GLIBCXX_PARALLEL_CONDITION</td>
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<p>Determine at compile(?)-time if the parallel variant of an algorithm should be called. </p>
<dl class="params"><dt>Parameters</dt><dd>
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<tr><td class="paramname">__c</td><td>A condition that is convertible to bool that is overruled by __gnu_parallel::_Settings::algorithm_strategy. Usually a decision based on the input size. </td></tr>
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<p class="definition">Definition at line <a class="el" href="a00962_source.html#l00095">95</a> of file <a class="el" href="a00962_source.html">settings.h</a>.</p>
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