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-<?xml version="1.0" encoding="UTF-8" standalone="no"?>
-<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Design</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.77.1" /><meta name="keywords" content="C++, library, parallel" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="parallel_mode.html" title="Chapter 18. Parallel Mode" /><link rel="prev" href="parallel_mode_using.html" title="Using" /><link rel="next" href="parallel_mode_test.html" title="Testing" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Design</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="parallel_mode_using.html">Prev</a> </td><th width="60%" align="center">Chapter 18. Parallel Mode</th><td width="20%" align="right"> <a accesskey="n" href="parallel_mode_test.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.ext.parallel_mode.design"></a>Design</h2></div></div></div><p>
- </p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="parallel_mode.design.intro"></a>Interface Basics</h3></div></div></div><p>
-All parallel algorithms are intended to have signatures that are
-equivalent to the ISO C++ algorithms replaced. For instance, the
-<code class="function">std::adjacent_find</code> function is declared as:
-</p><pre class="programlisting">
-namespace std
-{
- template&lt;typename _FIter&gt;
- _FIter
- adjacent_find(_FIter, _FIter);
-}
-</pre><p>
-Which means that there should be something equivalent for the parallel
-version. Indeed, this is the case:
-</p><pre class="programlisting">
-namespace std
-{
- namespace __parallel
- {
- template&lt;typename _FIter&gt;
- _FIter
- adjacent_find(_FIter, _FIter);
-
- ...
- }
-}
-</pre><p>But.... why the ellipses?
-</p><p> The ellipses in the example above represent additional overloads
-required for the parallel version of the function. These additional
-overloads are used to dispatch calls from the ISO C++ function
-signature to the appropriate parallel function (or sequential
-function, if no parallel functions are deemed worthy), based on either
-compile-time or run-time conditions.
-</p><p> The available signature options are specific for the different
-algorithms/algorithm classes.</p><p> The general view of overloads for the parallel algorithms look like this:
-</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>ISO C++ signature</p></li><li class="listitem"><p>ISO C++ signature + sequential_tag argument</p></li><li class="listitem"><p>ISO C++ signature + algorithm-specific tag type
- (several signatures)</p></li></ul></div><p> Please note that the implementation may use additional functions
-(designated with the <code class="code">_switch</code> suffix) to dispatch from the
-ISO C++ signature to the correct parallel version. Also, some of the
-algorithms do not have support for run-time conditions, so the last
-overload is therefore missing.
-</p></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="parallel_mode.design.tuning"></a>Configuration and Tuning</h3></div></div></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="parallel_mode.design.tuning.omp"></a>Setting up the OpenMP Environment</h4></div></div></div><p>
-Several aspects of the overall runtime environment can be manipulated
-by standard OpenMP function calls.
-</p><p>
-To specify the number of threads to be used for the algorithms globally,
-use the function <code class="function">omp_set_num_threads</code>. An example:
-</p><pre class="programlisting">
-#include &lt;stdlib.h&gt;
-#include &lt;omp.h&gt;
-
-int main()
-{
- // Explicitly set number of threads.
- const int threads_wanted = 20;
- omp_set_dynamic(false);
- omp_set_num_threads(threads_wanted);
-
- // Call parallel mode algorithms.
-
- return 0;
-}
-</pre><p>
- Some algorithms allow the number of threads being set for a particular call,
- by augmenting the algorithm variant.
- See the next section for further information.
-</p><p>
-Other parts of the runtime environment able to be manipulated include
-nested parallelism (<code class="function">omp_set_nested</code>), schedule kind
-(<code class="function">omp_set_schedule</code>), and others. See the OpenMP
-documentation for more information.
-</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="parallel_mode.design.tuning.compile"></a>Compile Time Switches</h4></div></div></div><p>
-To force an algorithm to execute sequentially, even though parallelism
-is switched on in general via the macro <code class="constant">_GLIBCXX_PARALLEL</code>,
-add <code class="classname">__gnu_parallel::sequential_tag()</code> to the end
-of the algorithm's argument list.
-</p><p>
-Like so:
-</p><pre class="programlisting">
-std::sort(v.begin(), v.end(), __gnu_parallel::sequential_tag());
-</pre><p>
-Some parallel algorithm variants can be excluded from compilation by
-preprocessor defines. See the doxygen documentation on
-<code class="code">compiletime_settings.h</code> and <code class="code">features.h</code> for details.
-</p><p>
-For some algorithms, the desired variant can be chosen at compile-time by
-appending a tag object. The available options are specific to the particular
-algorithm (class).
-</p><p>
-For the "embarrassingly parallel" algorithms, there is only one "tag object
-type", the enum _Parallelism.
-It takes one of the following values,
-<code class="code">__gnu_parallel::parallel_tag</code>,
-<code class="code">__gnu_parallel::balanced_tag</code>,
-<code class="code">__gnu_parallel::unbalanced_tag</code>,
-<code class="code">__gnu_parallel::omp_loop_tag</code>,
-<code class="code">__gnu_parallel::omp_loop_static_tag</code>.
-This means that the actual parallelization strategy is chosen at run-time.
-(Choosing the variants at compile-time will come soon.)
-</p><p>
-For the following algorithms in general, we have
-<code class="code">__gnu_parallel::parallel_tag</code> and
-<code class="code">__gnu_parallel::default_parallel_tag</code>, in addition to
-<code class="code">__gnu_parallel::sequential_tag</code>.
-<code class="code">__gnu_parallel::default_parallel_tag</code> chooses the default
-algorithm at compiletime, as does omitting the tag.
-<code class="code">__gnu_parallel::parallel_tag</code> postpones the decision to runtime
-(see next section).
-For all tags, the number of threads desired for this call can optionally be
-passed to the respective tag's constructor.
-</p><p>
-The <code class="code">multiway_merge</code> algorithm comes with the additional choices,
-<code class="code">__gnu_parallel::exact_tag</code> and
-<code class="code">__gnu_parallel::sampling_tag</code>.
-Exact and sampling are the two available splitting strategies.
-</p><p>
-For the <code class="code">sort</code> and <code class="code">stable_sort</code> algorithms, there are
-several additional choices, namely
-<code class="code">__gnu_parallel::multiway_mergesort_tag</code>,
-<code class="code">__gnu_parallel::multiway_mergesort_exact_tag</code>,
-<code class="code">__gnu_parallel::multiway_mergesort_sampling_tag</code>,
-<code class="code">__gnu_parallel::quicksort_tag</code>, and
-<code class="code">__gnu_parallel::balanced_quicksort_tag</code>.
-Multiway mergesort comes with the two splitting strategies for multi-way
-merging. The quicksort options cannot be used for <code class="code">stable_sort</code>.
-</p></div><div class="section"><div class="titlepage"><div><div><h4 class="title"><a id="parallel_mode.design.tuning.settings"></a>Run Time Settings and Defaults</h4></div></div></div><p>
-The default parallelization strategy, the choice of specific algorithm
-strategy, the minimum threshold limits for individual parallel
-algorithms, and aspects of the underlying hardware can be specified as
-desired via manipulation
-of <code class="classname">__gnu_parallel::_Settings</code> member data.
-</p><p>
-First off, the choice of parallelization strategy: serial, parallel,
-or heuristically deduced. This corresponds
-to <code class="code">__gnu_parallel::_Settings::algorithm_strategy</code> and is a
-value of enum <span class="type">__gnu_parallel::_AlgorithmStrategy</span>
-type. Choices
-include: <span class="type">heuristic</span>, <span class="type">force_sequential</span>,
-and <span class="type">force_parallel</span>. The default is <span class="type">heuristic</span>.
-</p><p>
-Next, the sub-choices for algorithm variant, if not fixed at compile-time.
-Specific algorithms like <code class="function">find</code> or <code class="function">sort</code>
-can be implemented in multiple ways: when this is the case,
-a <code class="classname">__gnu_parallel::_Settings</code> member exists to
-pick the default strategy. For
-example, <code class="code">__gnu_parallel::_Settings::sort_algorithm</code> can
-have any values of
-enum <span class="type">__gnu_parallel::_SortAlgorithm</span>: <span class="type">MWMS</span>, <span class="type">QS</span>,
-or <span class="type">QS_BALANCED</span>.
-</p><p>
-Likewise for setting the minimal threshold for algorithm
-parallelization. Parallelism always incurs some overhead. Thus, it is
-not helpful to parallelize operations on very small sets of
-data. Because of this, measures are taken to avoid parallelizing below
-a certain, pre-determined threshold. For each algorithm, a minimum
-problem size is encoded as a variable in the
-active <code class="classname">__gnu_parallel::_Settings</code> object. This
-threshold variable follows the following naming scheme:
-<code class="code">__gnu_parallel::_Settings::[algorithm]_minimal_n</code>. So,
-for <code class="function">fill</code>, the threshold variable
-is <code class="code">__gnu_parallel::_Settings::fill_minimal_n</code>,
-</p><p>
-Finally, hardware details like L1/L2 cache size can be hardwired
-via <code class="code">__gnu_parallel::_Settings::L1_cache_size</code> and friends.
-</p><p>
-</p><p>
-All these configuration variables can be changed by the user, if
-desired.
-There exists one global instance of the class <code class="classname">_Settings</code>,
-i. e. it is a singleton. It can be read and written by calling
-<code class="code">__gnu_parallel::_Settings::get</code> and
-<code class="code">__gnu_parallel::_Settings::set</code>, respectively.
-Please note that the first call return a const object, so direct manipulation
-is forbidden.
-See <a class="link" href="http://gcc.gnu.org/onlinedocs/libstdc++/latest-doxygen/a01005.html" target="_top">
- <code class="filename">settings.h</code></a>
-for complete details.
-</p><p>
-A small example of tuning the default:
-</p><pre class="programlisting">
-#include &lt;parallel/algorithm&gt;
-#include &lt;parallel/settings.h&gt;
-
-int main()
-{
- __gnu_parallel::_Settings s;
- s.algorithm_strategy = __gnu_parallel::force_parallel;
- __gnu_parallel::_Settings::set(s);
-
- // Do work... all algorithms will be parallelized, always.
-
- return 0;
-}
-</pre></div></div><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="parallel_mode.design.impl"></a>Implementation Namespaces</h3></div></div></div><p> One namespace contain versions of code that are always
-explicitly sequential:
-<code class="code">__gnu_serial</code>.
-</p><p> Two namespaces contain the parallel mode:
-<code class="code">std::__parallel</code> and <code class="code">__gnu_parallel</code>.
-</p><p> Parallel implementations of standard components, including
-template helpers to select parallelism, are defined in <code class="code">namespace
-std::__parallel</code>. For instance, <code class="function">std::transform</code> from <code class="filename">algorithm</code> has a parallel counterpart in
-<code class="function">std::__parallel::transform</code> from <code class="filename">parallel/algorithm</code>. In addition, these parallel
-implementations are injected into <code class="code">namespace
-__gnu_parallel</code> with using declarations.
-</p><p> Support and general infrastructure is in <code class="code">namespace
-__gnu_parallel</code>.
-</p><p> More information, and an organized index of types and functions
-related to the parallel mode on a per-namespace basis, can be found in
-the generated source documentation.
-</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="parallel_mode_using.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="parallel_mode.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="parallel_mode_test.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Using </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Testing</td></tr></table></div></body></html> \ No newline at end of file