1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Function all_reduce</title> 6<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css"> 7<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 8<link rel="home" href="../../index.html" title="The Boost C++ Libraries BoostBook Documentation Subset"> 9<link rel="up" href="../../mpi/reference.html#header.boost.mpi.collectives_hpp" title="Header <boost/mpi/collectives.hpp>"> 10<link rel="prev" href="all_gather.html" title="Function all_gather"> 11<link rel="next" href="all_to_all.html" title="Function all_to_all"> 12</head> 13<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 14<table cellpadding="2" width="100%"><tr> 15<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td> 16<td 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class="refnamediv"> 29<h2><span class="refentrytitle">Function all_reduce</span></h2> 30<p>boost::mpi::all_reduce — Combine the values stored by each process into a single value available to all processes. </p> 31</div> 32<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2> 33<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: <<a class="link" href="../../mpi/reference.html#header.boost.mpi.collectives_hpp" title="Header <boost/mpi/collectives.hpp>">boost/mpi/collectives.hpp</a>> 34 35</span> 36<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span> 37 <span class="keyword">void</span> <span class="identifier">all_reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">*</span> value<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span> 38 <span class="identifier">T</span> <span class="special">*</span> out_value<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span> 39<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span> 40 <span class="keyword">void</span> <span class="identifier">all_reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> value<span class="special">,</span> <span class="identifier">T</span> <span class="special">&</span> out_value<span class="special">,</span> 41 <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span> 42<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span> 43 <span class="identifier">T</span> <span class="identifier">all_reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <span class="keyword">const</span> <span class="identifier">T</span> <span class="special">&</span> value<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span> 44<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span> 45 <span class="keyword">void</span> <span class="identifier">all_reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <a class="link" href="inplace_t.html" title="Struct template inplace_t">inplace_t</a><span class="special"><</span> <span class="identifier">T</span> <span class="special">*</span> <span class="special">></span> value<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span> 46 <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span> 47<span class="keyword">template</span><span class="special"><</span><span class="keyword">typename</span> T<span class="special">,</span> <span class="keyword">typename</span> Op<span class="special">></span> 48 <span class="keyword">void</span> <span class="identifier">all_reduce</span><span class="special">(</span><span class="keyword">const</span> <a class="link" href="communicator.html" title="Class communicator">communicator</a> <span class="special">&</span> comm<span class="special">,</span> <a class="link" href="inplace_t.html" title="Struct template inplace_t">inplace_t</a><span class="special"><</span> <span class="identifier">T</span> <span class="special">></span> value<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">)</span><span class="special">;</span></pre></div> 49<div class="refsect1"> 50<a name="id-1.3.27.7.5.5.4"></a><h2>Description</h2> 51<p><code class="computeroutput">all_reduce</code> is a collective algorithm that combines the values stored by each process into a single value available to all processes. The values are combined in a user-defined way, specified via a function object. The type <code class="computeroutput">T</code> of the values may be any type that is serializable or has an associated MPI data type. One can think of this operation as a <code class="computeroutput">all_gather</code>, followed by an <code class="computeroutput">std::accumulate()</code> over the gather values and using the operation <code class="computeroutput">op</code>.</p> 52<p>When the type <code class="computeroutput">T</code> has an associated MPI data type, this routine invokes <code class="computeroutput">MPI_Allreduce</code> to perform the reduction. If possible, built-in MPI operations will be used; otherwise, <code class="computeroutput">all_reduce()</code> will create a custom MPI_Op for the call to MPI_Allreduce.</p> 53<p> 54If wrapped in a <code class="computeroutput"><code class="computeroutput"><a class="link" href="inplace_t.html" title="Struct template inplace_t">inplace_t</a></code></code> object, combine the usage of both input and $c out_value and the local value will be overwritten (a convenience function <code class="computeroutput">inplace</code> is provided for the wrapping).</p> 55<p> 56 57</p> 58<div class="variablelist"><table border="0" class="variablelist compact"> 59<colgroup> 60<col align="left" valign="top"> 61<col> 62</colgroup> 63<tbody> 64<tr> 65<td><p><span class="term">Parameters:</span></p></td> 66<td><div class="variablelist"><table border="0" class="variablelist compact"> 67<colgroup> 68<col align="left" valign="top"> 69<col> 70</colgroup> 71<tbody> 72<tr> 73<td><p><span class="term"><code class="computeroutput">comm</code></span></p></td> 74<td><p>The communicator over which the reduction will occur. </p></td> 75</tr> 76<tr> 77<td><p><span class="term"><code class="computeroutput">n</code></span></p></td> 78<td><p>Indicated the size of the buffers of array type. </p></td> 79</tr> 80<tr> 81<td><p><span class="term"><code class="computeroutput">op</code></span></p></td> 82<td><p>The binary operation that combines two values of type <code class="computeroutput">T</code> and returns a third value of type <code class="computeroutput">T</code>. For types <code class="computeroutput">T</code> that has ssociated MPI data types, <code class="computeroutput">op</code> will either be translated into an <code class="computeroutput">MPI_Op</code> (via <code class="computeroutput">MPI_Op_create</code>) or, if possible, mapped directly to a built-in MPI operation. See <code class="computeroutput"><code class="computeroutput"><a class="link" href="is_mpi_op.html" title="Struct template is_mpi_op">is_mpi_op</a></code></code> in the <code class="computeroutput">operations.hpp</code> header for more details on this mapping. For any non-built-in operation, commutativity will be determined by the <code class="computeroutput">is_commmutative</code> trait (also in <code class="computeroutput">operations.hpp</code>): users are encouraged to mark commutative operations as such, because it gives the implementation additional lattitude to optimize the reduction operation.</p></td> 83</tr> 84<tr> 85<td><p><span class="term"><code class="computeroutput">out_value</code></span></p></td> 86<td><p>Will receive the result of the reduction operation. If this parameter is omitted, the outgoing value will instead be returned.</p></td> 87</tr> 88<tr> 89<td><p><span class="term"><code class="computeroutput">value</code></span></p></td> 90<td><p>The local value to be combined with the local values of every other process. For reducing arrays, <code class="computeroutput">in_values</code> is a pointer to the local values to be reduced and <code class="computeroutput">n</code> is the number of values to reduce. See <code class="computeroutput">reduce</code> for more information.</p></td> 91</tr> 92</tbody> 93</table></div></td> 94</tr> 95<tr> 96<td><p><span class="term">Returns:</span></p></td> 97<td><p>If no <code class="computeroutput">out_value</code> parameter is supplied, returns the result of the reduction operation. </p></td> 98</tr> 99</tbody> 100</table></div> 101</div> 102</div> 103<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 104<td align="left"></td> 105<td align="right"><div class="copyright-footer">Copyright © 2005-2007 Douglas Gregor, 106 Matthias Troyer, Trustees of Indiana University<p> 107 Distributed under the Boost Software License, Version 1.0. (See accompanying 108 file LICENSE_1_0.txt or copy at <ulink url="http://www.boost.org/LICENSE_1_0.txt"> 109 http://www.boost.org/LICENSE_1_0.txt </ulink>) 110 </p> 111</div></td> 112</tr></table> 113<hr> 114<div class="spirit-nav"> 115<a accesskey="p" href="all_gather.html"><img src="../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../mpi/reference.html#header.boost.mpi.collectives_hpp"><img src="../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../index.html"><img src="../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="all_to_all.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 116</div> 117</body> 118</html> 119
