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 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="scatterv.html" title="Function scatterv"> 11<link rel="next" href="scan.html" title="Function scan"> 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 align="center"><a href="../../../../index.html">Home</a></td> 17<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td> 18<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 19<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 20<td align="center"><a href="../../../../more/index.htm">More</a></td> 21</tr></table> 22<hr> 23<div class="spirit-nav"> 24<a accesskey="p" href="scatterv.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="scan.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 25</div> 26<div class="refentry"> 27<a name="boost.mpi.reduce"></a><div class="titlepage"></div> 28<div class="refnamediv"> 29<h2><span class="refentrytitle">Function reduce</span></h2> 30<p>boost::mpi::reduce — Combine the values stored by each process into a single value at the root. </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">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> in_value<span class="special">,</span> <span class="identifier">T</span> <span class="special">&</span> out_value<span class="special">,</span> 38 <span class="identifier">Op</span> op<span class="special">,</span> <span class="keyword">int</span> root<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">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> in_value<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">,</span> <span class="keyword">int</span> root<span class="special">)</span><span class="special">;</span> 41<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> 42 <span class="keyword">void</span> <span class="identifier">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> in_values<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span> 43 <span class="identifier">T</span> <span class="special">*</span> out_values<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">,</span> <span class="keyword">int</span> root<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">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> in_values<span class="special">,</span> <span class="keyword">int</span> n<span class="special">,</span> <span class="identifier">Op</span> op<span class="special">,</span> 46 <span class="keyword">int</span> root<span class="special">)</span><span class="special">;</span></pre></div> 47<div class="refsect1"> 48<a name="id-1.3.27.7.5.12.4"></a><h2>Description</h2> 49<p><code class="computeroutput">reduce</code> is a collective algorithm that combines the values stored by each process into a single value at the <code class="computeroutput">root</code>. The values can be combined arbitrarily, 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">gather</code> to the <code class="computeroutput">root</code>, followed by an <code class="computeroutput">std::accumulate()</code> over the gathered values and using the operation <code class="computeroutput">op</code>.</p> 50<p>When the type <code class="computeroutput">T</code> has an associated MPI data type, this routine invokes <code class="computeroutput">MPI_Reduce</code> to perform the reduction. If possible, built-in MPI operations will be used; otherwise, <code class="computeroutput">reduce()</code> will create a custom MPI_Op for the call to MPI_Reduce.</p> 51<p> 52</p> 53<div class="variablelist"><table border="0" class="variablelist compact"> 54<colgroup> 55<col align="left" valign="top"> 56<col> 57</colgroup> 58<tbody><tr> 59<td><p><span class="term">Parameters:</span></p></td> 60<td><div class="variablelist"><table border="0" class="variablelist compact"> 61<colgroup> 62<col align="left" valign="top"> 63<col> 64</colgroup> 65<tbody> 66<tr> 67<td><p><span class="term"><code class="computeroutput">comm</code></span></p></td> 68<td><p>The communicator over which the reduction will occur.</p></td> 69</tr> 70<tr> 71<td><p><span class="term"><code class="computeroutput">in_value</code></span></p></td> 72<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> contains a pointer to the local values. In this case, <code class="computeroutput">n</code> is the number of values that will be reduced. Reduction occurs independently for each of the <code class="computeroutput">n</code> values referenced by <code class="computeroutput">in_values</code>, e.g., calling reduce on an array of <code class="computeroutput">n</code> values is like calling <code class="computeroutput">reduce</code> <code class="computeroutput">n</code> separate times, one for each location in <code class="computeroutput">in_values</code> and <code class="computeroutput">out_values</code>.</p></td> 73</tr> 74<tr> 75<td><p><span class="term"><code class="computeroutput">op</code></span></p></td> 76<td><p>The binary operation that combines two values of type <code class="computeroutput">T</code> into 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> 77</tr> 78<tr> 79<td><p><span class="term"><code class="computeroutput">out_value</code></span></p></td> 80<td><p>Will receive the result of the reduction operation, but only for the <code class="computeroutput">root</code> process. Non-root processes may omit if parameter; if they choose to supply the parameter, it will be unchanged. For reducing arrays, <code class="computeroutput">out_values</code> contains a pointer to the storage for the output values.</p></td> 81</tr> 82<tr> 83<td><p><span class="term"><code class="computeroutput">root</code></span></p></td> 84<td><p>The process ID number that will receive the final, combined value. This value must be the same on all processes. </p></td> 85</tr> 86</tbody> 87</table></div></td> 88</tr></tbody> 89</table></div> 90</div> 91</div> 92<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 93<td align="left"></td> 94<td align="right"><div class="copyright-footer">Copyright © 2005-2007 Douglas Gregor, 95 Matthias Troyer, Trustees of Indiana University<p> 96 Distributed under the Boost Software License, Version 1.0. (See accompanying 97 file LICENSE_1_0.txt or copy at <ulink url="http://www.boost.org/LICENSE_1_0.txt"> 98 http://www.boost.org/LICENSE_1_0.txt </ulink>) 99 </p> 100</div></td> 101</tr></table> 102<hr> 103<div class="spirit-nav"> 104<a accesskey="p" href="scatterv.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="scan.html"><img src="../../../../doc/src/images/next.png" alt="Next"></a> 105</div> 106</body> 107</html> 108
