58.html

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<!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" xml:lang="en" lang="en"><head>	<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />	<style type="text/css">	body { font-family: Verdana, Arial, Helvetica, sans-serif;}	a.at-term {	font-style: italic; }	</style>	<title>Global Reduction</title>	<meta name="Generator" content="ATutor">	<meta name="Keywords" content=""></head><body> <p>In addition to broadcasting data from one process to many, there are additional routines that allow you to scatter, or distribute, parts of an array to individual processors.&nbsp; Corresponding routines exist that allow you to collect, or gather, data from all of the processes into an array on the root process.&nbsp; The reduction operations are an extension to the gather routines.&nbsp; They allow you to gather the data and perform some
operation on it before it is stored on the root process.</p>

<p>Several examples of the reduction operations that are available are:</p>

<ul>
<li>Global sum or product</li>

<li>Global maximum or minimum</li>

<li>Global user-defined operation</li>
</ul>

<h3>General Form</h3>

<p class="codelang">C:</p>

<pre><code>int MPI_Reduce(void* sendbuf, void* recvbuf,int count, MPI_Datatype datatype, MPI_Op op, int root, MPI_Comm comm)</code></pre>

<p class="codelang">Fortran:</p>

<pre><code>CALL MPI_REDUCE(SENDBUF, RECVBUF, COUNT,DATATYPE, OP, ROOT, COMM, ERROR)
<type> SENDBUF(*), RECVBUF(*)</code></pre>

<ul>
<li><em>COUNT</em>: is the number of "ops" done on consecutive elements of sendbuf (it is also size of recvbuf)</li>
<li><em>OP</em>: is an associative operator that takes two operands of type datatype and returns a result of the same type</li>
</ul>
<img SRC="reduce1.gif" height="419" width="420"></body></html>

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