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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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+<html lang="en">
+<head>
+<title>An improved replacement for MPI_Alltoall - FFTW 3.3.3</title>
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+<meta name="description" content="FFTW 3.3.3">
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+<link rel="up" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" title="FFTW MPI Transposes">
+<link rel="prev" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface" title="Advanced distributed-transpose interface">
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+<!--
+This manual is for FFTW
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+Copyright (C) 2003 Matteo Frigo.
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+Copyright (C) 2003 Massachusetts Institute of Technology.
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+<div class="node">
+<a name="An-improved-replacement-for-MPI_Alltoall"></a>
+<a name="An-improved-replacement-for-MPI_005fAlltoall"></a>
+<p>
+Previous:&nbsp;<a rel="previous" accesskey="p" href="Advanced-distributed_002dtranspose-interface.html#Advanced-distributed_002dtranspose-interface">Advanced distributed-transpose interface</a>,
+Up:&nbsp;<a rel="up" accesskey="u" href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes">FFTW MPI Transposes</a>
+<hr>
+</div>
+
+<h4 class="subsection">6.7.3 An improved replacement for MPI_Alltoall</h4>
+
+<p>We close this section by noting that FFTW's MPI transpose routines can
+be thought of as a generalization for the <code>MPI_Alltoall</code> function
+(albeit only for floating-point types), and in some circumstances can
+function as an improved replacement. 
+<a name="index-MPI_005fAlltoall-406"></a>
+
+   <p><code>MPI_Alltoall</code> is defined by the MPI standard as:
+
+<pre class="example">     int MPI_Alltoall(void *sendbuf, int sendcount, MPI_Datatype sendtype,
+                      void *recvbuf, int recvcnt, MPI_Datatype recvtype,
+                      MPI_Comm comm);
+</pre>
+   <p>In particular, for <code>double*</code> arrays <code>in</code> and <code>out</code>,
+consider the call:
+
+<pre class="example">     MPI_Alltoall(in, howmany, MPI_DOUBLE, out, howmany MPI_DOUBLE, comm);
+</pre>
+   <p>This is completely equivalent to:
+
+<pre class="example">     MPI_Comm_size(comm, &amp;P);
+     plan = fftw_mpi_plan_many_transpose(P, P, howmany, 1, 1, in, out, comm, FFTW_ESTIMATE);
+     fftw_execute(plan);
+     fftw_destroy_plan(plan);
+</pre>
+   <p>That is, computing a P&nbsp;&times;&nbsp;P transpose on <code>P</code> processes,
+with a block size of 1, is just a standard all-to-all communication.
+
+   <p>However, using the FFTW routine instead of <code>MPI_Alltoall</code> may
+have certain advantages.  First of all, FFTW's routine can operate
+in-place (<code>in == out</code>) whereas <code>MPI_Alltoall</code> can only
+operate out-of-place. 
+<a name="index-in_002dplace-407"></a>
+
+   <p>Second, even for out-of-place plans, FFTW's routine may be faster,
+especially if you need to perform the all-to-all communication many
+times and can afford to use <code>FFTW_MEASURE</code> or
+<code>FFTW_PATIENT</code>.  It should certainly be no slower, not including
+the time to create the plan, since one of the possible algorithms that
+FFTW uses for an out-of-place transpose <em>is</em> simply to call
+<code>MPI_Alltoall</code>.  However, FFTW also considers several other
+possible algorithms that, depending on your MPI implementation and
+your hardware, may be faster. 
+<a name="index-FFTW_005fMEASURE-408"></a><a name="index-FFTW_005fPATIENT-409"></a>
+<!--  -->
+
+   </body></html>
+