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Tidy up for ROLI
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Tue, 17 May 2016 18:50:19 +0100
parents 636c989477e7
children
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--- a/Lib/fftw-3.2.1/doc/html/.svn/text-base/An-improved-replacement-for-MPI_005fAlltoall.html.svn-base	Wed May 04 11:02:59 2016 +0100
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-<title>An improved replacement for MPI_Alltoall - FFTW 3.2alpha3</title>
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-
-<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-376"></a>
-<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-377"></a>
-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-378"></a><a name="index-FFTW_005fPATIENT-379"></a>
-<!--  -->
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