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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 2cd0e3b3e1fd
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 6 Copyright (C) 2003 Matteo Frigo.
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: FFTW MPI Fortran Interface</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: FFTW MPI Fortran Interface">
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Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="FFTW-MPI-Fortran-Interface"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Previous: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="FFTW-MPI-Fortran-Interface-1"></a>
Chris@42 79 <h3 class="section">6.13 FFTW MPI Fortran Interface</h3>
Chris@42 80 <a name="index-Fortran-interface-1"></a>
Chris@42 81
Chris@42 82 <a name="index-iso_005fc_005fbinding"></a>
Chris@42 83 <p>The FFTW MPI interface is callable from modern Fortran compilers
Chris@42 84 supporting the Fortran 2003 <code>iso_c_binding</code> standard for calling
Chris@42 85 C functions. As described in <a href="Calling-FFTW-from-Modern-Fortran.html#Calling-FFTW-from-Modern-Fortran">Calling FFTW from Modern Fortran</a>,
Chris@42 86 this means that you can directly call FFTW&rsquo;s C interface from Fortran
Chris@42 87 with only minor changes in syntax. There are, however, a few things
Chris@42 88 specific to the MPI interface to keep in mind:
Chris@42 89 </p>
Chris@42 90 <ul>
Chris@42 91 <li> Instead of including <code>fftw3.f03</code> as in <a href="Overview-of-Fortran-interface.html#Overview-of-Fortran-interface">Overview of Fortran interface</a>, you should <code>include 'fftw3-mpi.f03'</code> (after
Chris@42 92 <code>use, intrinsic :: iso_c_binding</code> as before). The
Chris@42 93 <code>fftw3-mpi.f03</code> file includes <code>fftw3.f03</code>, so you should
Chris@42 94 <em>not</em> <code>include</code> them both yourself. (You will also want to
Chris@42 95 include the MPI header file, usually via <code>include 'mpif.h'</code> or
Chris@42 96 similar, although though this is not needed by <code>fftw3-mpi.f03</code>
Chris@42 97 <i>per se</i>.) (To use the &lsquo;<samp>fftwl_</samp>&rsquo; <code>long double</code> extended-precision routines in supporting compilers, you should include <code>fftw3f-mpi.f03</code> in <em>addition</em> to <code>fftw3-mpi.f03</code>. See <a href="Extended-and-quadruple-precision-in-Fortran.html#Extended-and-quadruple-precision-in-Fortran">Extended and quadruple precision in Fortran</a>.)
Chris@42 98
Chris@42 99 </li><li> Because of the different storage conventions between C and Fortran,
Chris@42 100 you reverse the order of your array dimensions when passing them to
Chris@42 101 FFTW (see <a href="Reversing-array-dimensions.html#Reversing-array-dimensions">Reversing array dimensions</a>). This is merely a
Chris@42 102 difference in notation and incurs no performance overhead. However,
Chris@42 103 it means that, whereas in C the <em>first</em> dimension is distributed,
Chris@42 104 in Fortran the <em>last</em> dimension of your array is distributed.
Chris@42 105
Chris@42 106 </li><li> <a name="index-MPI-communicator-3"></a>
Chris@42 107 In Fortran, communicators are stored as <code>integer</code> types; there is
Chris@42 108 no <code>MPI_Comm</code> type, nor is there any way to access a C
Chris@42 109 <code>MPI_Comm</code>. Fortunately, this is taken care of for you by the
Chris@42 110 FFTW Fortran interface: whenever the C interface expects an
Chris@42 111 <code>MPI_Comm</code> type, you should pass the Fortran communicator as an
Chris@42 112 <code>integer</code>.<a name="DOCF8" href="#FOOT8"><sup>8</sup></a>
Chris@42 113
Chris@42 114 </li><li> Because you need to call the &lsquo;<samp>local_size</samp>&rsquo; function to find out
Chris@42 115 how much space to allocate, and this may be <em>larger</em> than the
Chris@42 116 local portion of the array (see <a href="MPI-Data-Distribution.html#MPI-Data-Distribution">MPI Data Distribution</a>), you should
Chris@42 117 <em>always</em> allocate your arrays dynamically using FFTW&rsquo;s allocation
Chris@42 118 routines as described in <a href="Allocating-aligned-memory-in-Fortran.html#Allocating-aligned-memory-in-Fortran">Allocating aligned memory in Fortran</a>.
Chris@42 119 (Coincidentally, this also provides the best performance by
Chris@42 120 guaranteeding proper data alignment.)
Chris@42 121
Chris@42 122 </li><li> Because all sizes in the MPI FFTW interface are declared as
Chris@42 123 <code>ptrdiff_t</code> in C, you should use <code>integer(C_INTPTR_T)</code> in
Chris@42 124 Fortran (see <a href="FFTW-Fortran-type-reference.html#FFTW-Fortran-type-reference">FFTW Fortran type reference</a>).
Chris@42 125
Chris@42 126 </li><li> <a name="index-fftw_005fexecute_005fdft-1"></a>
Chris@42 127 <a name="index-fftw_005fmpi_005fexecute_005fdft-1"></a>
Chris@42 128 <a name="index-new_002darray-execution-3"></a>
Chris@42 129 In Fortran, because of the language semantics, we generally recommend
Chris@42 130 using the new-array execute functions for all plans, even in the
Chris@42 131 common case where you are executing the plan on the same arrays for
Chris@42 132 which the plan was created (see <a href="Plan-execution-in-Fortran.html#Plan-execution-in-Fortran">Plan execution in Fortran</a>).
Chris@42 133 However, note that in the MPI interface these functions are changed:
Chris@42 134 <code>fftw_execute_dft</code> becomes <code>fftw_mpi_execute_dft</code>,
Chris@42 135 etcetera. See <a href="Using-MPI-Plans.html#Using-MPI-Plans">Using MPI Plans</a>.
Chris@42 136
Chris@42 137 </li></ul>
Chris@42 138
Chris@42 139 <p>For example, here is a Fortran code snippet to perform a distributed
Chris@42 140 L&nbsp;&times;&nbsp;M complex DFT in-place. (This assumes you have already
Chris@42 141 initialized MPI with <code>MPI_init</code> and have also performed
Chris@42 142 <code>call fftw_mpi_init</code>.)
Chris@42 143 </p>
Chris@42 144 <div class="example">
Chris@42 145 <pre class="example"> use, intrinsic :: iso_c_binding
Chris@42 146 include 'fftw3-mpi.f03'
Chris@42 147 integer(C_INTPTR_T), parameter :: L = ...
Chris@42 148 integer(C_INTPTR_T), parameter :: M = ...
Chris@42 149 type(C_PTR) :: plan, cdata
Chris@42 150 complex(C_DOUBLE_COMPLEX), pointer :: data(:,:)
Chris@42 151 integer(C_INTPTR_T) :: i, j, alloc_local, local_M, local_j_offset
Chris@42 152
Chris@42 153 ! <span class="roman">get local data size and allocate (note dimension reversal)</span>
Chris@42 154 alloc_local = fftw_mpi_local_size_2d(M, L, MPI_COMM_WORLD, &amp;
Chris@42 155 local_M, local_j_offset)
Chris@42 156 cdata = fftw_alloc_complex(alloc_local)
Chris@42 157 call c_f_pointer(cdata, data, [L,local_M])
Chris@42 158
Chris@42 159 ! <span class="roman">create MPI plan for in-place forward DFT (note dimension reversal)</span>
Chris@42 160 plan = fftw_mpi_plan_dft_2d(M, L, data, data, MPI_COMM_WORLD, &amp;
Chris@42 161 FFTW_FORWARD, FFTW_MEASURE)
Chris@42 162
Chris@42 163 ! <span class="roman">initialize data to some function</span> my_function(i,j)
Chris@42 164 do j = 1, local_M
Chris@42 165 do i = 1, L
Chris@42 166 data(i, j) = my_function(i, j + local_j_offset)
Chris@42 167 end do
Chris@42 168 end do
Chris@42 169
Chris@42 170 ! <span class="roman">compute transform (as many times as desired)</span>
Chris@42 171 call fftw_mpi_execute_dft(plan, data, data)
Chris@42 172
Chris@42 173 call fftw_destroy_plan(plan)
Chris@42 174 call fftw_free(cdata)
Chris@42 175 </pre></div>
Chris@42 176
Chris@42 177 <p>Note that when we called <code>fftw_mpi_local_size_2d</code> and
Chris@42 178 <code>fftw_mpi_plan_dft_2d</code> with the dimensions in reversed order,
Chris@42 179 since a L&nbsp;&times;&nbsp;M Fortran array is viewed by FFTW in C as a
Chris@42 180 M&nbsp;&times;&nbsp;L array. This means that the array was distributed over
Chris@42 181 the <code>M</code> dimension, the local portion of which is a
Chris@42 182 L&nbsp;&times;&nbsp;local_M array in Fortran. (You must <em>not</em> use an
Chris@42 183 <code>allocate</code> statement to allocate an L&nbsp;&times;&nbsp;local_M array,
Chris@42 184 however; you must allocate <code>alloc_local</code> complex numbers, which
Chris@42 185 may be greater than <code>L * local_M</code>, in order to reserve space for
Chris@42 186 intermediate steps of the transform.) Finally, we mention that
Chris@42 187 because C&rsquo;s array indices are zero-based, the <code>local_j_offset</code>
Chris@42 188 argument can conveniently be interpreted as an offset in the 1-based
Chris@42 189 <code>j</code> index (rather than as a starting index as in C).
Chris@42 190 </p>
Chris@42 191 <p>If instead you had used the <code>ior(FFTW_MEASURE,
Chris@42 192 FFTW_MPI_TRANSPOSED_OUT)</code> flag, the output of the transform would be a
Chris@42 193 transposed M&nbsp;&times;&nbsp;local_L array, associated with the <em>same</em>
Chris@42 194 <code>cdata</code> allocation (since the transform is in-place), and which
Chris@42 195 you could declare with:
Chris@42 196 </p>
Chris@42 197 <div class="example">
Chris@42 198 <pre class="example"> complex(C_DOUBLE_COMPLEX), pointer :: tdata(:,:)
Chris@42 199 ...
Chris@42 200 call c_f_pointer(cdata, tdata, [M,local_L])
Chris@42 201 </pre></div>
Chris@42 202
Chris@42 203 <p>where <code>local_L</code> would have been obtained by changing the
Chris@42 204 <code>fftw_mpi_local_size_2d</code> call to:
Chris@42 205 </p>
Chris@42 206 <div class="example">
Chris@42 207 <pre class="example"> alloc_local = fftw_mpi_local_size_2d_transposed(M, L, MPI_COMM_WORLD, &amp;
Chris@42 208 local_M, local_j_offset, local_L, local_i_offset)
Chris@42 209 </pre></div>
Chris@42 210 <div class="footnote">
Chris@42 211 <hr>
Chris@42 212 <h4 class="footnotes-heading">Footnotes</h4>
Chris@42 213
Chris@42 214 <h3><a name="FOOT8" href="#DOCF8">(8)</a></h3>
Chris@42 215 <p>Technically, this is because you aren&rsquo;t
Chris@42 216 actually calling the C functions directly. You are calling wrapper
Chris@42 217 functions that translate the communicator with <code>MPI_Comm_f2c</code>
Chris@42 218 before calling the ordinary C interface. This is all done
Chris@42 219 transparently, however, since the <code>fftw3-mpi.f03</code> interface file
Chris@42 220 renames the wrappers so that they are called in Fortran with the same
Chris@42 221 names as the C interface functions.</p>
Chris@42 222 </div>
Chris@42 223 <hr>
Chris@42 224 <div class="header">
Chris@42 225 <p>
Chris@42 226 Previous: <a href="FFTW-MPI-Reference.html#FFTW-MPI-Reference" accesskey="p" rel="prev">FFTW MPI Reference</a>, Up: <a href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" accesskey="u" rel="up">Distributed-memory FFTW with MPI</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
Chris@42 227 </div>
Chris@42 228
Chris@42 229
Chris@42 230
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Chris@42 232 </html>