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comparison src/fftw-3.3.8/doc/html/FFTW-MPI-Wisdom.html @ 167:bd3cc4d1df30
Add FFTW 3.3.8 source, and a Linux build
author | Chris Cannam <cannam@all-day-breakfast.com> |
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date | Tue, 19 Nov 2019 14:52:55 +0000 |
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1 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> | |
2 <html> | |
3 <!-- This manual is for FFTW | |
4 (version 3.3.8, 24 May 2018). | |
5 | |
6 Copyright (C) 2003 Matteo Frigo. | |
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8 Copyright (C) 2003 Massachusetts Institute of Technology. | |
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22 approved by the Free Software Foundation. --> | |
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24 <head> | |
25 <title>FFTW 3.3.8: FFTW MPI Wisdom</title> | |
26 | |
27 <meta name="description" content="FFTW 3.3.8: FFTW MPI Wisdom"> | |
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34 <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> | |
35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> | |
36 <link href="Distributed_002dmemory-FFTW-with-MPI.html#Distributed_002dmemory-FFTW-with-MPI" rel="up" title="Distributed-memory FFTW with MPI"> | |
37 <link href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" rel="next" title="Avoiding MPI Deadlocks"> | |
38 <link href="An-improved-replacement-for-MPI_005fAlltoall.html#An-improved-replacement-for-MPI_005fAlltoall" rel="prev" title="An improved replacement for MPI_Alltoall"> | |
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65 </style> | |
66 | |
67 | |
68 </head> | |
69 | |
70 <body lang="en"> | |
71 <a name="FFTW-MPI-Wisdom"></a> | |
72 <div class="header"> | |
73 <p> | |
74 Next: <a href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" accesskey="n" rel="next">Avoiding MPI Deadlocks</a>, Previous: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="p" rel="prev">FFTW MPI Transposes</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> [<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> | |
75 </div> | |
76 <hr> | |
77 <a name="FFTW-MPI-Wisdom-1"></a> | |
78 <h3 class="section">6.8 FFTW MPI Wisdom</h3> | |
79 <a name="index-wisdom-2"></a> | |
80 <a name="index-saving-plans-to-disk-2"></a> | |
81 | |
82 <p>FFTW’s “wisdom” facility (see <a href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans">Words of Wisdom-Saving Plans</a>) can | |
83 be used to save MPI plans as well as to save uniprocessor plans. | |
84 However, for MPI there are several unavoidable complications. | |
85 </p> | |
86 <a name="index-MPI-I_002fO-1"></a> | |
87 <p>First, the MPI standard does not guarantee that every process can | |
88 perform file I/O (at least, not using C stdio routines)—in general, | |
89 we may only assume that process 0 is capable of I/O.<a name="DOCF7" href="#FOOT7"><sup>7</sup></a> So, if we | |
90 want to export the wisdom from a single process to a file, we must | |
91 first export the wisdom to a string, then send it to process 0, then | |
92 write it to a file. | |
93 </p> | |
94 <p>Second, in principle we may want to have separate wisdom for every | |
95 process, since in general the processes may run on different hardware | |
96 even for a single MPI program. However, in practice FFTW’s MPI code | |
97 is designed for the case of homogeneous hardware (see <a href="Load-balancing.html#Load-balancing">Load balancing</a>), and in this case it is convenient to use the same wisdom | |
98 for every process. Thus, we need a mechanism to synchronize the wisdom. | |
99 </p> | |
100 <p>To address both of these problems, FFTW provides the following two | |
101 functions: | |
102 </p> | |
103 <div class="example"> | |
104 <pre class="example">void fftw_mpi_broadcast_wisdom(MPI_Comm comm); | |
105 void fftw_mpi_gather_wisdom(MPI_Comm comm); | |
106 </pre></div> | |
107 <a name="index-fftw_005fmpi_005fgather_005fwisdom"></a> | |
108 <a name="index-fftw_005fmpi_005fbroadcast_005fwisdom"></a> | |
109 | |
110 <p>Given a communicator <code>comm</code>, <code>fftw_mpi_broadcast_wisdom</code> | |
111 will broadcast the wisdom from process 0 to all other processes. | |
112 Conversely, <code>fftw_mpi_gather_wisdom</code> will collect wisdom from all | |
113 processes onto process 0. (If the plans created for the same problem | |
114 by different processes are not the same, <code>fftw_mpi_gather_wisdom</code> | |
115 will arbitrarily choose one of the plans.) Both of these functions | |
116 may result in suboptimal plans for different processes if the | |
117 processes are running on non-identical hardware. Both of these | |
118 functions are <em>collective</em> calls, which means that they must be | |
119 executed by all processes in the communicator. | |
120 <a name="index-collective-function-1"></a> | |
121 </p> | |
122 | |
123 <p>So, for example, a typical code snippet to import wisdom from a file | |
124 and use it on all processes would be: | |
125 </p> | |
126 <div class="example"> | |
127 <pre class="example">{ | |
128 int rank; | |
129 | |
130 fftw_mpi_init(); | |
131 MPI_Comm_rank(MPI_COMM_WORLD, &rank); | |
132 if (rank == 0) fftw_import_wisdom_from_filename("mywisdom"); | |
133 fftw_mpi_broadcast_wisdom(MPI_COMM_WORLD); | |
134 } | |
135 </pre></div> | |
136 | |
137 <p>(Note that we must call <code>fftw_mpi_init</code> before importing any | |
138 wisdom that might contain MPI plans.) Similarly, a typical code | |
139 snippet to export wisdom from all processes to a file is: | |
140 <a name="index-fftw_005fmpi_005finit-2"></a> | |
141 </p> | |
142 <div class="example"> | |
143 <pre class="example">{ | |
144 int rank; | |
145 | |
146 fftw_mpi_gather_wisdom(MPI_COMM_WORLD); | |
147 MPI_Comm_rank(MPI_COMM_WORLD, &rank); | |
148 if (rank == 0) fftw_export_wisdom_to_filename("mywisdom"); | |
149 } | |
150 </pre></div> | |
151 | |
152 <div class="footnote"> | |
153 <hr> | |
154 <h4 class="footnotes-heading">Footnotes</h4> | |
155 | |
156 <h3><a name="FOOT7" href="#DOCF7">(7)</a></h3> | |
157 <p>In fact, | |
158 even this assumption is not technically guaranteed by the standard, | |
159 although it seems to be universal in actual MPI implementations and is | |
160 widely assumed by MPI-using software. Technically, you need to query | |
161 the <code>MPI_IO</code> attribute of <code>MPI_COMM_WORLD</code> with | |
162 <code>MPI_Attr_get</code>. If this attribute is <code>MPI_PROC_NULL</code>, no | |
163 I/O is possible. If it is <code>MPI_ANY_SOURCE</code>, any process can | |
164 perform I/O. Otherwise, it is the rank of a process that can perform | |
165 I/O ... but since it is not guaranteed to yield the <em>same</em> rank | |
166 on all processes, you have to do an <code>MPI_Allreduce</code> of some kind | |
167 if you want all processes to agree about which is going to do I/O. | |
168 And even then, the standard only guarantees that this process can | |
169 perform output, but not input. See e.g. <cite>Parallel Programming | |
170 with MPI</cite> by P. S. Pacheco, section 8.1.3. Needless to say, in our | |
171 experience virtually no MPI programmers worry about this.</p> | |
172 </div> | |
173 <hr> | |
174 <div class="header"> | |
175 <p> | |
176 Next: <a href="Avoiding-MPI-Deadlocks.html#Avoiding-MPI-Deadlocks" accesskey="n" rel="next">Avoiding MPI Deadlocks</a>, Previous: <a href="FFTW-MPI-Transposes.html#FFTW-MPI-Transposes" accesskey="p" rel="prev">FFTW MPI Transposes</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> [<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> | |
177 </div> | |
178 | |
179 | |
180 | |
181 </body> | |
182 </html> |