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200 lines
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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<title>Usage of Multi-threaded FFTW (FFTW 3.3.10)</title>
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<span id="Usage-of-Multi_002dthreaded-FFTW"></span><div class="header">
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<p>
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Next: <a href="How-Many-Threads-to-Use_003f.html" accesskey="n" rel="next">How Many Threads to Use?</a>, Previous: <a href="Installation-and-Supported-Hardware_002fSoftware.html" accesskey="p" rel="prev">Installation and Supported Hardware/Software</a>, Up: <a href="Multi_002dthreaded-FFTW.html" accesskey="u" rel="up">Multi-threaded FFTW</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Usage-of-Multi_002dthreaded-FFTW-1"></span><h3 class="section">5.2 Usage of Multi-threaded FFTW</h3>
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<p>Here, it is assumed that the reader is already familiar with the usage
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of the uniprocessor FFTW routines, described elsewhere in this manual.
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We only describe what one has to change in order to use the
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multi-threaded routines.
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</p>
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<span id="index-OpenMP-1"></span>
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<p>First, programs using the parallel complex transforms should be linked
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with <code>-lfftw3_threads -lfftw3 -lm</code> on Unix, or <code>-lfftw3_omp
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-lfftw3 -lm</code> if you compiled with OpenMP. You will also need to link
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with whatever library is responsible for threads on your system
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(e.g. <code>-lpthread</code> on GNU/Linux) or include whatever compiler flag
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enables OpenMP (e.g. <code>-fopenmp</code> with gcc).
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<span id="index-linking-on-Unix"></span>
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</p>
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<p>Second, before calling <em>any</em> FFTW routines, you should call the
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function:
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</p>
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<div class="example">
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<pre class="example">int fftw_init_threads(void);
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</pre></div>
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<span id="index-fftw_005finit_005fthreads"></span>
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<p>This function, which need only be called once, performs any one-time
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initialization required to use threads on your system. It returns zero
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if there was some error (which should not happen under normal
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circumstances) and a non-zero value otherwise.
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</p>
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<p>Third, before creating a plan that you want to parallelize, you should
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call:
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</p>
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<div class="example">
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<pre class="example">void fftw_plan_with_nthreads(int nthreads);
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</pre></div>
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<span id="index-fftw_005fplan_005fwith_005fnthreads"></span>
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<p>The <code>nthreads</code> argument indicates the number of threads you want
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FFTW to use (or actually, the maximum number). All plans subsequently
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created with any planner routine will use that many threads. You can
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call <code>fftw_plan_with_nthreads</code>, create some plans, call
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<code>fftw_plan_with_nthreads</code> again with a different argument, and
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create some more plans for a new number of threads. Plans already created
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before a call to <code>fftw_plan_with_nthreads</code> are unaffected. If you
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pass an <code>nthreads</code> argument of <code>1</code> (the default), threads are
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disabled for subsequent plans.
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</p>
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<p>You can determine the current number of threads that the planner can
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use by calling:
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</p>
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<div class="example">
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<pre class="example">int fftw_planner_nthreads(void);
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</pre></div>
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<span id="index-fftw_005fplanner_005fnthreads"></span>
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<span id="index-OpenMP-2"></span>
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<p>With OpenMP, to configure FFTW to use all of the currently running
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OpenMP threads (set by <code>omp_set_num_threads(nthreads)</code> or by the
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<code>OMP_NUM_THREADS</code> environment variable), you can do:
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<code>fftw_plan_with_nthreads(omp_get_max_threads())</code>. (The ‘<samp>omp_</samp>’
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OpenMP functions are declared via <code>#include <omp.h></code>.)
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</p>
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<span id="index-thread-safety"></span>
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<p>Given a plan, you then execute it as usual with
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<code>fftw_execute(plan)</code>, and the execution will use the number of
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threads specified when the plan was created. When done, you destroy
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it as usual with <code>fftw_destroy_plan</code>. As described in
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<a href="Thread-safety.html">Thread safety</a>, plan <em>execution</em> is thread-safe, but plan
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creation and destruction are <em>not</em>: you should create/destroy
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plans only from a single thread, but can safely execute multiple plans
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in parallel.
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</p>
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<p>There is one additional routine: if you want to get rid of all memory
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and other resources allocated internally by FFTW, you can call:
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</p>
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<div class="example">
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<pre class="example">void fftw_cleanup_threads(void);
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</pre></div>
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<span id="index-fftw_005fcleanup_005fthreads"></span>
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<p>which is much like the <code>fftw_cleanup()</code> function except that it
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also gets rid of threads-related data. You must <em>not</em> execute any
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previously created plans after calling this function.
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</p>
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<p>We should also mention one other restriction: if you save wisdom from a
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program using the multi-threaded FFTW, that wisdom <em>cannot be used</em>
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by a program using only the single-threaded FFTW (i.e. not calling
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<code>fftw_init_threads</code>). See <a href="Words-of-Wisdom_002dSaving-Plans.html">Words of Wisdom-Saving Plans</a>.
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</p>
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<p>Finally, FFTW provides a optional callback interface that allows you to
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replace its parallel threading backend at runtime:
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</p>
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<div class="example">
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<pre class="example">void fftw_threads_set_callback(
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void (*parallel_loop)(void *(*work)(void *), char *jobdata, size_t elsize, int njobs, void *data),
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void *data);
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</pre></div>
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<span id="index-fftw_005fthreads_005fset_005fcallback"></span>
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<p>This routine (which is <em>not</em> threadsafe and should generally be called before creating
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any FFTW plans) allows you to provide a function <code>parallel_loop</code> that executes
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parallel work for FFTW: it should call the function <code>work(jobdata + elsize*i)</code> for
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<code>i</code> from <code>0</code> to <code>njobs-1</code>, possibly in parallel. (The ‘data‘ pointer
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supplied to <code>fftw_threads_set_callback</code> is passed through to your <code>parallel_loop</code>
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function.) For example, if you link to an FFTW threads library built to use POSIX threads,
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but you want it to use OpenMP instead (because you are using OpenMP elsewhere in your program
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and want to avoid competing threads), you can call <code>fftw_threads_set_callback</code> with
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the callback function:
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</p>
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<div class="example">
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<pre class="example">void parallel_loop(void *(*work)(char *), char *jobdata, size_t elsize, int njobs, void *data)
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{
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#pragma omp parallel for
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for (int i = 0; i < njobs; ++i)
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work(jobdata + elsize * i);
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}
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</pre></div>
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<p>The same mechanism could be used in order to make FFTW use a threading backend
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implemented via Intel TBB, Apple GCD, or Cilk, for example.
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="How-Many-Threads-to-Use_003f.html" accesskey="n" rel="next">How Many Threads to Use?</a>, Previous: <a href="Installation-and-Supported-Hardware_002fSoftware.html" accesskey="p" rel="prev">Installation and Supported Hardware/Software</a>, Up: <a href="Multi_002dthreaded-FFTW.html" accesskey="u" rel="up">Multi-threaded FFTW</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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