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<title>Avoiding MPI Deadlocks - FFTW 3.3</title>
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<h3 class="section">6.9 Avoiding MPI Deadlocks</h3>
<p><a name="index-deadlock-417"></a>
An MPI program can <em>deadlock</em> if one process is waiting for a
message from another process that never gets sent. To avoid deadlocks
when using FFTW's MPI routines, it is important to know which
functions are <em>collective</em>: that is, which functions must
<em>always</em> be called in the <em>same order</em> from <em>every</em>
process in a given communicator. (For example, <code>MPI_Barrier</code> is
the canonical example of a collective function in the MPI standard.)
<a name="index-collective-function-418"></a><a name="index-MPI_005fBarrier-419"></a>
<p>The functions in FFTW that are <em>always</em> collective are: every
function beginning with ‘<samp><span class="samp">fftw_mpi_plan</span></samp>’, as well as
<code>fftw_mpi_broadcast_wisdom</code> and <code>fftw_mpi_gather_wisdom</code>.
Also, the following functions from the ordinary FFTW interface are
collective when they are applied to a plan created by an
‘<samp><span class="samp">fftw_mpi_plan</span></samp>’ function: <code>fftw_execute</code>,
<code>fftw_destroy_plan</code>, and <code>fftw_flops</code>.
<a name="index-fftw_005fexecute-420"></a><a name="index-fftw_005fdestroy_005fplan-421"></a><a name="index-fftw_005fflops-422"></a>
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