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<div class="section" id="tutorial">
<span id="id1"></span><h1>Tutorial<a class="headerlink" href="#tutorial" title="Permalink to this headline">¶</a></h1>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">Under construction. Contributions very welcome!</p>
</div>
<p><em>MPI for Python</em> supports convenient, <em>pickle</em>-based communication of
generic Python object as well as fast, near C-speed, direct array data
communication of buffer-provider objects (e.g., NumPy arrays).</p>
<ul>
<li><p class="first">Communication of generic Python objects</p>
<p>You have to use <strong>all-lowercase</strong> methods (of the <code class="xref py py-class docutils literal notranslate"><span class="pre">Comm</span></code>
class), like <code class="xref py py-meth docutils literal notranslate"><span class="pre">send()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">recv()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">bcast()</span></code>. An
object to be sent is passed as a paramenter to the communication
call, and the received object is simply the return value.</p>
<p>The <code class="xref py py-meth docutils literal notranslate"><span class="pre">isend()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">irecv()</span></code> methods return
<code class="xref py py-class docutils literal notranslate"><span class="pre">Request</span></code> instances; completion of these methods can be
managed using the <code class="xref py py-meth docutils literal notranslate"><span class="pre">test()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">wait()</span></code> methods of the
<code class="xref py py-class docutils literal notranslate"><span class="pre">Request</span></code> class.</p>
<p>The <code class="xref py py-meth docutils literal notranslate"><span class="pre">recv()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">irecv()</span></code> methods may be passed a buffer
object that can be repeatedly used to receive messages avoiding
internal memory allocation. This buffer must be sufficiently large
to accommodate the transmitted messages; hence, any buffer passed to
<code class="xref py py-meth docutils literal notranslate"><span class="pre">recv()</span></code> or <code class="xref py py-meth docutils literal notranslate"><span class="pre">irecv()</span></code> must be at least as long as the
<em>pickled</em> data transmitted to the receiver.</p>
<p>Collective calls like <code class="xref py py-meth docutils literal notranslate"><span class="pre">scatter()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">gather()</span></code>,
<code class="xref py py-meth docutils literal notranslate"><span class="pre">allgather()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">alltoall()</span></code> expect a single value or a
sequence of <code class="xref py py-attr docutils literal notranslate"><span class="pre">Comm.size</span></code> elements at the root or all
process. They return a single value, a list of <code class="xref py py-attr docutils literal notranslate"><span class="pre">Comm.size</span></code>
elements, or <code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code>.</p>
</li>
<li><p class="first">Communication of buffer-like objects</p>
<p>You have to use method names starting with an <strong>upper-case</strong> letter
(of the <code class="xref py py-class docutils literal notranslate"><span class="pre">Comm</span></code> class), like <code class="xref py py-meth docutils literal notranslate"><span class="pre">Send()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">Recv()</span></code>,
<code class="xref py py-meth docutils literal notranslate"><span class="pre">Bcast()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">Scatter()</span></code>, <code class="xref py py-meth docutils literal notranslate"><span class="pre">Gather()</span></code>.</p>
<p>In general, buffer arguments to these calls must be explicitly
specified by using a 2/3-list/tuple like <code class="docutils literal notranslate"><span class="pre">[data,</span> <span class="pre">MPI.DOUBLE]</span></code>, or
<code class="docutils literal notranslate"><span class="pre">[data,</span> <span class="pre">count,</span> <span class="pre">MPI.DOUBLE]</span></code> (the former one uses the byte-size of
<code class="docutils literal notranslate"><span class="pre">data</span></code> and the extent of the MPI datatype to define <code class="docutils literal notranslate"><span class="pre">count</span></code>).</p>
<p>For vector collectives communication operations like
<code class="xref py py-meth docutils literal notranslate"><span class="pre">Scatterv()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">Gatherv()</span></code>, buffer arguments are
specified as <code class="docutils literal notranslate"><span class="pre">[data,</span> <span class="pre">count,</span> <span class="pre">displ,</span> <span class="pre">datatype]</span></code>, where <code class="docutils literal notranslate"><span class="pre">count</span></code> and
<code class="docutils literal notranslate"><span class="pre">displ</span></code> are sequences of integral values.</p>
<p>Automatic MPI datatype discovery for NumPy arrays and PEP-3118
buffers is supported, but limited to basic C types (all C/C99-native
signed/unsigned integral types and single/double precision
real/complex floating types) and availability of matching datatypes
in the underlying MPI implementation. In this case, the
buffer-provider object can be passed directly as a buffer argument,
the count and MPI datatype will be inferred.</p>
</li>
</ul>
<div class="section" id="running-python-scripts-with-mpi">
<h2>Running Python scripts with MPI<a class="headerlink" href="#running-python-scripts-with-mpi" title="Permalink to this headline">¶</a></h2>
<p>Most MPI programs can be run with the command <strong class="program">mpiexec</strong>. In practice, running
Python programs looks like:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ mpiexec -n 4 python script.py
</pre></div>
</div>
<p>to run the program with 4 processors.</p>
</div>
<div class="section" id="point-to-point-communication">
<h2>Point-to-Point Communication<a class="headerlink" href="#point-to-point-communication" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">Python objects (<a class="reference external" href="https://docs.python.org/3/library/pickle.html#module-pickle" title="(in Python v3.8)"><code class="xref py py-mod docutils literal notranslate"><span class="pre">pickle</span></code></a> under the hood):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'a'</span><span class="p">:</span> <span class="mi">7</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">:</span> <span class="mf">3.14</span><span class="p">}</span>
<span class="n">comm</span><span class="o">.</span><span class="n">send</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">11</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">recv</span><span class="p">(</span><span class="n">source</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">11</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p class="first">Python objects with non-blocking communication:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'a'</span><span class="p">:</span> <span class="mi">7</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">:</span> <span class="mf">3.14</span><span class="p">}</span>
<span class="n">req</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">isend</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">11</span><span class="p">)</span>
<span class="n">req</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
<span class="k">elif</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="n">req</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">irecv</span><span class="p">(</span><span class="n">source</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">11</span><span class="p">)</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">req</span><span class="o">.</span><span class="n">wait</span><span class="p">()</span>
</pre></div>
</div>
</li>
<li><p class="first">NumPy arrays (the fast way!):</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="c1"># passing MPI datatypes explicitly</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">1000</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Send</span><span class="p">([</span><span class="n">data</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">INT</span><span class="p">],</span> <span class="n">dest</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">77</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="mi">1000</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Recv</span><span class="p">([</span><span class="n">data</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">INT</span><span class="p">],</span> <span class="n">source</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">77</span><span class="p">)</span>
<span class="c1"># automatic MPI datatype discovery</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="n">numpy</span><span class="o">.</span><span class="n">float64</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Send</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">13</span><span class="p">)</span>
<span class="k">elif</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">1</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="n">numpy</span><span class="o">.</span><span class="n">float64</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Recv</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">source</span><span class="o">=</span><span class="mi">0</span><span class="p">,</span> <span class="n">tag</span><span class="o">=</span><span class="mi">13</span><span class="p">)</span>
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="collective-communication">
<h2>Collective Communication<a class="headerlink" href="#collective-communication" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">Broadcasting a Python dictionary:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">{</span><span class="s1">'key1'</span> <span class="p">:</span> <span class="p">[</span><span class="mi">7</span><span class="p">,</span> <span class="mf">2.72</span><span class="p">,</span> <span class="mi">2</span><span class="o">+</span><span class="mi">3</span><span class="n">j</span><span class="p">],</span>
<span class="s1">'key2'</span> <span class="p">:</span> <span class="p">(</span> <span class="s1">'abc'</span><span class="p">,</span> <span class="s1">'xyz'</span><span class="p">)}</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="kc">None</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">bcast</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p class="first">Scattering Python objects:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">size</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">[(</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">size</span><span class="p">)]</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="kc">None</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">scatter</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">data</span> <span class="o">==</span> <span class="p">(</span><span class="n">rank</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span>
</pre></div>
</div>
</li>
<li><p class="first">Gathering Python objects:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">size</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="n">data</span> <span class="o">=</span> <span class="p">(</span><span class="n">rank</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">gather</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">size</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">data</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">==</span> <span class="p">(</span><span class="n">i</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span><span class="o">**</span><span class="mi">2</span>
<span class="k">else</span><span class="p">:</span>
<span class="k">assert</span> <span class="n">data</span> <span class="ow">is</span> <span class="kc">None</span>
</pre></div>
</div>
</li>
<li><p class="first">Broadcasting a NumPy array:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Bcast</span><span class="p">(</span><span class="n">data</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">100</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">data</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">==</span> <span class="n">i</span>
</pre></div>
</div>
</li>
<li><p class="first">Scattering NumPy arrays:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">size</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="n">sendbuf</span> <span class="o">=</span> <span class="kc">None</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">sendbuf</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">([</span><span class="n">size</span><span class="p">,</span> <span class="mi">100</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">sendbuf</span><span class="o">.</span><span class="n">T</span><span class="p">[:,:]</span> <span class="o">=</span> <span class="nb">range</span><span class="p">(</span><span class="n">size</span><span class="p">)</span>
<span class="n">recvbuf</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Scatter</span><span class="p">(</span><span class="n">sendbuf</span><span class="p">,</span> <span class="n">recvbuf</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="k">assert</span> <span class="n">np</span><span class="o">.</span><span class="n">allclose</span><span class="p">(</span><span class="n">recvbuf</span><span class="p">,</span> <span class="n">rank</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p class="first">Gathering NumPy arrays:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">size</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="n">sendbuf</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span> <span class="o">+</span> <span class="n">rank</span>
<span class="n">recvbuf</span> <span class="o">=</span> <span class="kc">None</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="n">recvbuf</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">([</span><span class="n">size</span><span class="p">,</span> <span class="mi">100</span><span class="p">],</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Gather</span><span class="p">(</span><span class="n">sendbuf</span><span class="p">,</span> <span class="n">recvbuf</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="k">if</span> <span class="n">rank</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">size</span><span class="p">):</span>
<span class="k">assert</span> <span class="n">np</span><span class="o">.</span><span class="n">allclose</span><span class="p">(</span><span class="n">recvbuf</span><span class="p">[</span><span class="n">i</span><span class="p">,:],</span> <span class="n">i</span><span class="p">)</span>
</pre></div>
</div>
</li>
<li><p class="first">Parallel matrix-vector product:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span>
<span class="k">def</span> <span class="nf">matvec</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="n">A</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="n">m</span> <span class="o">=</span> <span class="n">A</span><span class="o">.</span><span class="n">shape</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="c1"># local rows</span>
<span class="n">p</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">xg</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">zeros</span><span class="p">(</span><span class="n">m</span><span class="o">*</span><span class="n">p</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'d'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Allgather</span><span class="p">([</span><span class="n">x</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">DOUBLE</span><span class="p">],</span>
<span class="p">[</span><span class="n">xg</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">DOUBLE</span><span class="p">])</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">dot</span><span class="p">(</span><span class="n">A</span><span class="p">,</span> <span class="n">xg</span><span class="p">)</span>
<span class="k">return</span> <span class="n">y</span>
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="mpi-io">
<h2>MPI-IO<a class="headerlink" href="#mpi-io" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">Collective I/O with NumPy arrays:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">amode</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">MODE_WRONLY</span><span class="o">|</span><span class="n">MPI</span><span class="o">.</span><span class="n">MODE_CREATE</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">fh</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">File</span><span class="o">.</span><span class="n">Open</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="s2">"./datafile.contig"</span><span class="p">,</span> <span class="n">amode</span><span class="p">)</span>
<span class="n">buffer</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="n">np</span><span class="o">.</span><span class="n">int</span><span class="p">)</span>
<span class="n">buffer</span><span class="p">[:]</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="n">offset</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span><span class="o">*</span><span class="n">buffer</span><span class="o">.</span><span class="n">nbytes</span>
<span class="n">fh</span><span class="o">.</span><span class="n">Write_at_all</span><span class="p">(</span><span class="n">offset</span><span class="p">,</span> <span class="n">buffer</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">Close</span><span class="p">()</span>
</pre></div>
</div>
</li>
<li><p class="first">Non-contiguous Collective I/O with NumPy arrays and datatypes:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span> <span class="k">as</span> <span class="nn">np</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="n">size</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">amode</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">MODE_WRONLY</span><span class="o">|</span><span class="n">MPI</span><span class="o">.</span><span class="n">MODE_CREATE</span>
<span class="n">fh</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">File</span><span class="o">.</span><span class="n">Open</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="s2">"./datafile.noncontig"</span><span class="p">,</span> <span class="n">amode</span><span class="p">)</span>
<span class="n">item_count</span> <span class="o">=</span> <span class="mi">10</span>
<span class="n">buffer</span> <span class="o">=</span> <span class="n">np</span><span class="o">.</span><span class="n">empty</span><span class="p">(</span><span class="n">item_count</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">buffer</span><span class="p">[:]</span> <span class="o">=</span> <span class="n">rank</span>
<span class="n">filetype</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">INT</span><span class="o">.</span><span class="n">Create_vector</span><span class="p">(</span><span class="n">item_count</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="n">size</span><span class="p">)</span>
<span class="n">filetype</span><span class="o">.</span><span class="n">Commit</span><span class="p">()</span>
<span class="n">displacement</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">INT</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span><span class="o">*</span><span class="n">rank</span>
<span class="n">fh</span><span class="o">.</span><span class="n">Set_view</span><span class="p">(</span><span class="n">displacement</span><span class="p">,</span> <span class="n">filetype</span><span class="o">=</span><span class="n">filetype</span><span class="p">)</span>
<span class="n">fh</span><span class="o">.</span><span class="n">Write_all</span><span class="p">(</span><span class="n">buffer</span><span class="p">)</span>
<span class="n">filetype</span><span class="o">.</span><span class="n">Free</span><span class="p">()</span>
<span class="n">fh</span><span class="o">.</span><span class="n">Close</span><span class="p">()</span>
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="dynamic-process-management">
<h2>Dynamic Process Management<a class="headerlink" href="#dynamic-process-management" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">Compute Pi - Master (or parent, or client) side:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env python</span>
<span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span>
<span class="kn">import</span> <span class="nn">sys</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_SELF</span><span class="o">.</span><span class="n">Spawn</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">executable</span><span class="p">,</span>
<span class="n">args</span><span class="o">=</span><span class="p">[</span><span class="s1">'cpi.py'</span><span class="p">],</span>
<span class="n">maxprocs</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span>
<span class="n">N</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="mi">100</span><span class="p">,</span> <span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Bcast</span><span class="p">([</span><span class="n">N</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">INT</span><span class="p">],</span> <span class="n">root</span><span class="o">=</span><span class="n">MPI</span><span class="o">.</span><span class="n">ROOT</span><span class="p">)</span>
<span class="n">PI</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="mf">0.0</span><span class="p">,</span> <span class="s1">'d'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Reduce</span><span class="p">(</span><span class="kc">None</span><span class="p">,</span> <span class="p">[</span><span class="n">PI</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">DOUBLE</span><span class="p">],</span>
<span class="n">op</span><span class="o">=</span><span class="n">MPI</span><span class="o">.</span><span class="n">SUM</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="n">MPI</span><span class="o">.</span><span class="n">ROOT</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">PI</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Disconnect</span><span class="p">()</span>
</pre></div>
</div>
</li>
<li><p class="first">Compute Pi - Worker (or child, or server) side:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="ch">#!/usr/bin/env python</span>
<span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="kn">import</span> <span class="nn">numpy</span>
<span class="n">comm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">Comm</span><span class="o">.</span><span class="n">Get_parent</span><span class="p">()</span>
<span class="n">size</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_size</span><span class="p">()</span>
<span class="n">rank</span> <span class="o">=</span> <span class="n">comm</span><span class="o">.</span><span class="n">Get_rank</span><span class="p">()</span>
<span class="n">N</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'i'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Bcast</span><span class="p">([</span><span class="n">N</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">INT</span><span class="p">],</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="n">h</span> <span class="o">=</span> <span class="mf">1.0</span> <span class="o">/</span> <span class="n">N</span><span class="p">;</span> <span class="n">s</span> <span class="o">=</span> <span class="mf">0.0</span>
<span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="n">rank</span><span class="p">,</span> <span class="n">N</span><span class="p">,</span> <span class="n">size</span><span class="p">):</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">h</span> <span class="o">*</span> <span class="p">(</span><span class="n">i</span> <span class="o">+</span> <span class="mf">0.5</span><span class="p">)</span>
<span class="n">s</span> <span class="o">+=</span> <span class="mf">4.0</span> <span class="o">/</span> <span class="p">(</span><span class="mf">1.0</span> <span class="o">+</span> <span class="n">x</span><span class="o">**</span><span class="mi">2</span><span class="p">)</span>
<span class="n">PI</span> <span class="o">=</span> <span class="n">numpy</span><span class="o">.</span><span class="n">array</span><span class="p">(</span><span class="n">s</span> <span class="o">*</span> <span class="n">h</span><span class="p">,</span> <span class="n">dtype</span><span class="o">=</span><span class="s1">'d'</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Reduce</span><span class="p">([</span><span class="n">PI</span><span class="p">,</span> <span class="n">MPI</span><span class="o">.</span><span class="n">DOUBLE</span><span class="p">],</span> <span class="kc">None</span><span class="p">,</span>
<span class="n">op</span><span class="o">=</span><span class="n">MPI</span><span class="o">.</span><span class="n">SUM</span><span class="p">,</span> <span class="n">root</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="n">comm</span><span class="o">.</span><span class="n">Disconnect</span><span class="p">()</span>
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="wrapping-with-swig">
<h2>Wrapping with SWIG<a class="headerlink" href="#wrapping-with-swig" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">C source:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cm">/* file: helloworld.c */</span>
<span class="kt">void</span> <span class="nf">sayhello</span><span class="p">(</span><span class="n">MPI_Comm</span> <span class="n">comm</span><span class="p">)</span>
<span class="p">{</span>
<span class="kt">int</span> <span class="n">size</span><span class="p">,</span> <span class="n">rank</span><span class="p">;</span>
<span class="n">MPI_Comm_size</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="o">&</span><span class="n">size</span><span class="p">);</span>
<span class="n">MPI_Comm_rank</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="o">&</span><span class="n">rank</span><span class="p">);</span>
<span class="n">printf</span><span class="p">(</span><span class="s">"Hello, World! "</span>
<span class="s">"I am process %d of %d.</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span>
<span class="n">rank</span><span class="p">,</span> <span class="n">size</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
</li>
<li><p class="first">SWIG interface file:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="c1">// file: helloworld.i</span>
<span class="o">%</span><span class="n">module</span> <span class="n">helloworld</span>
<span class="o">%</span><span class="p">{</span>
<span class="cp">#include</span> <span class="cpf"><mpi.h></span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">"helloworld.c"</span><span class="cp"></span>
<span class="p">}</span><span class="o">%</span>
<span class="o">%</span><span class="n">include</span> <span class="n">mpi4py</span><span class="o">/</span><span class="n">mpi4py</span><span class="p">.</span><span class="n">i</span>
<span class="o">%</span><span class="n">mpi4py_typemap</span><span class="p">(</span><span class="n">Comm</span><span class="p">,</span> <span class="n">MPI_Comm</span><span class="p">);</span>
<span class="kt">void</span> <span class="nf">sayhello</span><span class="p">(</span><span class="n">MPI_Comm</span> <span class="n">comm</span><span class="p">);</span>
</pre></div>
</div>
</li>
<li><p class="first">Try it in the Python prompt:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">helloworld</span>
<span class="gp">>>> </span><span class="n">helloworld</span><span class="o">.</span><span class="n">sayhello</span><span class="p">(</span><span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span><span class="p">)</span>
<span class="go">Hello, World! I am process 0 of 1.</span>
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="wrapping-with-f2py">
<h2>Wrapping with F2Py<a class="headerlink" href="#wrapping-with-f2py" title="Permalink to this headline">¶</a></h2>
<ul>
<li><p class="first">Fortran 90 source:</p>
<div class="highlight-fortran notranslate"><div class="highlight"><pre><span></span><span class="c">! file: helloworld.f90</span>
<span class="k">subroutine </span><span class="n">sayhello</span><span class="p">(</span><span class="n">comm</span><span class="p">)</span>
<span class="k">use </span><span class="n">mpi</span>
<span class="k">implicit none</span>
<span class="k"> </span><span class="kt">integer</span> <span class="kd">::</span> <span class="n">comm</span><span class="p">,</span> <span class="n">rank</span><span class="p">,</span> <span class="n">size</span><span class="p">,</span> <span class="n">ierr</span>
<span class="k">call </span><span class="n">MPI_Comm_size</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="n">size</span><span class="p">,</span> <span class="n">ierr</span><span class="p">)</span>
<span class="k">call </span><span class="n">MPI_Comm_rank</span><span class="p">(</span><span class="n">comm</span><span class="p">,</span> <span class="n">rank</span><span class="p">,</span> <span class="n">ierr</span><span class="p">)</span>
<span class="k">print</span> <span class="o">*</span><span class="p">,</span> <span class="s1">'Hello, World! I am process '</span><span class="p">,</span><span class="n">rank</span><span class="p">,</span><span class="s1">' of '</span><span class="p">,</span><span class="n">size</span><span class="p">,</span><span class="s1">'.'</span>
<span class="k">end subroutine </span><span class="n">sayhello</span>
</pre></div>
</div>
</li>
<li><p class="first">Compiling example using f2py</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ f2py -c --f90exec=mpif90 helloworld.f90 -m helloworld
</pre></div>
</div>
</li>
<li><p class="first">Try it in the Python prompt:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">mpi4py</span> <span class="k">import</span> <span class="n">MPI</span>
<span class="gp">>>> </span><span class="kn">import</span> <span class="nn">helloworld</span>
<span class="gp">>>> </span><span class="n">fcomm</span> <span class="o">=</span> <span class="n">MPI</span><span class="o">.</span><span class="n">COMM_WORLD</span><span class="o">.</span><span class="n">py2f</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">helloworld</span><span class="o">.</span><span class="n">sayhello</span><span class="p">(</span><span class="n">fcomm</span><span class="p">)</span>
<span class="go">Hello, World! I am process 0 of 1.</span>
</pre></div>
</div>
</li>
</ul>
</div>
</div>
</div>
</div>
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<li><a class="reference internal" href="#">Tutorial</a><ul>
<li><a class="reference internal" href="#running-python-scripts-with-mpi">Running Python scripts with MPI</a></li>
<li><a class="reference internal" href="#point-to-point-communication">Point-to-Point Communication</a></li>
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<li><a class="reference internal" href="#dynamic-process-management">Dynamic Process Management</a></li>
<li><a class="reference internal" href="#wrapping-with-swig">Wrapping with SWIG</a></li>
<li><a class="reference internal" href="#wrapping-with-f2py">Wrapping with F2Py</a></li>
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