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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="mpi.python"></a><a class="link" href="python.html" title="Python Bindings">Python Bindings</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="python.html#mpi.python.quickstart">Quickstart</a></span></dt>
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<dt><span class="section"><a href="python.html#mpi.python.user_data">Transmitting User-Defined Data</a></span></dt>
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<dt><span class="section"><a href="python.html#mpi.python.collectives">Collectives</a></span></dt>
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<dt><span class="section"><a href="python.html#mpi.python.skeleton_content">Skeleton/Content Mechanism</a></span></dt>
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<dt><span class="section"><a href="python.html#mpi.python.compatibility">C++/Python MPI Compatibility</a></span></dt>
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<dt><span class="section"><a href="python.html#mpi.python.reference">Reference</a></span></dt>
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</dl></div>
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<p>
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Boost.MPI provides an alternative MPI interface from the <a href="http://www.python.org" target="_top">Python</a>
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programming language via the <code class="computeroutput"><span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span></code> module.
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The Boost.MPI Python bindings, built on top of the C++ Boost.MPI using the
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<a href="http://www.boost.org/libs/python/doc" target="_top">Boost.Python</a> library,
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provide nearly all of the functionality of Boost.MPI within a dynamic, object-oriented
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language.
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</p>
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<p>
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The Boost.MPI Python module can be built and installed from the <code class="computeroutput"><span class="identifier">libs</span><span class="special">/</span><span class="identifier">mpi</span><span class="special">/</span><span class="identifier">build</span></code> directory.
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Just follow the <a class="link" href="getting_started.html#mpi.getting_started.config" title="Configure and Build">configuration</a>
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and <a class="link" href="getting_started.html#mpi.getting_started.config.installation" title="Installation">installation</a>
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instructions for the C++ Boost.MPI. Once you have installed the Python module,
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be sure that the installation location is in your <code class="computeroutput"><span class="identifier">PYTHONPATH</span></code>.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="mpi.python.quickstart"></a><a class="link" href="python.html#mpi.python.quickstart" title="Quickstart">Quickstart</a>
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</h3></div></div></div>
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<p>
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Getting started with the Boost.MPI Python module is as easy as importing
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<code class="computeroutput"><span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span></code>. Our first "Hello, World!"
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program is just two lines long:
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</p>
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<pre class="programlisting"><span class="keyword">import</span> <span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span> <span class="keyword">as</span> <span class="identifier">mpi</span>
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<span class="keyword">print</span> <span class="string">"I am process %d of %d."</span> <span class="special">%</span> <span class="special">(</span><span class="identifier">mpi</span><span class="special">.</span><span class="identifier">rank</span><span class="special">,</span> <span class="identifier">mpi</span><span class="special">.</span><span class="identifier">size</span><span class="special">)</span>
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</pre>
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<p>
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Go ahead and run this program with several processes. Be sure to invoke the
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<code class="computeroutput"><span class="identifier">python</span></code> interpreter from
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<code class="computeroutput"><span class="identifier">mpirun</span></code>, e.g.,
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</p>
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<pre class="programlisting">mpirun -np 5 python hello_world.py
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</pre>
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<p>
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This will return output such as:
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</p>
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<pre class="programlisting">I am process 1 of 5.
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I am process 3 of 5.
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I am process 2 of 5.
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I am process 4 of 5.
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I am process 0 of 5.
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</pre>
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<p>
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Point-to-point operations in Boost.MPI have nearly the same syntax in Python
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as in C++. We can write a simple two-process Python program that prints "Hello,
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world!" by transmitting Python strings:
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</p>
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<pre class="programlisting"><span class="keyword">import</span> <span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span> <span class="keyword">as</span> <span class="identifier">mpi</span>
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<span class="keyword">if</span> <span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">rank</span> <span class="special">==</span> <span class="number">0</span><span class="special">:</span>
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<span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">send</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="string">'Hello'</span><span class="special">)</span>
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<span class="identifier">msg</span> <span class="special">=</span> <span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">recv</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">1</span><span class="special">)</span>
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<span class="keyword">print</span> <span class="identifier">msg</span><span class="special">,</span><span class="string">'!'</span>
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<span class="keyword">else</span><span class="special">:</span>
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<span class="identifier">msg</span> <span class="special">=</span> <span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">recv</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
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<span class="keyword">print</span> <span class="special">(</span><span class="identifier">msg</span> <span class="special">+</span> <span class="string">', '</span><span class="special">),</span>
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<span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">send</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="number">1</span><span class="special">,</span> <span class="string">'world'</span><span class="special">)</span>
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</pre>
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<p>
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There are only a few notable differences between this Python code and the
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example <a class="link" href="tutorial.html#mpi.tutorial.point_to_point" title="Point-to-Point communication">in the C++ tutorial</a>.
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First of all, we don't need to write any initialization code in Python: just
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loading the <code class="computeroutput"><span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span></code> module makes the appropriate <code class="computeroutput"><span class="identifier">MPI_Init</span></code> and <code class="computeroutput"><span class="identifier">MPI_Finalize</span></code>
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calls. Second, we're passing Python objects from one process to another through
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MPI. Any Python object that can be pickled can be transmitted; the next section
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will describe in more detail how the Boost.MPI Python layer transmits objects.
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Finally, when we receive objects with <code class="computeroutput"><span class="identifier">recv</span></code>,
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we don't need to specify the type because transmission of Python objects
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is polymorphic.
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</p>
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<p>
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When experimenting with Boost.MPI in Python, don't forget that help is always
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available via <code class="computeroutput"><span class="identifier">pydoc</span></code>: just
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pass the name of the module or module entity on the command line (e.g.,
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<code class="computeroutput"><span class="identifier">pydoc</span> <span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span><span class="special">.</span><span class="identifier">communicator</span></code>) to receive complete reference
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||
documentation. When in doubt, try it!
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="mpi.python.user_data"></a><a class="link" href="python.html#mpi.python.user_data" title="Transmitting User-Defined Data">Transmitting User-Defined Data</a>
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</h3></div></div></div>
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<p>
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Boost.MPI can transmit user-defined data in several different ways. Most
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importantly, it can transmit arbitrary <a href="http://www.python.org" target="_top">Python</a>
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objects by pickling them at the sender and unpickling them at the receiver,
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allowing arbitrarily complex Python data structures to interoperate with
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MPI.
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</p>
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<p>
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Boost.MPI also supports efficient serialization and transmission of C++ objects
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(that have been exposed to Python) through its C++ interface. Any C++ type
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that provides (de-)serialization routines that meet the requirements of the
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Boost.Serialization library is eligible for this optimization, but the type
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||
must be registered in advance. To register a C++ type, invoke the C++ function
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<code class="computeroutput"><a class="link" href="../boost/mpi/python/register_serialized.html" title="Function template register_serialized">register_serialized</a></code>.
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If your C++ types come from other Python modules (they probably will!), those
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||
modules will need to link against the <code class="computeroutput"><span class="identifier">boost_mpi</span></code>
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||
and <code class="computeroutput"><span class="identifier">boost_mpi_python</span></code> libraries
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||
as described in the <a class="link" href="getting_started.html#mpi.getting_started.config.installation" title="Installation">installation
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||
section</a>. Note that you do <span class="bold"><strong>not</strong></span> need
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||
to link against the Boost.MPI Python extension module.
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||
</p>
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<p>
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Finally, Boost.MPI supports separation of the structure of an object from
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the data it stores, allowing the two pieces to be transmitted separately.
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This "skeleton/content" mechanism, described in more detail in
|
||
a later section, is a communication optimization suitable for problems with
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||
fixed data structures whose internal data changes frequently.
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||
</p>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h3 class="title">
|
||
<a name="mpi.python.collectives"></a><a class="link" href="python.html#mpi.python.collectives" title="Collectives">Collectives</a>
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||
</h3></div></div></div>
|
||
<p>
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||
Boost.MPI supports all of the MPI collectives (<code class="computeroutput"><span class="identifier">scatter</span></code>,
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<code class="computeroutput"><span class="identifier">reduce</span></code>, <code class="computeroutput"><span class="identifier">scan</span></code>,
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||
<code class="computeroutput"><span class="identifier">broadcast</span></code>, etc.) for any
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||
type of data that can be transmitted with the point-to-point communication
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||
operations. For the MPI collectives that require a user-specified operation
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||
(e.g., <code class="computeroutput"><span class="identifier">reduce</span></code> and <code class="computeroutput"><span class="identifier">scan</span></code>), the operation can be an arbitrary
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||
Python function. For instance, one could concatenate strings with <code class="computeroutput"><span class="identifier">all_reduce</span></code>:
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||
</p>
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||
<pre class="programlisting"><span class="identifier">mpi</span><span class="special">.</span><span class="identifier">all_reduce</span><span class="special">(</span><span class="identifier">my_string</span><span class="special">,</span> <span class="keyword">lambda</span> <span class="identifier">x</span><span class="special">,</span><span class="identifier">y</span><span class="special">:</span> <span class="identifier">x</span> <span class="special">+</span> <span class="identifier">y</span><span class="special">)</span>
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||
</pre>
|
||
<p>
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The following module-level functions implement MPI collectives: all_gather
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||
Gather the values from all processes. all_reduce Combine the results from
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all processes. all_to_all Every process sends data to every other process.
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||
broadcast Broadcast data from one process to all other processes. gather
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||
Gather the values from all processes to the root. reduce Combine the results
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||
from all processes to the root. scan Prefix reduction of the values from
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||
all processes. scatter Scatter the values stored at the root to all processes.
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||
</p>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h3 class="title">
|
||
<a name="mpi.python.skeleton_content"></a><a class="link" href="python.html#mpi.python.skeleton_content" title="Skeleton/Content Mechanism">Skeleton/Content Mechanism</a>
|
||
</h3></div></div></div>
|
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<p>
|
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Boost.MPI provides a skeleton/content mechanism that allows the transfer
|
||
of large data structures to be split into two separate stages, with the skeleton
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(or, "shape") of the data structure sent first and the content
|
||
(or, "data") of the data structure sent later, potentially several
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times, so long as the structure has not changed since the skeleton was transferred.
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The skeleton/content mechanism can improve performance when the data structure
|
||
is large and its shape is fixed, because while the skeleton requires serialization
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||
(it has an unknown size), the content transfer is fixed-size and can be done
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||
without extra copies.
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||
</p>
|
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<p>
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To use the skeleton/content mechanism from Python, you must first register
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||
the type of your data structure with the skeleton/content mechanism <span class="bold"><strong>from C++</strong></span>. The registration function is <code class="computeroutput"><a class="link" href="../boost/mpi/python/regist_1_3_27_7_20_2_1_1_2.html" title="Function template register_skeleton_and_content">register_skeleton_and_content</a></code>
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||
and resides in the <code class="computeroutput"><a class="link" href="reference.html#header.boost.mpi.python_hpp" title="Header <boost/mpi/python.hpp>"><boost/mpi/python.hpp></a></code>
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||
header.
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||
</p>
|
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<p>
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Once you have registered your C++ data structures, you can extract the skeleton
|
||
for an instance of that data structure with <code class="computeroutput"><span class="identifier">skeleton</span><span class="special">()</span></code>. The resulting <code class="computeroutput"><span class="identifier">skeleton_proxy</span></code>
|
||
can be transmitted via the normal send routine, e.g.,
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</p>
|
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<pre class="programlisting"><span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">send</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">skeleton</span><span class="special">(</span><span class="identifier">my_data_structure</span><span class="special">))</span>
|
||
</pre>
|
||
<p>
|
||
<code class="computeroutput"><span class="identifier">skeleton_proxy</span></code> objects can
|
||
be received on the other end via <code class="computeroutput"><span class="identifier">recv</span><span class="special">()</span></code>, which stores a newly-created instance
|
||
of your data structure with the same "shape" as the sender in its
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<code class="computeroutput"><span class="string">"object"</span></code> attribute:
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||
</p>
|
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<pre class="programlisting"><span class="identifier">shape</span> <span class="special">=</span> <span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">recv</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="number">0</span><span class="special">)</span>
|
||
<span class="identifier">my_data_structure</span> <span class="special">=</span> <span class="identifier">shape</span><span class="special">.</span><span class="identifier">object</span>
|
||
</pre>
|
||
<p>
|
||
Once the skeleton has been transmitted, the content (accessed via <code class="computeroutput"><span class="identifier">get_content</span></code>) can be transmitted in much
|
||
the same way. Note, however, that the receiver also specifies <code class="computeroutput"><span class="identifier">get_content</span><span class="special">(</span><span class="identifier">my_data_structure</span><span class="special">)</span></code>
|
||
in its call to receive:
|
||
</p>
|
||
<pre class="programlisting"><span class="keyword">if</span> <span class="identifier">mpi</span><span class="special">.</span><span class="identifier">rank</span> <span class="special">==</span> <span class="number">0</span><span class="special">:</span>
|
||
<span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">send</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">get_content</span><span class="special">(</span><span class="identifier">my_data_structure</span><span class="special">))</span>
|
||
<span class="keyword">else</span><span class="special">:</span>
|
||
<span class="identifier">mpi</span><span class="special">.</span><span class="identifier">world</span><span class="special">.</span><span class="identifier">recv</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">get_content</span><span class="special">(</span><span class="identifier">my_data_structure</span><span class="special">))</span>
|
||
</pre>
|
||
<p>
|
||
Of course, this transmission of content can occur repeatedly, if the values
|
||
in the data structure--but not its shape--changes.
|
||
</p>
|
||
<p>
|
||
The skeleton/content mechanism is a structured way to exploit the interaction
|
||
between custom-built MPI datatypes and <code class="computeroutput"><span class="identifier">MPI_BOTTOM</span></code>,
|
||
to eliminate extra buffer copies.
|
||
</p>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h3 class="title">
|
||
<a name="mpi.python.compatibility"></a><a class="link" href="python.html#mpi.python.compatibility" title="C++/Python MPI Compatibility">C++/Python MPI Compatibility</a>
|
||
</h3></div></div></div>
|
||
<p>
|
||
Boost.MPI is a C++ library whose facilities have been exposed to Python via
|
||
the Boost.Python library. Since the Boost.MPI Python bindings are build directly
|
||
on top of the C++ library, and nearly every feature of C++ library is available
|
||
in Python, hybrid C++/Python programs using Boost.MPI can interact, e.g.,
|
||
sending a value from Python but receiving that value in C++ (or vice versa).
|
||
However, doing so requires some care. Because Python objects are dynamically
|
||
typed, Boost.MPI transfers type information along with the serialized form
|
||
of the object, so that the object can be received even when its type is not
|
||
known. This mechanism differs from its C++ counterpart, where the static
|
||
types of transmitted values are always known.
|
||
</p>
|
||
<p>
|
||
The only way to communicate between the C++ and Python views on Boost.MPI
|
||
is to traffic entirely in Python objects. For Python, this is the normal
|
||
state of affairs, so nothing will change. For C++, this means sending and
|
||
receiving values of type <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span></code>,
|
||
from the <a href="http://www.boost.org/libs/python/doc" target="_top">Boost.Python</a>
|
||
library. For instance, say we want to transmit an integer value from Python:
|
||
</p>
|
||
<pre class="programlisting"><span class="identifier">comm</span><span class="special">.</span><span class="identifier">send</span><span class="special">(</span><span class="number">1</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="number">17</span><span class="special">)</span>
|
||
</pre>
|
||
<p>
|
||
In C++, we would receive that value into a Python object and then <code class="computeroutput"><span class="identifier">extract</span></code> an integer value:
|
||
</p>
|
||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">object</span> <span class="identifier">value</span><span class="special">;</span>
|
||
<span class="identifier">comm</span><span class="special">.</span><span class="identifier">recv</span><span class="special">(</span><span class="number">0</span><span class="special">,</span> <span class="number">0</span><span class="special">,</span> <span class="identifier">value</span><span class="special">);</span>
|
||
<span class="keyword">int</span> <span class="identifier">int_value</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">python</span><span class="special">::</span><span class="identifier">extract</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">value</span><span class="special">);</span>
|
||
</pre>
|
||
<p>
|
||
In the future, Boost.MPI will be extended to allow improved interoperability
|
||
with the C++ Boost.MPI and the C MPI bindings.
|
||
</p>
|
||
</div>
|
||
<div class="section">
|
||
<div class="titlepage"><div><div><h3 class="title">
|
||
<a name="mpi.python.reference"></a><a class="link" href="python.html#mpi.python.reference" title="Reference">Reference</a>
|
||
</h3></div></div></div>
|
||
<p>
|
||
The Boost.MPI Python module, <code class="computeroutput"><span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span></code>, has
|
||
its own <a href="../boost.mpi.html" target="_top">reference documentation</a>, which
|
||
is also available using <code class="computeroutput"><span class="identifier">pydoc</span></code>
|
||
(from the command line) or <code class="computeroutput"><span class="identifier">help</span><span class="special">(</span><span class="identifier">boost</span><span class="special">.</span><span class="identifier">mpi</span><span class="special">)</span></code>
|
||
(from the Python interpreter).
|
||
</p>
|
||
</div>
|
||
</div>
|
||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||
<td align="left"></td>
|
||
<td align="right"><div class="copyright-footer">Copyright © 2005-2007 Douglas Gregor,
|
||
Matthias Troyer, Trustees of Indiana University<p>
|
||
Distributed under the Boost Software License, Version 1.0. (See accompanying
|
||
file LICENSE_1_0.txt or copy at <ulink url="http://www.boost.org/LICENSE_1_0.txt">
|
||
http://www.boost.org/LICENSE_1_0.txt </ulink>)
|
||
</p>
|
||
</div></td>
|
||
</tr></table>
|
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