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<div class="document" id="parallel-shortest-paths">
<h1 class="title">Parallel Shortest Paths</h1>

<!-- Copyright (C) 2004-2008 The Trustees of Indiana University.
Use, modification and distribution is subject to the Boost Software
License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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<p>To illustrate the use of the Parallel Boost Graph Library, we
illustrate the use of both the sequential and parallel BGL to find
the shortest paths from vertex A to every other vertex in the
following simple graph:</p>
<img alt="../dijkstra_seq_graph.png" src="../dijkstra_seq_graph.png" />
<p>With the sequential <a class="reference external" href="http://www.boost.org/libs/graph/doc">BGL</a>, the program to calculate shortest paths has
three stages. Readers familiar with the BGL may wish to skip ahead to
the section <a class="reference internal" href="#distributing-the-graph">Distributing the graph</a>.</p>
<blockquote>
<ul class="simple">
<li><a class="reference internal" href="#define-the-graph-type">Define the graph type</a></li>
<li><a class="reference internal" href="#construct-the-graph">Construct the graph</a></li>
<li><a class="reference internal" href="#invoke-dijkstra-s-algorithm">Invoke Dijkstra's algorithm</a></li>
</ul>
</blockquote>
<div class="section" id="define-the-graph-type">
<h1>Define the graph type</h1>
<p>For this problem we use an adjacency list representation of the graph,
using the BGL <tt class="docutils literal"><span class="pre">adjacency_list``_</span> <span class="pre">class</span> <span class="pre">template.</span> <span class="pre">It</span> <span class="pre">will</span> <span class="pre">be</span> <span class="pre">a</span> <span class="pre">directed</span>
<span class="pre">graph</span> <span class="pre">(``directedS</span></tt> parameter ) whose vertices are stored in an
<tt class="docutils literal"><span class="pre">std::vector</span></tt> (<tt class="docutils literal"><span class="pre">vecS</span></tt> parameter) where the outgoing edges of each
vertex are stored in an <tt class="docutils literal"><span class="pre">std::list</span></tt> (<tt class="docutils literal"><span class="pre">listS</span></tt> parameter). To each
of the edges we attach an integral weight.</p>
<pre class="literal-block">
typedef adjacency_list&lt;listS, vecS, directedS,
                       no_property,                 // Vertex properties
                       property&lt;edge_weight_t, int&gt; // Edge properties
                       &gt; graph_t;
typedef graph_traits &lt; graph_t &gt;::vertex_descriptor vertex_descriptor;
typedef graph_traits &lt; graph_t &gt;::edge_descriptor edge_descriptor;
</pre>
</div>
<div class="section" id="construct-the-graph">
<h1>Construct the graph</h1>
<p>To build the graph, we declare an enumeration containing the node
names (for our own use) and create two arrays: the first,
<tt class="docutils literal"><span class="pre">edge_array</span></tt>, contains the source and target of each edge, whereas
the second, <tt class="docutils literal"><span class="pre">weights</span></tt>, contains the integral weight of each
edge. We pass the contents of the arrays via pointers (used here as
iterators) to the graph constructor to build our graph:</p>
<pre class="literal-block">
typedef std::pair&lt;int, int&gt; Edge;
const int num_nodes = 5;
enum nodes { A, B, C, D, E };
char name[] = &quot;ABCDE&quot;;
Edge edge_array[] = { Edge(A, C), Edge(B, B), Edge(B, D), Edge(B, E),
  Edge(C, B), Edge(C, D), Edge(D, E), Edge(E, A), Edge(E, B)
};
int weights[] = { 1, 2, 1, 2, 7, 3, 1, 1, 1 };
int num_arcs = sizeof(edge_array) / sizeof(Edge);

graph_t g(edge_array, edge_array + num_arcs, weights, num_nodes);
</pre>
</div>
<div class="section" id="invoke-dijkstra-s-algorithm">
<h1>Invoke Dijkstra's algorithm</h1>
<p>To invoke Dijkstra's algorithm, we need to first decide how we want
to receive the results of the algorithm, namely the distance to each
vertex and the predecessor of each vertex (allowing reconstruction of
the shortest paths themselves). In our case, we will create two
vectors, <tt class="docutils literal"><span class="pre">p</span></tt> for predecessors and <tt class="docutils literal"><span class="pre">d</span></tt> for distances.</p>
<p>Next, we determine our starting vertex <tt class="docutils literal"><span class="pre">s</span></tt> using the <tt class="docutils literal"><span class="pre">vertex</span></tt>
operation on the <tt class="docutils literal"><span class="pre">adjacency_list``_</span> <span class="pre">and</span> <span class="pre">call</span>
<span class="pre">``dijkstra_shortest_paths``_</span> <span class="pre">with</span> <span class="pre">the</span> <span class="pre">graph</span> <span class="pre">``g</span></tt>, starting vertex
<tt class="docutils literal"><span class="pre">s</span></tt>, and two <tt class="docutils literal"><span class="pre">property</span> <span class="pre">maps``_</span> <span class="pre">that</span> <span class="pre">instruct</span> <span class="pre">the</span> <span class="pre">algorithm</span> <span class="pre">to</span>
<span class="pre">store</span> <span class="pre">predecessors</span> <span class="pre">in</span> <span class="pre">the</span> <span class="pre">``p</span></tt> vector and distances in the <tt class="docutils literal"><span class="pre">d</span></tt>
vector. The algorithm automatically uses the edge weights stored
within the graph, although this capability can be overridden.</p>
<pre class="literal-block">
// Keeps track of the predecessor of each vertex
std::vector&lt;vertex_descriptor&gt; p(num_vertices(g));
// Keeps track of the distance to each vertex
std::vector&lt;int&gt; d(num_vertices(g));

vertex_descriptor s = vertex(A, g);
dijkstra_shortest_paths
  (g, s,
   predecessor_map(
     make_iterator_property_map(p.begin(), get(vertex_index, g))).
   distance_map(
     make_iterator_property_map(d.begin(), get(vertex_index, g)))
   );
</pre>
</div>
<div class="section" id="distributing-the-graph">
<h1>Distributing the graph</h1>
<p>The prior computation is entirely sequential, with the graph stored
within a single address space. To distribute the graph across several
processors without a shared address space, we need to represent the
processors and communication among them and alter the graph type.</p>
<p>Processors and their interactions are abstracted via a <em>process
group</em>. In our case, we will use <a class="reference external" href="http://www-unix.mcs.anl.gov/mpi/">MPI</a> for communication with
inter-processor messages sent immediately:</p>
<pre class="literal-block">
typedef mpi::process_group&lt;mpi::immediateS&gt; process_group_type;
</pre>
<p>Next, we instruct the <tt class="docutils literal"><span class="pre">adjacency_list</span></tt> template
to distribute the vertices of the graph across our process group,
storing the local vertices in an <tt class="docutils literal"><span class="pre">std::vector</span></tt>:</p>
<pre class="literal-block">
typedef adjacency_list&lt;listS,
                       distributedS&lt;process_group_type, vecS&gt;,
                       directedS,
                       no_property,                 // Vertex properties
                       property&lt;edge_weight_t, int&gt; // Edge properties
                       &gt; graph_t;
typedef graph_traits &lt; graph_t &gt;::vertex_descriptor vertex_descriptor;
typedef graph_traits &lt; graph_t &gt;::edge_descriptor edge_descriptor;
</pre>
<p>Note that the only difference from the sequential BGL is the use of
the <tt class="docutils literal"><span class="pre">distributedS</span></tt> selector, which identifies a distributed
graph. The vertices of the graph will be distributed among the
various processors, and the processor that owns a vertex also stores
the edges outgoing from that vertex and any properties associated
with that vertex or its edges. With three processors and the default
block distribution, the graph would be distributed in this manner:</p>
<img alt="../dijkstra_dist3_graph.png" src="../dijkstra_dist3_graph.png" />
<p>Processor 0 (red) owns vertices A and B, including all edges outoing
from those vertices, processor 1 (green) owns vertices C and D (and
their edges), and processor 2 (blue) owns vertex E. Constructing this
graph uses the same syntax is the sequential graph, as in the section
<a class="reference internal" href="#construct-the-graph">Construct the graph</a>.</p>
<p>The call to <tt class="docutils literal"><span class="pre">dijkstra_shortest_paths</span></tt> is syntactically equivalent to
the sequential call, but the mechanisms used are very different. The
property maps passed to <tt class="docutils literal"><span class="pre">dijkstra_shortest_paths</span></tt> are actually
<em>distributed</em> property maps, which store properties for local edges
or vertices and perform implicit communication to access properties
of remote edges or vertices when needed. The formulation of Dijkstra's
algorithm is also slightly different, because each processor can
only attempt to relax edges outgoing from local vertices: as shorter
paths to a vertex are discovered, messages to the processor owning
that vertex indicate that the vertex may require reprocessing.</p>
<hr class="docutils" />
<p>Return to the <a class="reference external" href="index.html">Parallel BGL home page</a></p>
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