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208 lines
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<meta name="generator" content="Docutils 0.6: http://docutils.sourceforge.net/" />
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<title>Parallel BGL Depth-First Visit</title>
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<link rel="stylesheet" href="../../../../rst.css" type="text/css" />
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<body>
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<div class="document" id="logo-depth-first-visit">
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<h1 class="title"><a class="reference external" href="http://www.osl.iu.edu/research/pbgl"><img align="middle" alt="Parallel BGL" class="align-middle" src="pbgl-logo.png" /></a> Depth-First Visit</h1>
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<!-- Copyright (C) 2004-2008 The Trustees of Indiana University.
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Use, modification and distribution is subject to the Boost Software
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License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt) -->
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<pre class="literal-block">
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template<typename DistributedGraph, typename DFSVisitor>
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void
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depth_first_visit(const DistributedGraph& g,
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typename graph_traits<DistributedGraph>::vertex_descriptor s,
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DFSVisitor vis);
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namespace graph {
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template<typename DistributedGraph, typename DFSVisitor,
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typename VertexIndexMap>
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void
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tsin_depth_first_visit(const DistributedGraph& g,
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typename graph_traits<DistributedGraph>::vertex_descriptor s,
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DFSVisitor vis);
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template<typename DistributedGraph, typename DFSVisitor,
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typename VertexIndexMap>
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void
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tsin_depth_first_visit(const DistributedGraph& g,
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typename graph_traits<DistributedGraph>::vertex_descriptor s,
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DFSVisitor vis, VertexIndexMap index_map);
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template<typename DistributedGraph, typename ColorMap, typename ParentMap,
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typename ExploreMap, typename NextOutEdgeMap, typename DFSVisitor>
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void
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tsin_depth_first_visit(const DistributedGraph& g,
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typename graph_traits<DistributedGraph>::vertex_descriptor s,
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DFSVisitor vis, ColorMap color, ParentMap parent, ExploreMap explore,
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NextOutEdgeMap next_out_edge);
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}
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</pre>
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<p>The <tt class="docutils literal"><span class="pre">depth_first_visit()</span></tt> function performs a distributed
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depth-first traversal of an undirected graph using Tsin's corrections
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<a class="citation-reference" href="#tsin02" id="id1">[Tsin02]</a> to Cidon's algorithm <a class="citation-reference" href="#cidon88" id="id2">[Cidon88]</a>. The distributed DFS is
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syntactically similar to its <a class="reference external" href="http://www.boost.org/libs/graph/doc/depth_first_visit.html">sequential counterpart</a>, which provides
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additional background and discussion. Differences in semantics are
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highlighted here, and we refer the reader to the documentation of the
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<a class="reference external" href="http://www.boost.org/libs/graph/doc/depth_first_visit.html">sequential depth-first search</a> for the remainder of the
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details. Visitors passed to depth-first search need to account for the
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distribution of vertices across processes, because events will be
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triggered on certain processes but not others. See the section
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<a class="reference internal" href="#visitor-event-points">Visitor Event Points</a> for details.</p>
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<div class="section" id="where-defined">
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<h1>Where Defined</h1>
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<p><<tt class="docutils literal"><span class="pre">boost/graph/distributed/depth_first_search.hpp</span></tt>></p>
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<p>also available in</p>
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<p><<tt class="docutils literal"><span class="pre">boost/graph/depth_first_search.hpp</span></tt>></p>
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</div>
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<div class="section" id="parameters">
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<h1>Parameters</h1>
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<dl class="docutils">
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<dt>IN: <tt class="docutils literal"><span class="pre">const</span> <span class="pre">Graph&</span> <span class="pre">g</span></tt></dt>
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<dd>The graph type must be a model of <a class="reference external" href="DistributedGraph.html">Distributed Graph</a>. The graph
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must be undirected.</dd>
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<dt>IN: <tt class="docutils literal"><span class="pre">vertex_descriptor</span> <span class="pre">s</span></tt></dt>
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<dd>The start vertex must be the same in every process.</dd>
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<dt>IN: <tt class="docutils literal"><span class="pre">DFSVisitor</span> <span class="pre">vis</span></tt></dt>
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<dd>The visitor must be a distributed DFS visitor. The suble differences
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between sequential and distributed DFS visitors are discussed in the
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section <a class="reference internal" href="#visitor-event-points">Visitor Event Points</a>.</dd>
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<dt>IN: <tt class="docutils literal"><span class="pre">VertexIndexMap</span> <span class="pre">map</span></tt></dt>
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<dd><p class="first">A model of <a class="reference external" href="http://www.boost.org/libs/property_map/ReadablePropertyMap.html">Readable Property Map</a> whose key type is the vertex
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descriptor type of the graph <tt class="docutils literal"><span class="pre">g</span></tt> and whose value type is an
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integral type. The property map should map from vertices to their
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(local) indices in the range <em>[0, num_vertices(g))</em>.</p>
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<p class="last"><strong>Default</strong>: <tt class="docutils literal"><span class="pre">get(vertex_index,</span> <span class="pre">g)</span></tt></p>
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</dd>
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<dt>UTIL/OUT: <tt class="docutils literal"><span class="pre">ColorMap</span> <span class="pre">color</span></tt></dt>
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<dd><p class="first">The color map must be a <a class="reference external" href="distributed_property_map.html">Distributed Property Map</a> with the same
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process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose colors must monotonically
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darken (white -> gray -> black).</p>
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<p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
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<tt class="docutils literal"><span class="pre">std::vector</span></tt> of <tt class="docutils literal"><span class="pre">default_color_type</span></tt>.</p>
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</dd>
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<dt>UTIL/OUT: <tt class="docutils literal"><span class="pre">ParentMap</span> <span class="pre">parent</span></tt></dt>
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<dd><p class="first">The parent map must be a <a class="reference external" href="distributed_property_map.html">Distributed Property Map</a> with the same
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process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose key and values types are the
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same as the vertex descriptor type of the graph <tt class="docutils literal"><span class="pre">g</span></tt>. This
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property map holds the parent of each vertex in the depth-first
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search tree.</p>
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<p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
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<tt class="docutils literal"><span class="pre">std::vector</span></tt> of the vertex descriptor type of the graph.</p>
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</dd>
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<dt>UTIL: <tt class="docutils literal"><span class="pre">ExploreMap</span> <span class="pre">explore</span></tt></dt>
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<dd><p class="first">The explore map must be a <a class="reference external" href="distributed_property_map.html">Distributed Property Map</a> with the same
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process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose key and values types are the
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same as the vertex descriptor type of the graph <tt class="docutils literal"><span class="pre">g</span></tt>.</p>
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<p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
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<tt class="docutils literal"><span class="pre">std::vector</span></tt> of the vertex descriptor type of the graph.</p>
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</dd>
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<dt>UTIL: <tt class="docutils literal"><span class="pre">NextOutEdgeMap</span> <span class="pre">next_out_edge</span></tt></dt>
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<dd><p class="first">The next out-edge map must be a <a class="reference external" href="distributed_property_map.html">Distributed Property Map</a> with the
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same process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose key type is the vertex
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descriptor of the graph <tt class="docutils literal"><span class="pre">g</span></tt> and whose value type is the
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<tt class="docutils literal"><span class="pre">out_edge_iterator</span></tt> type of the graph. It is used internally to
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keep track of the next edge that should be traversed from each
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vertex.</p>
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<p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
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<tt class="docutils literal"><span class="pre">std::vector</span></tt> of the <tt class="docutils literal"><span class="pre">out_edge_iterator</span></tt> type of the graph.</p>
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</dd>
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</dl>
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</div>
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<div class="section" id="complexity">
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<h1>Complexity</h1>
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<p>Depth-first search is inherently sequential, so parallel speedup is
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very poor. Regardless of the number of processors, the algorithm will
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not be faster than <em>O(V)</em>; see <a class="citation-reference" href="#tsin02" id="id3">[Tsin02]</a> for more details.</p>
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</div>
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<div class="section" id="visitor-event-points">
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<h1>Visitor Event Points</h1>
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<p>The <a class="reference external" href="http://www.boost.org/libs/graph/doc/DFSVisitor.html">DFS Visitor</a> concept defines 8 event points that will be
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triggered by the <a class="reference external" href="http://www.boost.org/libs/graph/doc/depth_first_visit.html">sequential depth-first search</a>. The distributed
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DFS retains these event points, but the sequence of events
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triggered and the process in which each event occurs will change
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depending on the distribution of the graph.</p>
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<dl class="docutils">
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<dt><tt class="docutils literal"><span class="pre">initialize_vertex(s,</span> <span class="pre">g)</span></tt></dt>
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<dd>This will be invoked by every process for each local vertex.</dd>
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<dt><tt class="docutils literal"><span class="pre">discover_vertex(u,</span> <span class="pre">g)</span></tt></dt>
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<dd>This will be invoked each time a process discovers a new vertex
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<tt class="docutils literal"><span class="pre">u</span></tt>.</dd>
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<dt><tt class="docutils literal"><span class="pre">examine_vertex(u,</span> <span class="pre">g)</span></tt></dt>
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<dd>This will be invoked by the process owning the vertex <tt class="docutils literal"><span class="pre">u</span></tt>.</dd>
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<dt><tt class="docutils literal"><span class="pre">examine_edge(e,</span> <span class="pre">g)</span></tt></dt>
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<dd>This will be invoked by the process owning the source vertex of
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<tt class="docutils literal"><span class="pre">e</span></tt>.</dd>
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<dt><tt class="docutils literal"><span class="pre">tree_edge(e,</span> <span class="pre">g)</span></tt></dt>
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<dd>Similar to <tt class="docutils literal"><span class="pre">examine_edge</span></tt>, this will be invoked by the process
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owning the source vertex and may be invoked only once.</dd>
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<dt><tt class="docutils literal"><span class="pre">back_edge(e,</span> <span class="pre">g)</span></tt></dt>
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<dd>Some edges that would be forward or cross edges in the sequential
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DFS may be detected as back edges by the distributed DFS, so extra
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<tt class="docutils literal"><span class="pre">back_edge</span></tt> events may be received.</dd>
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<dt><tt class="docutils literal"><span class="pre">forward_or_cross_edge(e,</span> <span class="pre">g)</span></tt></dt>
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<dd>Some edges that would be forward or cross edges in the sequential
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DFS may be detected as back edges by the distributed DFS, so fewer
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<tt class="docutils literal"><span class="pre">forward_or_cross_edge</span></tt> events may be received in the distributed
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algorithm than in the sequential case.</dd>
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<dt><tt class="docutils literal"><span class="pre">finish_vertex(e,</span> <span class="pre">g)</span></tt></dt>
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<dd>See documentation for <tt class="docutils literal"><span class="pre">examine_vertex</span></tt>.</dd>
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</dl>
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<div class="section" id="making-visitors-safe-for-distributed-dfs">
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<h2>Making Visitors Safe for Distributed DFS</h2>
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<p>The three most important things to remember when updating an existing
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DFS visitor for distributed DFS or writing a new distributed DFS
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visitor are:</p>
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<ol class="arabic simple">
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<li>Be sure that all state is either entirely local or in a
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distributed data structure (most likely a property map!) using
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the same process group as the graph.</li>
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<li>Be sure that the visitor doesn't require precise event sequences
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that cannot be guaranteed by distributed DFS, e.g., requiring
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<tt class="docutils literal"><span class="pre">back_edge</span></tt> and <tt class="docutils literal"><span class="pre">forward_or_cross_edge</span></tt> events to be completely
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distinct.</li>
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<li>Be sure that the visitor can operate on incomplete
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information. This often includes using an appropriate reduction
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operation in a <a class="reference external" href="distributed_property_map.html">distributed property map</a> and verifying that
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results written are "better" that what was previously written.</li>
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</ol>
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</div>
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</div>
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<div class="section" id="bibliography">
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<h1>Bibliography</h1>
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<table class="docutils citation" frame="void" id="cidon88" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<tr><td class="label"><a class="fn-backref" href="#id2">[Cidon88]</a></td><td>Isreal Cidon. Yet another distributed depth-first-search
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algorithm. Information Processing Letters, 26(6):301--305, 1988.</td></tr>
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</tbody>
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</table>
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<table class="docutils citation" frame="void" id="tsin02" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<tr><td class="label">[Tsin02]</td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id3">2</a>)</em> Y. H. Tsin. Some remarks on distributed depth-first
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search. Information Processing Letters, 82(4):173--178, 2002.</td></tr>
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</tbody>
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</table>
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<hr class="docutils" />
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<p>Copyright (C) 2005 The Trustees of Indiana University.</p>
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<p>Authors: Douglas Gregor and Andrew Lumsdaine</p>
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</div>
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</div>
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<hr class="footer" />
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Generated on: 2009-05-31 00:21 UTC.
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Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source.
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