ceph/src/boost/libs/graph_parallel/doc/html/tsin_depth_first_visit.html

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  11 <div class="document" id="logo-depth-first-visit">
  12 <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>
  13
  14 <!-- Copyright (C) 2004-2008 The Trustees of Indiana University.
  15 Use, modification and distribution is subject to the Boost Software
  16 License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
  17 http://www.boost.org/LICENSE_1_0.txt) -->
  18 <pre class="literal-block">
  19 template&lt;typename DistributedGraph, typename DFSVisitor&gt;
  20 void
  21 depth_first_visit(const DistributedGraph&amp; g,
  22                   typename graph_traits&lt;DistributedGraph&gt;::vertex_descriptor s,
  23                   DFSVisitor vis);
  24
  25 namespace graph {
  26   template&lt;typename DistributedGraph, typename DFSVisitor,
  27          typename VertexIndexMap&gt;
  28   void
  29   tsin_depth_first_visit(const DistributedGraph&amp; g,
  30                          typename graph_traits&lt;DistributedGraph&gt;::vertex_descriptor s,
  31                          DFSVisitor vis);
  32
  33   template&lt;typename DistributedGraph, typename DFSVisitor,
  34            typename VertexIndexMap&gt;
  35   void
  36   tsin_depth_first_visit(const DistributedGraph&amp; g,
  37                          typename graph_traits&lt;DistributedGraph&gt;::vertex_descriptor s,
  38                          DFSVisitor vis, VertexIndexMap index_map);
  39
  40   template&lt;typename DistributedGraph, typename ColorMap, typename ParentMap,
  41            typename ExploreMap, typename NextOutEdgeMap, typename DFSVisitor&gt;
  42   void
  43   tsin_depth_first_visit(const DistributedGraph&amp; g,
  44                          typename graph_traits&lt;DistributedGraph&gt;::vertex_descriptor s,
  45                          DFSVisitor vis, ColorMap color, ParentMap parent, ExploreMap explore,
  46                          NextOutEdgeMap next_out_edge);
  47 }
  48 </pre>
  49 <p>The <tt class="docutils literal"><span class="pre">depth_first_visit()</span></tt> function performs a distributed
  50 depth-first traversal of an undirected graph using Tsin's corrections
  51 <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
  52 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
  53 additional background and discussion. Differences in semantics are
  54 highlighted here, and we refer the reader to the documentation of the
  55 <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
  56 details. Visitors passed to depth-first search need to account for the
  57 distribution of vertices across processes, because events will be
  58 triggered on certain processes but not others. See the section
  59 <a class="reference internal" href="#visitor-event-points">Visitor Event Points</a> for details.</p>
  60 <div class="section" id="where-defined">
  61 <h1>Where Defined</h1>
  62 <p>&lt;<tt class="docutils literal"><span class="pre">boost/graph/distributed/depth_first_search.hpp</span></tt>&gt;</p>
  63 <p>also available in</p>
  64 <p>&lt;<tt class="docutils literal"><span class="pre">boost/graph/depth_first_search.hpp</span></tt>&gt;</p>
  65 </div>
  66 <div class="section" id="parameters">
  67 <h1>Parameters</h1>
  68 <dl class="docutils">
  69 <dt>IN: <tt class="docutils literal"><span class="pre">const</span> <span class="pre">Graph&amp;</span> <span class="pre">g</span></tt></dt>
  70 <dd>The graph type must be a model of <a class="reference external" href="DistributedGraph.html">Distributed Graph</a>. The graph
  71 must be undirected.</dd>
  72 <dt>IN: <tt class="docutils literal"><span class="pre">vertex_descriptor</span> <span class="pre">s</span></tt></dt>
  73 <dd>The start vertex must be the same in every process.</dd>
  74 <dt>IN: <tt class="docutils literal"><span class="pre">DFSVisitor</span> <span class="pre">vis</span></tt></dt>
  75 <dd>The visitor must be a distributed DFS visitor. The suble differences
  76 between sequential and distributed DFS visitors are discussed in the
  77 section <a class="reference internal" href="#visitor-event-points">Visitor Event Points</a>.</dd>
  78 <dt>IN: <tt class="docutils literal"><span class="pre">VertexIndexMap</span> <span class="pre">map</span></tt></dt>
  79 <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
  80 descriptor type of the graph <tt class="docutils literal"><span class="pre">g</span></tt> and whose value type is an
  81 integral type. The property map should map from vertices to their
  82 (local) indices in the range <em>[0, num_vertices(g))</em>.</p>
  83 <p class="last"><strong>Default</strong>: <tt class="docutils literal"><span class="pre">get(vertex_index,</span> <span class="pre">g)</span></tt></p>
  84 </dd>
  85 <dt>UTIL/OUT: <tt class="docutils literal"><span class="pre">ColorMap</span> <span class="pre">color</span></tt></dt>
  86 <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
  87 process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose colors must monotonically
  88 darken (white -&gt; gray -&gt; black).</p>
  89 <p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
  90 <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>
  91 </dd>
  92 <dt>UTIL/OUT: <tt class="docutils literal"><span class="pre">ParentMap</span> <span class="pre">parent</span></tt></dt>
  93 <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
  94 process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose key and values types are the
  95 same as the vertex descriptor type of the graph <tt class="docutils literal"><span class="pre">g</span></tt>. This
  96 property map holds the parent of each vertex in the depth-first
  97 search tree.</p>
  98 <p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
  99 <tt class="docutils literal"><span class="pre">std::vector</span></tt> of the vertex descriptor type of the graph.</p>
 100 </dd>
 101 <dt>UTIL: <tt class="docutils literal"><span class="pre">ExploreMap</span> <span class="pre">explore</span></tt></dt>
 102 <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
 103 process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose key and values types are the
 104 same as the vertex descriptor type of the graph <tt class="docutils literal"><span class="pre">g</span></tt>.</p>
 105 <p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
 106 <tt class="docutils literal"><span class="pre">std::vector</span></tt> of the vertex descriptor type of the graph.</p>
 107 </dd>
 108 <dt>UTIL: <tt class="docutils literal"><span class="pre">NextOutEdgeMap</span> <span class="pre">next_out_edge</span></tt></dt>
 109 <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
 110 same process group as the graph <tt class="docutils literal"><span class="pre">g</span></tt> whose key type is the vertex
 111 descriptor of the graph <tt class="docutils literal"><span class="pre">g</span></tt> and whose value type is the
 112 <tt class="docutils literal"><span class="pre">out_edge_iterator</span></tt> type of the graph. It is used internally to
 113 keep track of the next edge that should be traversed from each
 114 vertex.</p>
 115 <p class="last"><strong>Default</strong>: A distributed <tt class="docutils literal"><span class="pre">iterator_property_map</span></tt> created from a
 116 <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>
 117 </dd>
 118 </dl>
 119 </div>
 120 <div class="section" id="complexity">
 121 <h1>Complexity</h1>
 122 <p>Depth-first search is inherently sequential, so parallel speedup is
 123 very poor. Regardless of the number of processors, the algorithm will
 124 not be faster than <em>O(V)</em>; see <a class="citation-reference" href="#tsin02" id="id3">[Tsin02]</a> for more details.</p>
 125 </div>
 126 <div class="section" id="visitor-event-points">
 127 <h1>Visitor Event Points</h1>
 128 <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
 129 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
 130 DFS retains these event points, but the sequence of events
 131 triggered and the process in which each event occurs will change
 132 depending on the distribution of the graph.</p>
 133 <dl class="docutils">
 134 <dt><tt class="docutils literal"><span class="pre">initialize_vertex(s,</span> <span class="pre">g)</span></tt></dt>
 135 <dd>This will be invoked by every process for each local vertex.</dd>
 136 <dt><tt class="docutils literal"><span class="pre">discover_vertex(u,</span> <span class="pre">g)</span></tt></dt>
 137 <dd>This will be invoked each time a process discovers a new vertex
 138 <tt class="docutils literal"><span class="pre">u</span></tt>.</dd>
 139 <dt><tt class="docutils literal"><span class="pre">examine_vertex(u,</span> <span class="pre">g)</span></tt></dt>
 140 <dd>This will be invoked by the process owning the vertex <tt class="docutils literal"><span class="pre">u</span></tt>.</dd>
 141 <dt><tt class="docutils literal"><span class="pre">examine_edge(e,</span> <span class="pre">g)</span></tt></dt>
 142 <dd>This will be invoked by the process owning the source vertex of
 143 <tt class="docutils literal"><span class="pre">e</span></tt>.</dd>
 144 <dt><tt class="docutils literal"><span class="pre">tree_edge(e,</span> <span class="pre">g)</span></tt></dt>
 145 <dd>Similar to <tt class="docutils literal"><span class="pre">examine_edge</span></tt>, this will be invoked by the process
 146 owning the source vertex and may be invoked only once.</dd>
 147 <dt><tt class="docutils literal"><span class="pre">back_edge(e,</span> <span class="pre">g)</span></tt></dt>
 148 <dd>Some edges that would be forward or cross edges in the sequential
 149 DFS may be detected as back edges by the distributed DFS, so extra
 150 <tt class="docutils literal"><span class="pre">back_edge</span></tt> events may be received.</dd>
 151 <dt><tt class="docutils literal"><span class="pre">forward_or_cross_edge(e,</span> <span class="pre">g)</span></tt></dt>
 152 <dd>Some edges that would be forward or cross edges in the sequential
 153 DFS may be detected as back edges by the distributed DFS, so fewer
 154 <tt class="docutils literal"><span class="pre">forward_or_cross_edge</span></tt> events may be received in the distributed
 155 algorithm than in the sequential case.</dd>
 156 <dt><tt class="docutils literal"><span class="pre">finish_vertex(e,</span> <span class="pre">g)</span></tt></dt>
 157 <dd>See documentation for <tt class="docutils literal"><span class="pre">examine_vertex</span></tt>.</dd>
 158 </dl>
 159 <div class="section" id="making-visitors-safe-for-distributed-dfs">
 160 <h2>Making Visitors Safe for Distributed DFS</h2>
 161 <p>The three most important things to remember when updating an existing
 162 DFS visitor for distributed DFS or writing a new distributed DFS
 163 visitor are:</p>
 164 <ol class="arabic simple">
 165 <li>Be sure that all state is either entirely local or in a
 166 distributed data structure (most likely a property map!) using
 167 the same process group as the graph.</li>
 168 <li>Be sure that the visitor doesn't require precise event sequences
 169 that cannot be guaranteed by distributed DFS, e.g., requiring
 170 <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
 171 distinct.</li>
 172 <li>Be sure that the visitor can operate on incomplete
 173 information. This often includes using an appropriate reduction
 174 operation in a <a class="reference external" href="distributed_property_map.html">distributed property map</a> and verifying that
 175 results written are &quot;better&quot; that what was previously written.</li>
 176 </ol>
 177 </div>
 178 </div>
 179 <div class="section" id="bibliography">
 180 <h1>Bibliography</h1>
 181 <table class="docutils citation" frame="void" id="cidon88" rules="none">
 182 <colgroup><col class="label" /><col /></colgroup>
 183 <tbody valign="top">
 184 <tr><td class="label"><a class="fn-backref" href="#id2">[Cidon88]</a></td><td>Isreal Cidon. Yet another distributed depth-first-search
 185 algorithm. Information Processing Letters, 26(6):301--305, 1988.</td></tr>
 186 </tbody>
 187 </table>
 188 <table class="docutils citation" frame="void" id="tsin02" rules="none">
 189 <colgroup><col class="label" /><col /></colgroup>
 190 <tbody valign="top">
 191 <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
 192 search. Information Processing Letters, 82(4):173--178, 2002.</td></tr>
 193 </tbody>
 194 </table>
 195 <hr class="docutils" />
 196 <p>Copyright (C) 2005 The Trustees of Indiana University.</p>
 197 <p>Authors: Douglas Gregor and Andrew Lumsdaine</p>
 198 </div>
 199 </div>
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