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

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   7 <title>Parallel BGL Distributed queue adaptor</title>
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  11 <div class="document" id="logo-distributed-queue-adaptor">
  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> Distributed queue adaptor</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 ProcessGroup, typename Buffer&gt;
  20 class distributed_queue
  21 {
  22 public:
  23   typedef ProcessGroup                     process_group_type;
  24   typedef Buffer                           buffer_type;
  25   typedef typename buffer_type::value_type value_type;
  26   typedef typename buffer_type::size_type  size_type;
  27
  28   explicit
  29   distributed_queue(const ProcessGroup&amp; process_group = ProcessGroup(),
  30                     const Buffer&amp; buffer = Buffer(),
  31                     bool polling = false);
  32
  33   distributed_queue(const ProcessGroup&amp; process_group, bool polling);
  34
  35   void push(const value_type&amp; x);
  36   void pop();
  37   value_type&amp; top();
  38   const value_type&amp; top() const;
  39   bool empty() const;
  40   size_type size() const;
  41 };
  42
  43 template&lt;typename ProcessGroup, typename Buffer&gt;
  44 inline distributed_queue&lt;ProcessGroup, Buffer&gt;
  45 make_distributed_queue(const ProcessGroup&amp; process_group, const Buffer&amp; buffer,
  46                        bool polling = false);
  47 </pre>
  48 <p>Class template <tt class="docutils literal"><span class="pre">distributed_queue</span></tt> implements a distributed queue
  49 across a process group. The distributed queue is an adaptor over an
  50 existing (local) queue, which must model the <a class="reference external" href="http://www.boost.org/libs/graph/doc/Buffer.html">Buffer</a> concept. Each
  51 process stores a distinct copy of the local queue, from which it draws
  52 or removes elements via the <tt class="docutils literal"><span class="pre">pop</span></tt> and <tt class="docutils literal"><span class="pre">top</span></tt> members.</p>
  53 <p>The value type of the local queue must be a model of the
  54 <a class="reference external" href="GlobalDescriptor.html">Global Descriptor</a> concept. The <tt class="docutils literal"><span class="pre">push</span></tt> operation of the
  55 distributed queue passes (via a message) the value to its owning
  56 processor. Thus, the elements within a particular local queue are
  57 guaranteed to have the process owning that local queue as an owner.</p>
  58 <p>Synchronization of distributed queues occurs in the <tt class="docutils literal"><span class="pre">empty</span></tt> and
  59 <tt class="docutils literal"><span class="pre">size</span></tt> functions, which will only return &quot;empty&quot; values (true or 0,
  60 respectively) when the entire distributed queue is empty. If the local
  61 queue is empty but the distributed queue is not, the operation will
  62 block until either condition changes. When the <tt class="docutils literal"><span class="pre">size</span></tt> function of a
  63 nonempty queue returns, it returns the size of the local queue. These
  64 semantics were selected so that sequential code that processes
  65 elements in the queue via the following idiom can be parallelized via
  66 introduction of a distributed queue:</p>
  67 <pre class="literal-block">
  68 distributed_queue&lt;...&gt; Q;
  69 Q.push(x);
  70 while (!Q.empty()) {
  71   // do something, that may push a value onto Q
  72 }
  73 </pre>
  74 <p>In the parallel version, the initial <tt class="docutils literal"><span class="pre">push</span></tt> operation will place
  75 the value <tt class="docutils literal"><span class="pre">x</span></tt> onto its owner's queue. All processes will
  76 synchronize at the call to empty, and only the process owning <tt class="docutils literal"><span class="pre">x</span></tt>
  77 will be allowed to execute the loop (<tt class="docutils literal"><span class="pre">Q.empty()</span></tt> returns
  78 false). This iteration may in turn push values onto other remote
  79 queues, so when that process finishes execution of the loop body
  80 and all processes synchronize again in <tt class="docutils literal"><span class="pre">empty</span></tt>, more processes
  81 may have nonempty local queues to execute. Once all local queues
  82 are empty, <tt class="docutils literal"><span class="pre">Q.empty()</span></tt> returns <tt class="docutils literal"><span class="pre">false</span></tt> for all processes.</p>
  83 <p>The distributed queue can receive messages at two different times:
  84 during synchronization and when polling <tt class="docutils literal"><span class="pre">empty</span></tt>. Messages are
  85 always received during synchronization, to ensure that accurate
  86 local queue sizes can be determines. However, whether <tt class="docutils literal"><span class="pre">empty</span></tt>
  87 should poll for messages is specified as an option to the
  88 constructor. Polling may be desired when the order in which
  89 elements in the queue are processed is not important, because it
  90 permits fewer synchronization steps and less communication
  91 overhead. However, when more strict ordering guarantees are
  92 required, polling may be semantically incorrect. By disabling
  93 polling, one ensures that parallel execution using the idiom above
  94 will not process an element at a later &quot;level&quot; before an earlier
  95 &quot;level&quot;.</p>
  96 <p>The distributed queue nearly models the <a class="reference external" href="http://www.boost.org/libs/graph/doc/Buffer.html">Buffer</a>
  97 concept. However, the <tt class="docutils literal"><span class="pre">push</span></tt> routine does not necessarily
  98 increase the result of <tt class="docutils literal"><span class="pre">size()</span></tt> by one (although the size of the
  99 global queue does increase by one).</p>
 100 <div class="section" id="member-functions">
 101 <h1>Member Functions</h1>
 102 <pre class="literal-block">
 103 explicit
 104 distributed_queue(const ProcessGroup&amp; process_group = ProcessGroup(),
 105                   const Buffer&amp; buffer = Buffer(),
 106                   bool polling = false);
 107 </pre>
 108 <p>Build a new distributed queue that communicates over the given
 109 <tt class="docutils literal"><span class="pre">process_group</span></tt>, whose local queue is initialized via <tt class="docutils literal"><span class="pre">buffer</span></tt> and
 110 which may or may not poll for messages.</p>
 111 <hr class="docutils" />
 112 <pre class="literal-block">
 113 distributed_queue(const ProcessGroup&amp; process_group, bool polling);
 114 </pre>
 115 <p>Build a new distributed queue that communicates over the given
 116 <tt class="docutils literal"><span class="pre">process_group</span></tt>, whose local queue is default-initalized and which
 117 may or may not poll for messages.</p>
 118 <hr class="docutils" />
 119 <pre class="literal-block">
 120 void push(const value_type&amp; x);
 121 </pre>
 122 <p>Push an element onto the distributed queue.</p>
 123 <p>The element will be sent to its owner process to be added to that
 124 process's local queue. If polling is enabled for this queue and
 125 the owner process is the current process, the value will be
 126 immediately pushed onto the local queue.</p>
 127 <p>Complexity: O(1) messages of size O(<tt class="docutils literal"><span class="pre">sizeof(value_type)</span></tt>) will be
 128 transmitted.</p>
 129 <hr class="docutils" />
 130 <pre class="literal-block">
 131 void pop();
 132 </pre>
 133 <p>Pop an element off the local queue. The queue must not be <tt class="docutils literal"><span class="pre">empty()</span></tt>.</p>
 134 <hr class="docutils" />
 135 <pre class="literal-block">
 136 value_type&amp; top();
 137 const value_type&amp; top();
 138 </pre>
 139 <p>Returns the top element in the local queue. The queue must not be
 140 <tt class="docutils literal"><span class="pre">empty()</span></tt>.</p>
 141 <hr class="docutils" />
 142 <pre class="literal-block">
 143 bool empty() const;
 144 </pre>
 145 <p>Determines if the queue is empty.</p>
 146 <p>When the local queue is nonempty, returns true. If the local queue is
 147 empty, synchronizes with all other processes in the process group
 148 until either (1) the local queue is nonempty (returns true) (2) the
 149 entire distributed queue is empty (returns false).</p>
 150 <hr class="docutils" />
 151 <pre class="literal-block">
 152 size_type size() const;
 153 </pre>
 154 <p>Determines the size of the local queue.</p>
 155 <p>The behavior of this routine is equivalent to the behavior of
 156 <tt class="docutils literal"><span class="pre">empty</span></tt>, except that when <tt class="docutils literal"><span class="pre">empty</span></tt> returns true this
 157 function returns the size of the local queue and when <tt class="docutils literal"><span class="pre">empty</span></tt>
 158 returns false this function returns zero.</p>
 159 </div>
 160 <div class="section" id="free-functions">
 161 <h1>Free Functions</h1>
 162 <pre class="literal-block">
 163 template&lt;typename ProcessGroup, typename Buffer&gt;
 164 inline distributed_queue&lt;ProcessGroup, Buffer&gt;
 165 make_distributed_queue(const ProcessGroup&amp; process_group, const Buffer&amp; buffer,
 166                        bool polling = false);
 167 </pre>
 168 <p>Constructs a distributed queue.</p>
 169 <hr class="docutils" />
 170 <p>Copyright (C) 2004, 2005 The Trustees of Indiana University.</p>
 171 <p>Authors: Douglas Gregor and Andrew Lumsdaine</p>
 172 </div>
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