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  <div class="section" id="background">
<span id="background"></span><h1>Background<a class="headerlink" href="#background" title="永久链接至标题">¶</a></h1>
<p>PaddlePaddle divides the description of neural network computation into two stages: compile time and runtime. At compile time, the neural network computation is described as a <code class="docutils literal"><span class="pre">ProgramDesc</span></code> whereas at runtime an <code class="docutils literal"><span class="pre">Executor</span></code> interprets the <code class="docutils literal"><span class="pre">ProgramDesc</span></code> to compute the operations.</p>
<p>PaddlePaddle use proto message to describe compile time program because</p>
<ol class="simple">
<li>The computation program description must be serializable and saved in a file.</li>
<li>During distributed training, the sreialized program will be sent to multiple workers. It should also be possible to break the program into different components, each of which can be executed on different workers.</li>
</ol>
<p>The computation <code class="docutils literal"><span class="pre">Program</span></code> consists of nested <code class="docutils literal"><span class="pre">Blocks</span></code>. Each <code class="docutils literal"><span class="pre">Block</span></code> will consist of data(i.e. <code class="docutils literal"><span class="pre">Variable</span></code>)  and  <code class="docutils literal"><span class="pre">Operations</span></code>. The concept to represent them is in the table below.</p>
<p>| |compile time|runtime|
|&#8212;|&#8212;|&#8212;|
|Data|VarDesc(proto)|Variable(cpp)|
|Operation|OpDesc(proto)|Operator(cpp)|</p>
</div>
<div class="section" id="definition-of-vardesc">
<span id="definition-of-vardesc"></span><h1>Definition of VarDesc<a class="headerlink" href="#definition-of-vardesc" title="永久链接至标题">¶</a></h1>
<p>A VarDesc should have a name, and value. The are two kinds of variable type in compile time, they are <code class="docutils literal"><span class="pre">LoDTensor</span></code> and <code class="docutils literal"><span class="pre">SelectedRows</span></code>.</p>
<div class="highlight-proto"><div class="highlight"><pre><span></span><span class="kd">message</span> <span class="nc">VarDesc</span> <span class="p">{</span>
  <span class="k">required</span> <span class="kt">string</span> <span class="na">name</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
  <span class="kd">enum</span> <span class="n">VarType</span> <span class="p">{</span>
    <span class="na">LOD_TENSOR</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
    <span class="na">SELECTED_ROWS</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
  <span class="p">}</span>
  <span class="k">required</span> <span class="n">VarType</span> <span class="na">type</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span>
  <span class="k">optional</span> <span class="n">LoDTensorDesc</span> <span class="na">lod_desc</span> <span class="o">=</span> <span class="mi">3</span><span class="p">;</span>
  <span class="k">optional</span> <span class="n">TensorDesc</span> <span class="na">selected_rows_desc</span> <span class="o">=</span> <span class="mi">4</span><span class="p">;</span>
  <span class="k">optional</span> <span class="kt">bool</span> <span class="na">persistable</span> <span class="o">=</span> <span class="mi">5</span> <span class="p">[</span> <span class="k">default</span> <span class="o">=</span> <span class="kc">false</span> <span class="p">];</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="definition-of-tensordesc">
<span id="definition-of-tensordesc"></span><h1>Definition of TensorDesc<a class="headerlink" href="#definition-of-tensordesc" title="永久链接至标题">¶</a></h1>
<div class="highlight-proto"><div class="highlight"><pre><span></span><span class="kd">enum</span> <span class="n">DataType</span> <span class="p">{</span>
  <span class="na">BOOL</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span>
  <span class="na">INT16</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
  <span class="na">INT32</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span>
  <span class="na">INT64</span> <span class="o">=</span> <span class="mi">3</span><span class="p">;</span>
  <span class="na">FP16</span> <span class="o">=</span> <span class="mi">4</span><span class="p">;</span>
  <span class="na">FP32</span> <span class="o">=</span> <span class="mi">5</span><span class="p">;</span>
  <span class="na">FP64</span> <span class="o">=</span> <span class="mi">6</span><span class="p">;</span>
<span class="p">}</span>

<span class="kd">message</span> <span class="nc">TensorDesc</span> <span class="p">{</span>
  <span class="k">required</span> <span class="n">DataType</span> <span class="na">data_type</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
  <span class="k">repeated</span> <span class="kt">int64</span> <span class="na">dims</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span> <span class="c1">// [UNK, 640, 480] is saved as [-1, 640, 480]</span>
<span class="p">}</span>
</pre></div>
</div>
<p>A TensorDesc describes <code class="docutils literal"><span class="pre">SelectedRows</span></code> and <code class="docutils literal"><span class="pre">LoDTensor</span></code>. For details of <code class="docutils literal"><span class="pre">SelectedRows</span></code>, please reference <a class="reference external" href="./selected_rows.md"><code class="docutils literal"><span class="pre">SelectedRows</span></code></a>.</p>
</div>
<div class="section" id="definition-of-lodtensordesc">
<span id="definition-of-lodtensordesc"></span><h1>Definition of LodTensorDesc<a class="headerlink" href="#definition-of-lodtensordesc" title="永久链接至标题">¶</a></h1>
<div class="highlight-proto"><div class="highlight"><pre><span></span><span class="kd">message</span> <span class="nc">LoDTensorDesc</span> <span class="p">{</span>
  <span class="k">required</span> <span class="n">TensorDesc</span> <span class="na">tensor</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
  <span class="k">optional</span> <span class="n">int</span> <span class="na">lod_level</span> <span class="o">=</span> <span class="mi">2</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>A LoDTensorDesc contains a tensor and a lod_level.</p>
</div>
<div class="section" id="definition-of-variable-in-python">
<span id="definition-of-variable-in-python"></span><h1>Definition of Variable in Python<a class="headerlink" href="#definition-of-variable-in-python" title="永久链接至标题">¶</a></h1>
<p>For Variable in Python, please reference <a class="reference external" href="./python_api.md"><code class="docutils literal"><span class="pre">Python</span> <span class="pre">API</span></code></a>.</p>
</div>


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