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  <div class="section" id="executor-design-doc">
<span id="executor-design-doc"></span><h1>Executor Design Doc<a class="headerlink" href="#executor-design-doc" title="Permalink to this headline">¶</a></h1>
<div class="section" id="motivation">
<span id="motivation"></span><h2>Motivation<a class="headerlink" href="#motivation" title="Permalink to this headline">¶</a></h2>
<p>We use executor to do the runtime evaluation of a <code class="docutils literal"><span class="pre">ProgramDesc</span></code>.</p>
</div>
<div class="section" id="overview">
<span id="overview"></span><h2>Overview<a class="headerlink" href="#overview" title="Permalink to this headline">¶</a></h2>
<p>An executor takes a <code class="docutils literal"><span class="pre">ProgramDesc</span></code>, a <code class="docutils literal"><span class="pre">block_id</span></code> and a <code class="docutils literal"><span class="pre">Scope</span></code>.  The <code class="docutils literal"><span class="pre">ProgramDesc</span></code> is a list of blocks and each block contains the protobuf definition of all the parameters and operators. The <code class="docutils literal"><span class="pre">block_id</span></code> specifies the entrance block. And the <code class="docutils literal"><span class="pre">Scope</span></code> is the container of all the variable instance, which is persistent throughout different runs.</p>
<div class="section" id="what-does-executor-do">
<span id="what-does-executor-do"></span><h3>What does executor do?<a class="headerlink" href="#what-does-executor-do" title="Permalink to this headline">¶</a></h3>
<p>It evaluates all the operators in the <code class="docutils literal"><span class="pre">block_id</span></code>th block of a <code class="docutils literal"><span class="pre">ProgramDesc</span></code>.</p>
</div>
<div class="section" id="what-does-executor-not-do">
<span id="what-does-executor-not-do"></span><h3>What does executor NOT do?<a class="headerlink" href="#what-does-executor-not-do" title="Permalink to this headline">¶</a></h3>
<p>It does not do runtime optimization, meaning intelligently parse the dependency of each op a choose which one to be run and in which order they should be run.</p>
<p>It does not do graph partitioning, meaning dividing the <code class="docutils literal"><span class="pre">ProgramDesc</span></code> into several small pieces and executing them on different devices.</p>
</div>
</div>
<div class="section" id="implementation">
<span id="implementation"></span><h2>Implementation<a class="headerlink" href="#implementation" title="Permalink to this headline">¶</a></h2>
<p><code class="docutils literal"><span class="pre">Executor</span></code> evaluates a <code class="docutils literal"><span class="pre">ProgramDesc</span></code>. Essentially, it instantiates Variables and Operators, then run all the operators in sequence. <a class="reference external" href="https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/executor.cc">[code]</a></p>
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