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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>In <a class="reference external" href="https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/fluid.md">fluid</a>, we encourage the user to use deep learning programming paradigms to describe the training process. When the user-written Python program is executed, it will first create a protobuf message
<a class="reference external" href="https://github.com/PaddlePaddle/Paddle/blob/a91efdde6910ce92a78e3aa7157412c4c88d9ee8/paddle/framework/framework.proto#L145"><code class="docutils literal"><span class="pre">ProgramDesc</span></code></a> that describes the process and is conceptually like an <a class="reference external" href="https://en.wikipedia.org/wiki/Abstract_syntax_tree">abstract syntax tree</a>.</p>
<p>The executor runs the <code class="docutils literal"><span class="pre">ProgramDesc</span></code> like an interpreter. <code class="docutils literal"><span class="pre">ProgramDesc</span></code> contains the intrinsics (operators in this case) and variables which will be used, executor explicitly executes the stored precompiled 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 in the block. 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 instances, which is persistent throughout different runs.</p>
</div>
<div class="section" id="executor">
<span id="executor"></span><h2>Executor<a class="headerlink" href="#executor" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal"><span class="pre">Executor</span></code> explicitly executes all the intrinsics (operators here) 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>. Essentially, it instantiates Variables and Operators, then runs all the operators in sequence one-by-one.
It is very similar to how a push stack frame works when entering a block, following which it cleans up all the temporary variables when a mini-batch is finished. It does not however, have the stack frame pop process.</p>
<div class="section" id="the-interface">
<span id="the-interface"></span><h3>The interface<a class="headerlink" href="#the-interface" title="Permalink to this headline">¶</a></h3>
<div class="highlight-c++"><div class="highlight"><pre><span></span>  <span class="n">Executor</span><span class="p">(</span><span class="n">places</span><span class="p">);</span>
</pre></div>
</div>
<p>A executor does not own any computing resources, a user can only construct an executor using the specified places.</p>
</div>
<div class="section" id="running-an-executor">
<span id="running-an-executor"></span><h3>Running an Executor<a class="headerlink" href="#running-an-executor" title="Permalink to this headline">¶</a></h3>
<div class="highlight-default"><div class="highlight"><pre><span></span>  <span class="n">void</span> <span class="n">Run</span><span class="p">(</span><span class="n">ProgramDesc</span><span class="p">,</span> <span class="n">Scope</span><span class="p">,</span> <span class="n">block_id</span><span class="p">,</span> <span class="n">create_local_scope</span><span class="p">);</span>
</pre></div>
</div>
<p>An <code class="docutils literal"><span class="pre">Executor</span></code> only provides a unified way to execute <code class="docutils literal"><span class="pre">ProgramDesc</span></code>. <code class="docutils literal"><span class="pre">ProgramDesc</span></code> is the target that will be executed, the <code class="docutils literal"><span class="pre">Scope</span></code> specifies the variable container, the <code class="docutils literal"><span class="pre">block_id</span></code> indicates the entrance block and <code class="docutils literal"><span class="pre">create_local_scope</span></code> is a boolean that states whether it will destroy the temporary variables after the execution is finished.</p>
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