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<title>Taskflow Algorithms &raquo; Data-parallel Pipeline | Taskflow QuickStart</title>
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<h1>
<span class="m-breadcrumb"><a href="Algorithms.html">Taskflow Algorithms</a> &raquo;</span>
Data-parallel Pipeline
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<h3>Contents</h3>
<ul>
<li><a href="#ParallelDataPipelineIncludeHeaderFile">Include the Header</a></li>
<li><a href="#CreateADataPipelineModuleTask">Create a Data Pipeline Module Task</a></li>
<li><a href="#UnderstandInternalDataStorage">Understand Internal Data Storage</a></li>
<li><a href="#DataParallelPipelineLearnMore">Learn More about Taskflow Pipeline</a></li>
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<p>Taskflow provides another variant, <a href="classtf_1_1DataPipeline.html" class="m-doc">tf::<wbr />DataPipeline</a>, on top of <a href="classtf_1_1Pipeline.html" class="m-doc">tf::<wbr />Pipeline</a> (see <a href="TaskParallelPipeline.html" class="m-doc">Task-parallel Pipeline</a>) to help you implement data-parallel pipeline algorithms while leaving data management to Taskflow. We recommend you finishing reading TaskParallelPipeline first before learning <a href="classtf_1_1DataPipeline.html" class="m-doc">tf::<wbr />DataPipeline</a>.</p><section id="ParallelDataPipelineIncludeHeaderFile"><h2><a href="#ParallelDataPipelineIncludeHeaderFile">Include the Header</a></h2><p>You need to include the header file, <code>taskflow/algorithm/data_pipeline.hpp</code>, for implementing data-parallel pipeline algorithms.</p><pre class="m-code"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/algorithm/data_pipeline.hpp&gt;</span></pre></section><section id="CreateADataPipelineModuleTask"><h2><a href="#CreateADataPipelineModuleTask">Create a Data Pipeline Module Task</a></h2><p>Similar to creating a task-parallel pipeline (<a href="classtf_1_1Pipeline.html" class="m-doc">tf::<wbr />Pipeline</a>), there are three steps to create a data-parallel pipeline application:</p><ol><li>Define the pipeline structure (e.g., pipe type, pipe callable, stopping rule, line count)</li><li>Define the data storage and layout, if needed for the application</li><li>Define the pipeline taskflow graph using composition</li></ol><p>The following example creates a data-parallel pipeline that generates a total of five dataflow tokens from <code>void</code> to <code>int</code> at the first stage, from <code>int</code> to <code>std::string</code> at the second stage, and <code>std::string</code> to <code>void</code> at the final stage. Data storage between stages is automatically managed by <a href="classtf_1_1DataPipeline.html" class="m-doc">tf::<wbr />DataPipeline</a>.</p><pre class="m-code"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/taskflow.hpp&gt;</span>
<span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/algorithm/data_pipeline.hpp&gt;</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// data flow =&gt; void -&gt; int -&gt; std::string -&gt; void </span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">(</span><span class="s">&quot;pipeline&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">num_lines</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">4</span><span class="p">;</span>
<span class="w"> </span>
<span class="w"> </span><span class="c1">// create a pipeline graph</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">DataPipeline</span><span class="w"> </span><span class="n">pl</span><span class="p">(</span><span class="n">num_lines</span><span class="p">,</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="o">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Pipeflow</span><span class="o">&amp;</span><span class="w"> </span><span class="n">pf</span><span class="p">)</span><span class="w"> </span><span class="o">-&gt;</span><span class="w"> </span><span class="kt">int</span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">pf</span><span class="p">.</span><span class="n">token</span><span class="p">()</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">5</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">stop</span><span class="p">();</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">&quot;first pipe returns %lu</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">token</span><span class="p">());</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">token</span><span class="p">();</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}),</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="o">&amp;</span><span class="w"> </span><span class="n">input</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">&quot;second pipe returns a string of %d</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">input</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">100</span><span class="p">);</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">to_string</span><span class="p">(</span><span class="n">input</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">100</span><span class="p">);</span>
<span class="w"> </span><span class="p">}),</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="p">,</span><span class="w"> </span><span class="kt">void</span><span class="o">&gt;</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span><span class="w"> </span><span class="n">input</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">&quot;third pipe receives the input string %s</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">input</span><span class="p">.</span><span class="n">c_str</span><span class="p">());</span>
<span class="w"> </span><span class="p">})</span>
<span class="w"> </span><span class="p">);</span>
<span class="w"> </span><span class="c1">// build the pipeline graph using composition</span>
<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">composed_of</span><span class="p">(</span><span class="n">pl</span><span class="p">).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;pipeline&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// dump the pipeline graph structure (with composition)</span>
<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">dump</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="p">);</span>
<span class="w"> </span><span class="c1">// run the pipeline</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span></pre><p>The interface of <a href="classtf_1_1DataPipeline.html" class="m-doc">tf::<wbr />DataPipeline</a> is very similar to <a href="classtf_1_1Pipeline.html" class="m-doc">tf::<wbr />Pipeline</a>, except that the library transparently manages the dataflow between pipes. To create a stage in a data-parallel pipeline, you should always use the helper function <a href="namespacetf.html#a8975fa5762088789adb0b60f38208309" class="m-doc">tf::<wbr />make_data_pipe</a>:</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">(</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span>
<span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="o">&amp;</span><span class="w"> </span><span class="n">input</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">to_string</span><span class="p">(</span><span class="n">input</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">100</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">);</span></pre><p>The helper function starts with a pair of an input and an output types in its template arguments. Both types will always be decayed to their original form using <a href="http://en.cppreference.com/w/cpp/types/decay.html" class="m-doc-external">std::<wbr />decay</a> (e.g., <code>const int&amp;</code> becomes <code>int</code>) for storage purpose. In terms of function arguments, the first argument specifies the direction of this data pipe, which can be either <a href="namespacetf.html#abb7a11e41fd457f69e7ff45d4c769564a7b804a28d6154ab8007287532037f1d0" class="m-doc">tf::<wbr />PipeType::<wbr />SERIAL</a> or <a href="namespacetf.html#abb7a11e41fd457f69e7ff45d4c769564adf13a99b035d6f0bce4f44ab18eec8eb" class="m-doc">tf::<wbr />PipeType::<wbr />PARALLEL</a>, and the second argument is a callable to invoke by the pipeline scheduler. The callable must take the input data type in its first argument and returns a value of the output data type. Additionally, the callable can take a <a href="classtf_1_1Pipeflow.html" class="m-doc">tf::<wbr />Pipeflow</a> reference in its second argument which allows you to query the runtime information of a stage task, such as its line number and token number.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&gt;</span><span class="p">(</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span>
<span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="o">&amp;</span><span class="w"> </span><span class="n">input</span><span class="p">,</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Pipeflow</span><span class="o">&amp;</span><span class="w"> </span><span class="n">pf</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">&quot;token=%lu, line=%lu</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">,</span><span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">token</span><span class="p">(),</span><span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">line</span><span class="p">());</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">to_string</span><span class="p">(</span><span class="n">input</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="mi">100</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">)</span></pre><aside class="m-note m-info"><h4>Note</h4><p>By default, <a href="classtf_1_1DataPipeline.html" class="m-doc">tf::<wbr />DataPipeline</a> passes the data in reference to your callable at which you can take it in copy or in reference depending on application needs.</p></aside><p>For the first pipe, the input type should always be <code>void</code> and the callable must take a <a href="classtf_1_1Pipeflow.html" class="m-doc">tf::<wbr />Pipeflow</a> reference in its argument. In this example, we will stop the pipeline when processing five tokens.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="kt">void</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="n">tf</span><span class="o">::</span><span class="n">Pipeflow</span><span class="o">&amp;</span><span class="w"> </span><span class="n">pf</span><span class="p">)</span><span class="w"> </span><span class="o">-&gt;</span><span class="w"> </span><span class="kt">int</span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">pf</span><span class="p">.</span><span class="n">token</span><span class="p">()</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">5</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">stop</span><span class="p">();</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="c1">// returns a dummy value</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">pf</span><span class="p">.</span><span class="n">token</span><span class="p">();</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}),</span></pre><p>Similarly, the output type of the last pipe should be <code>void</code> as no more data will go out of the final pipe.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">make_data_pipe</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="p">,</span><span class="w"> </span><span class="kt">void</span><span class="o">&gt;</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">PipeType</span><span class="o">::</span><span class="n">SERIAL</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span><span class="w"> </span><span class="n">input</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">input</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">})</span></pre><p>Finally, you need to compose the pipeline graph by creating a module task (i.e., tf::Taskflow::compoased_of).</p><pre class="m-code"><span class="c1">// build the pipeline graph using composition</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">composed_of</span><span class="p">(</span><span class="n">pl</span><span class="p">).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;pipeline&quot;</span><span class="p">);</span>
<span class="c1">// dump the pipeline graph structure (with composition)</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">dump</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="p">);</span>
<span class="c1">// run the pipeline</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span></pre><p><br /></p><div class="m-graph"><svg style="width: 41.600rem; height: 17.800rem;" viewBox="0.00 0.00 416.00 178.00">
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</div></section><section id="UnderstandInternalDataStorage"><h2><a href="#UnderstandInternalDataStorage">Understand Internal Data Storage</a></h2><p>By default, <a href="classtf_1_1DataPipeline.html" class="m-doc">tf::<wbr />DataPipeline</a> uses <a href="https://en.cppreference.com/w/cpp/utility/variant">std::<wbr />variant</a> to store a type-safe union of all input and output data types extracted from the given data pipes. To avoid false sharing, each line keeps a variant that is aligned with the cacheline size. When invoking a pipe callable, the input data is acquired in reference from the variant using <a href="https://en.cppreference.com/w/cpp/utility/variant/get">std::<wbr />get</a>. When returning from a pipe callable, the output data is stored back to the variant using assignment operator.</p></section><section id="DataParallelPipelineLearnMore"><h2><a href="#DataParallelPipelineLearnMore">Learn More about Taskflow Pipeline</a></h2><p>Visit the following pages to learn more about pipeline:</p><ol><li><a href="TaskParallelPipeline.html" class="m-doc">Task-parallel Pipeline</a></li><li><a href="TaskParallelScalablePipeline.html" class="m-doc">Task-parallel Scalable Pipeline</a></li><li><a href="TextProcessingPipeline.html" class="m-doc">Text Processing Pipeline</a></li><li><a href="GraphProcessingPipeline.html" class="m-doc">Graph Processing Pipeline</a></li><li><a href="TaskflowProcessingPipeline.html" class="m-doc">Taskflow Processing Pipeline</a></li></ol></section>
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