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TaskBasedParallelism Task-based Parallelism</h1>

<p>The traditional loop-level parallelism is simple but hardly allows users to exploit parallelism in more irregular applications such as graph algorithms, incremental flows, recursion, and dynamically-allocated data structures. To address these challenges, parallel programming and libraries are evolving from the tradition loop-based parallelism to the <em>task-based</em> model.</p>

<div class="image">

<p>The above figure shows an example <em>task dependency graph</em>. Each node in the graph represents a task unit at function level and each edge indicates the task dependency between a pair of tasks. Task-based model offers a powerful means to express both regular and irregular parallelism in a top-down manner, and provides transparent scaling to large number of cores. In fact, it has been proven, both by the research community and the evolution of parallel programming standards, task-based approach scales the best with future processor generations and architectures.</p>

<h1><a class="anchor" id="ChallengesOfTaskBasedParallelProgramming"></a>

<div class="header">

<div class="headertitle">

<div class="contents">

<div class="textblock"><p>In many situations, you will need to execute a task dependency multiple times to create a <em>reusable</em> and <em>stateful</em> execution. The <em>framework</em> - <a class="el" href="classtf_1_1Framework.html" title="A reusable task dependency graph. ">tf::Framework</a> is designed for this purpose. This chapter introduces the concept of a framework and shows its usage.</p>

<h1><a class="anchor" id="C7_CreateAFramework"></a>

Create a Framework</h1>

Execute a Framework</h1>

<p>To execute a framework, you use one of following methods: <a class="el" href="classtf_1_1BasicTaskflow.html#afc7f9ac974ee803b2d3b5577a6c8a2bc" title="runs the framework once ">tf::Taskflow::run</a>, tf::Taskflow::silent_run, <a class="el" href="classtf_1_1BasicTaskflow.html#abe9f7964777c2d65052b2dda7307efb6" title="runs the framework for N times ">tf::Taskflow::run_n</a> or tf::Taskflow::silent_run_n. The methods <a class="el" href="classtf_1_1BasicTaskflow.html#afc7f9ac974ee803b2d3b5577a6c8a2bc" title="runs the framework once ">tf::Taskflow::run</a> and tf::Taskflow::silent_run execute the framework only once. The methods <a class="el" href="classtf_1_1BasicTaskflow.html#abe9f7964777c2d65052b2dda7307efb6" title="runs the framework for N times ">tf::Taskflow::run_n</a> and tf::Taskflow::silent_run_n execute the framework for <code>N</code> times. The prefix <code>silent_</code> indicates a void return while others will return a <a class="elRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/shared_future.html">std::shared_future</a> object for users to access the execution status. All methods accept an optional callback to invoke after the execution completes. The code below shows how to run a framework.</p>

<div class="fragment"><div class="line"> 1: <span class="comment">// Declare a framework </span></div><div class="line"> 2: <a class="code" href="classtf_1_1Framework.html">tf::Framework</a> f;</div><div class="line"> 3: </div><div class="line"> 4: <span class="comment">// Add three tasks into the framework</span></div><div class="line"> 5: <span class="keyword">auto</span> [A, B, C] = f.<a class="code" href="classtf_1_1FlowBuilder.html#a7285613836c840e22b8511d447734c87">silent_emplace</a>(</div><div class="line"> 6: [] () { <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> &lt;&lt; <span class="stringliteral">&quot;This is TaskA\n&quot;</span>; },</div><div class="line"> 7: [] () { <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> &lt;&lt; <span class="stringliteral">&quot;This is TaskB\n&quot;</span>; },</div><div class="line"> 8: [] () { <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> &lt;&lt; <span class="stringliteral">&quot;This is TaskC\n&quot;</span>; },</div><div class="line"> 9: );</div><div class="line">10: </div><div class="line">11: <span class="comment">// Build precedence between tasks</span></div><div class="line">12: A.precede(B, C); </div><div class="line">13: </div><div class="line">14: <span class="comment">// Declare a taskflow object</span></div><div class="line">15: <a class="code" href="classtf_1_1BasicTaskflow.html">tf::Taskflow</a> taskflow;</div><div class="line">16: </div><div class="line">17: <span class="keyword">auto</span> fu = taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#afc7f9ac974ee803b2d3b5577a6c8a2bc">run</a>(f);</div><div class="line">18: fu.get();</div><div class="line">19: taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#afc7f9ac974ee803b2d3b5577a6c8a2bc">run</a>(f, [](){ <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> &lt;&lt; <span class="stringliteral">&quot;end of one execution\n&quot;</span>; }).<span class="keyword">get</span>();</div><div class="line">20: </div><div class="line">21: taskflow.silent_run_n(f, 4);</div><div class="line">22: taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#a37ef86998f23ee7315be032c40fe815e">wait_for_all</a>();</div><div class="line">23: </div><div class="line">24: taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#abe9f7964777c2d65052b2dda7307efb6">run_n</a>(f, 4, [](){ <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> &lt;&lt; <span class="stringliteral">&quot;end of four executions\n&quot;</span>; }).<span class="keyword">get</span>();</div><div class="line">25:</div></div><!-- fragment --><p>Debrief:</p>