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<div class="title">C4: Conditional Tasking </div> </div>
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<div class="textblock"><p>Real parallel workloads often require dynamic flow controls. Cpp-Taskflow supports an very efficient interface of conditional tasking for users to implement dynamic and cyclic control flows that are otherwise difficult to do with existing task programming frameworks.</p>
<h1><a class="anchor" id="C4_CreateAConditionTask"></a>
Create a Condition Task</h1>
<p>A condition task evalutes a set of instructions and returns an integer index of the next immediate successor to execute. The index is defined with respect to the order of its successor construction.</p>
<div class="fragment"><div class="line"> 1: <a class="code" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"> 2: </div><div class="line"> 3: <a class="code" href="classtf_1_1Task.html">tf::Task</a> init = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"init"</span>);</div><div class="line"> 4: <a class="code" href="classtf_1_1Task.html">tf::Task</a> stop = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"stop"</span>);</div><div class="line"> 5:</div><div class="line"> 6: <span class="comment">// creates a condition task that returns 0 or 1</span></div><div class="line"> 7: <a class="code" href="classtf_1_1Task.html">tf::Task</a> cond = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){</div><div class="line"> 8: <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"flipping a coin\n"</span>;</div><div class="line"> 9: <span class="keywordflow">return</span> <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/numeric/random/rand.html">std::rand</a>() % 2;</div><div class="line">10: }).name(<span class="stringliteral">"cond"</span>);</div><div class="line">11:</div><div class="line">12: <span class="comment">// creates a feedback loop</span></div><div class="line">13: init.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(cond);</div><div class="line">14: cond.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(cond, stop); <span class="comment">// cond--0-->cond, cond--1-->stop</span></div><div class="line">15:</div><div class="line">16: executor.<a class="code" href="classtf_1_1Executor.html#a81f35d5b0a20ac0646447eb80d97c0aa">run</a>(taskflow).wait();</div></div><!-- fragment --><div class="image">
<object type="image/svg+xml" data="conditional-tasking-1.svg" width="45%">conditional-tasking-1.svg</object>
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<p>The example above implements a simple yet commonly used feedback loop through a condition task (line 7-10) that returns a random binary value. If the return value from <code>cond</code> is 0, it loops back to itself, or otherwise to <code>stop</code>. Our conditional tasking interface is very neat and expressive. A complex flow control often just takes a few lines of code to implement. The code below creates another taskflow with three condition tasks.</p>
<div class="fragment"><div class="line"> 1: <a class="code" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"> 2:</div><div class="line"> 3: <a class="code" href="classtf_1_1Task.html">tf::Task</a> A = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"A"</span>);</div><div class="line"> 4: <a class="code" href="classtf_1_1Task.html">tf::Task</a> B = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"B"</span>);</div><div class="line"> 5: <a class="code" href="classtf_1_1Task.html">tf::Task</a> C = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"C"</span>);</div><div class="line"> 6: <a class="code" href="classtf_1_1Task.html">tf::Task</a> D = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"D"</span>);</div><div class="line"> 7: <a class="code" href="classtf_1_1Task.html">tf::Task</a> E = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"E"</span>);</div><div class="line"> 8: <a class="code" href="classtf_1_1Task.html">tf::Task</a> F = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"F"</span>);</div><div class="line"> 9: <a class="code" href="classtf_1_1Task.html">tf::Task</a> G = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"G"</span>);</div><div class="line">10: <a class="code" href="classtf_1_1Task.html">tf::Task</a> H = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"H"</span>);</div><div class="line">11: <a class="code" href="classtf_1_1Task.html">tf::Task</a> I = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"I"</span>);</div><div class="line">12: <a class="code" href="classtf_1_1Task.html">tf::Task</a> K = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"K"</span>);</div><div class="line">13: <a class="code" href="classtf_1_1Task.html">tf::Task</a> L = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"L"</span>);</div><div class="line">14: <a class="code" href="classtf_1_1Task.html">tf::Task</a> M = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){}).name(<span class="stringliteral">"M"</span>);</div><div class="line">15: <a class="code" href="classtf_1_1Task.html">tf::Task</a> cond_1 = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){ <span class="keywordflow">return</span> <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/numeric/random/rand.html">std::rand</a>()%2; }).name(<span class="stringliteral">"cond_1"</span>);</div><div class="line">16: <a class="code" href="classtf_1_1Task.html">tf::Task</a> cond_2 = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){ <span class="keywordflow">return</span> <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/numeric/random/rand.html">std::rand</a>()%2; }).name(<span class="stringliteral">"cond_2"</span>);</div><div class="line">17: <a class="code" href="classtf_1_1Task.html">tf::Task</a> cond_3 = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){ <span class="keywordflow">return</span> <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/numeric/random/rand.html">std::rand</a>()%2; }).name(<span class="stringliteral">"cond_3"</span>);</div><div class="line">18:</div><div class="line">19: A.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(B, F);</div><div class="line">20: B.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(C);</div><div class="line">21: C.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(D);</div><div class="line">22: D.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(cond_1);</div><div class="line">23: E.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(K);</div><div class="line">24: F.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(cond_2);</div><div class="line">25: H.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(I);</div><div class="line">26: I.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(cond_3);</div><div class="line">27: L.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(M);</div><div class="line">28:</div><div class="line">29: cond_1.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(B, E);</div><div class="line">30: cond_2.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(G, H);</div><div class="line">31: cond_3.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(cond_3, L);</div><div class="line">32:</div><div class="line">33: taskflow.<a class="code" href="classtf_1_1Taskflow.html#ac433018262e44b12c4cc9f0c4748d758">dump</a>(<a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a>);</div></div><!-- fragment --><p>Debrief: </p><ul>
<li>Line 29 creates a condition task that loops back to B on return 0, and proceeds to E on return 1 </li>
<li>Line 30 creates a condition task that goes to G and H on return 0 and 1, respectively </li>
<li>Line 31 creates a condition task that loops back to itself on return 0, and proceeds to L on return 1</li>
</ul>
<div class="image">
<object type="image/svg+xml" data="conditional-tasking-2.svg" width="100%">conditional-tasking-2.svg</object>
</div>
<p>You can use condition tasks to create cycles as long as the graph does not introduce task race during execution. However, cycles are not allowed in non-condition tasks.</p>
<h1><a class="anchor" id="C4_StrongDependencyVSWeakDependency"></a>
Strong Dependency vs Weak Dependency</h1>
<p>In order to understand how an executor schedules condition tasks, we define two dependency types, <em>strong dependency</em> and <em>weak dependency</em>. A strong dependency is a preceding link from a non-condition task to another task. A weak dependency is a preceding link from a condition task to another task. The number of dependents of a task is the sum of strong dependency and weak dependency. The table below lists the strong dependency and weak dependency numbers of each task in the previous example.</p>
<div align="center"> <table class="markdownTable">
<tr class="markdownTableHead">
<th class="markdownTableHeadCenter">task </th><th class="markdownTableHeadCenter">strong dependency </th><th class="markdownTableHeadCenter">weak dependency </th><th class="markdownTableHeadNone">dependents </th></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">A </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">0 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">B </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyNone">2 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">C </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">D </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">E </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">F </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">G </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">H </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">I </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">K </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">L </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">M </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">cond_1 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowEven">
<td class="markdownTableBodyCenter">cond_2 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">0 </td><td class="markdownTableBodyNone">1 </td></tr>
<tr class="markdownTableBody" class="markdownTableRowOdd">
<td class="markdownTableBodyCenter">cond_3 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyCenter">1 </td><td class="markdownTableBodyNone">2 </td></tr>
</table>
</div><p>You can query the number of strong dependents, the number of weak dependents, and the number of dependents of a task.</p>
<div class="fragment"><div class="line">1: <a class="code" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div><div class="line">2: </div><div class="line">3: <a class="code" href="classtf_1_1Task.html">tf::Task</a> task = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a796e29175380f70246cf2a5639adc437">emplace</a>([](){});</div><div class="line">4: </div><div class="line">5: <span class="comment">// ... add more tasks and preceding links</span></div><div class="line">6:</div><div class="line">7: <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << task.<a class="code" href="classtf_1_1Task.html#a974dc1d738b62b829ad261beeafbd67c">num_dependents</a>() << <span class="charliteral">'\n'</span>;</div><div class="line">8: <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << task.<a class="code" href="classtf_1_1Task.html#ad49a92e8858c3c298bed0215e341b66b">num_strong_dependents</a>() << <span class="charliteral">'\n'</span>; </div><div class="line">9: <a class="codeRef" doxygen="/Users/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << task.<a class="code" href="classtf_1_1Task.html#af3bf886291af7f39957d43d17083fe07">num_weak_dependents</a>() << <span class="charliteral">'\n'</span>;</div></div><!-- fragment --><p>When you submit a task to an executor, the scheduler starts with tasks of <em>zero dependents</em> (both zero strong and weak dependencies) and continues to execute successive tasks whenever their <em>strong dependencies</em> are met. However, the scheduler skips this rule when executing a condition task and jumps directly to its successors indexed by the return value.</p>
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<p>Each task has an <em>atomic</em> join counter to keep track of strong dependents that are met at runtime. When a task completes, the join counter is restored to the task's strong dependency number in the graph, such that the subsequent execution can reuse the counter again.</p>
<h1><a class="anchor" id="C4_CommonPitfalls"></a>
Common Pitfalls</h1>
<p>Condition tasks are handy in creasing dynamic and cyclic control flows, but they are also easy to make mistakes. It is your responsibility to ensure a taskflow is properly conditioned. Top things to avoid include <em>no source tasks</em> to start with and <em>task race</em>. The figure below shows common pitfalls and their remedies.</p>
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<p>In the error1 scenario, there is no source task for the scheduler to start with, and the simplest fix is to add a task S that has no dependents. In the error2 scenario, D might be scheduled twice by E through the strong dependency and C through the weak dependency (on return 1). To fix this problem, you can add an auxiliary task D-aux to break the mixed use of strong dependency and weak dependency. In the risky scenario, task X may not be raced if P and M is exclusively branching to X.</p>
<p>A good practice for avoiding mistakes of conditional tasking is to infer the execution flow of your graphs based on our scheduling rules. Make sure there is no task race. </p>
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