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<!DOCTYPE html>

<head>

<title>Cookbook » Subflow Tasking | Taskflow QuickStart</title>

</head>

<body>

<a href="https://taskflow.github.io"><img src="taskflow_logo.png" alt="" />Taskflow</a> | <a href="index.html" class="m-thin">QuickStart</a>

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<h1>

<a href="Cookbook.html">Cookbook</a> »

Subflow Tasking

</h1>

<h3>Contents</h3>

<ul>

<li><a href="#CreateASubflow">Create a Subflow</a></li>

<li><a href="#RetainASubflow">Retain a Subflow</a></li>

<li><a href="#JoinASubflow">Join a Subflow Explicitly</a></li>

<li><a href="#CreateANestedSubflow">Create a Nested Subflow</a></li>

</ul>

</nav>

It is very common for a parallel program to spawn task dependency graphs at runtime. In Taskflow, we call this subflow tasking.<section id="CreateASubflow"><h2><a href="#CreateASubflow">Create a Subflow</a></h2>Subflow tasks are those created during the execution of a graph. These tasks are spawned from a parent task and are grouped together to a subflow dependency graph. To create a subflow, emplace a callable that takes an argument of type <a href="classtf_1_1Subflow.html" class="m-doc">tf::Subflow</a>. A <a href="classtf_1_1Subflow.html" class="m-doc">tf::Subflow</a> object will be created and forwarded to the execution context of the task. All methods you find in <a href="classtf_1_1Taskflow.html" class="m-doc">tf::Taskflow</a> are applicable for <a href="classtf_1_1Subflow.html" class="m-doc">tf::Subflow</a>.<pre class="m-code"> 1: tf::Taskflow taskflow;

2: tf::Executor executor;

3:

4: tf::Task A = taskflow.emplace([] () {}).name("A"); // static task A

5: tf::Task C = taskflow.emplace([] () {}).name("C"); // static task C

6: tf::Task D = taskflow.emplace([] () {}).name("D"); // static task D

7:

8: tf::Task B = taskflow.emplace([] (tf::Subflow& subflow) {

9: tf::Task B1 = subflow.emplace([] () {}).name("B1"); // subflow task B1

10: tf::Task B2 = subflow.emplace([] () {}).name("B2"); // subflow task B2

11: tf::Task B3 = subflow.emplace([] () {}).name("B3"); // subflow task B3

12: B1.precede(B3); // B1 runs before B3

13: B2.precede(B3); // B2 runs before B3

14: }).name("B");

15:

16: A.precede(B); // B runs after A

17: A.precede(C); // C runs after A

18: B.precede(D); // D runs after B

19: C.precede(D); // D runs after C

20:

21: executor.run(taskflow).get(); // execute the graph to spawn the subflow</pre><div><div class="m-graph"><svg style="width: 37.200rem; height: 22.800rem;" viewBox="0.00 0.00 372.00 228.00">

<title>Taskflow</title>

<title>cluster_p0x7ffee9781810</title>

<text text-anchor="middle" x="182" y="-200" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow</text>

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<title>cluster_p0x7f9866c01b70</title>

<text text-anchor="middle" x="141" y="-173" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: B</text>

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</div></div>Debrief:<ul><li>Lines 1-2 create a taskflow and an executor</li><li>Lines 4-6 create three tasks, A, C, and D</li><li>Lines 8-14 create a task B that spawns a task dependency graph of three tasks B1, B2, and B3</li><li>Lines 16-19 add dependencies among A, B, C, and D</li><li>Line 21 submits the graph to an executor and waits until it finishes</li></ul>Lines 8-14 are the main block to enable subflow tasking at task B. The runtime will create a <a href="classtf_1_1Subflow.html" class="m-doc">tf::Subflow</a> passing it to task B, and spawn a dependency graph as described by the associated callable. This new subflow graph will be added to the topology of its parent task B.</section><section id="RetainASubflow"><h2><a href="#RetainASubflow">Retain a Subflow</a></h2>By default, a <a href="classtf_1_1Subflow.html" class="m-doc">tf::Subflow</a> automatically clears its internal task graph once it is joined. After a subflow joins, its structure and associated resources are no longer accessible. This behavior is designed to reduce memory usage, particularly in applications that recursively spawn many subflows. For applications that require post-processing, such as visualizing the subflow through <a href="classtf_1_1Taskflow.html#ac433018262e44b12c4cc9f0c4748d758" class="m-doc">tf::Taskflow::dump</a>, users can disable this default cleanup behavior by calling <a href="classtf_1_1Subflow.html#ac585638d8ca8fb2f34c4826cb0d4f39f" class="m-doc">tf::Subflow::retain</a> on <code>true</code>. This instructs the runtime to retain the subflow's task graph even after it has joined, enabling further inspection or visualization.<pre class="m-code">tf::Taskflow taskflow;

tf::Executor executor;

taskflow.emplace([&](tf::Subflow& sf){

sf.retain(true); // retain the subflow after join for visualization

auto A = sf.emplace([](){ std::cout << "A\n"; });

auto B = sf.emplace([](){ std::cout << "B\n"; });

auto C = sf.emplace([](){ std::cout << "C\n"; });

A.precede(B, C); // A runs before B and C

}); // subflow implicitly joins here

// The subflow graph is now retained and can be visualized using taskflow.dump(...)

taskflow.dump(std::cout);</pre></section><section id="JoinASubflow"><h2><a href="#JoinASubflow">Join a Subflow Explicitly</a></h2>By default, a subflow implicitly joins its parent task when execution leaves its context. All terminal nodes (i.e., nodes with no outgoing edges) in the subflow are guaranteed to precede the parent task. Upon joining, the subflow's task graph and associated resources are automatically cleaned up. If your application needs to access variables defined within the subflow after it joins, you can explicitly join the subflow and handle post-processing accordingly. A common use case is parallelizing recursive computations such as the Fibonacci sequence:<pre class="m-code">int spawn(int n, tf::Subflow& sbf) {

if (n < 2) return n;

int res1, res2;

sbf.emplace([&res1, n] (tf::Subflow& sbf) { res1 = spawn(n - 1, sbf); } );

sbf.emplace([&res2, n] (tf::Subflow& sbf) { res2 = spawn(n - 2, sbf); } );

sbf.join(); // join to materialize the subflow immediately

return res1 + res2;

}

taskflow.emplace([&res] (tf::Subflow& sbf) {

res = spawn(5, sbf);

});

executor.run(taskflow).wait();</pre>The code above computes the fifth Fibonacci number using recursive subflow. Calling <a href="classtf_1_1Subflow.html#a59fcac1323e70d920088dd37bd0be245" class="m-doc">tf::Subflow::join</a> immediately materializes the subflow by executing all associated tasks to recursively compute Fibonacci numbers. The taskflow graph is shown below:<div class="m-graph"><svg style="width: 67.000rem; height: 52.500rem;" viewBox="0.00 0.00 670.00 525.00">

<title>Taskflow</title>

<title>cluster_p0x7ffd972c0cd0</title>

<text text-anchor="middle" x="331" y="-497" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow: fibonacci</text>

</g>

<title>cluster_p0xa445c0</title>

<text text-anchor="middle" x="331" y="-470" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 5</text>

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<title>cluster_p0x7fe918000b90</title>

<text text-anchor="middle" x="445" y="-443" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 4</text>

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<title>cluster_p0x7fe910000b90</title>

<text text-anchor="middle" x="520" y="-416" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 3</text>

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<title>cluster_p0x7fe918000fe0</title>

<text text-anchor="middle" x="551" y="-389" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 2</text>

</g>

<title>cluster_p0x7fe910000c48</title>

<text text-anchor="middle" x="331" y="-291" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 2</text>

</g>

<title>cluster_p0x7fe918000c48</title>

<text text-anchor="middle" x="134" y="-318" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 3</text>

</g>

<title>cluster_p0x7fe918000d00</title>

<text text-anchor="middle" x="165" y="-291" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: 2</text>

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</div><aside class="m-note m-warning"><h4>Attention</h4>Using <a href="classtf_1_1Subflow.html" class="m-doc">tf::Subflow</a> to implement recursive parallelism like finding Fibonacci numbers may not be as efficient as <a href="classtf_1_1Runtime.html" class="m-doc">tf::Runtime</a> due to additional task graph overhead. For more details, readers can refer to <a href="ExamplesFibonacciNumber.html" class="m-doc">Fibonacci Number</a></aside></section><section id="CreateANestedSubflow"><h2><a href="#CreateANestedSubflow">Create a Nested Subflow</a></h2>A subflow can be nested or recursive. You can create another subflow from the execution of a subflow and so on.<pre class="m-code"> 1: tf::Taskflow taskflow;

2:

3: tf::Task A = taskflow.emplace([] (tf::Subflow& sf){

4: std::cout << "A spawns A1 & subflow A2\n";

5: tf::Task A1 = sf.emplace([] () {

6: std::cout << "subtask A1\n";

7: }).name("A1");

8:

9: tf::Task A2 = sf.emplace([] (tf::Subflow& sf2){

10: std::cout << "A2 spawns A2_1 & A2_2\n";

11: tf::Task A2_1 = sf2.emplace([] () {

12: std::cout << "subtask A2_1\n";

13: }).name("A2_1");

14: tf::Task A2_2 = sf2.emplace([] () {

15: std::cout << "subtask A2_2\n";

16: }).name("A2_2");

17: A2_1.precede(A2_2);

18: }).name("A2");

19: A1.precede(A2);

20: }).name("A");

21:

22: // execute the graph to spawn the subflow

23: tf::Executor().run(taskflow).get();</pre><div class="m-graph"><svg style="width: 38.800rem; height: 21.100rem;" viewBox="0.00 0.00 388.00 211.00">

<title>Taskflow</title>

<title>cluster_p0x7ffeeca03810</title>

<text text-anchor="middle" x="190" y="-183" font-family="Helvetica,sans-Serif" font-size="10.00">Taskflow</text>

</g>

<title>cluster_p0x7fbc40c02830</title>

<text text-anchor="middle" x="190" y="-156" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: A</text>

</g>

<title>cluster_p0x7fbc40d00240</title>

<text text-anchor="middle" x="149" y="-129" font-family="Helvetica,sans-Serif" font-size="10.00">Subflow: A2</text>

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</div>Debrief:<ul><li>Line 1 creates a taskflow object</li><li>Lines 3-20 create a task to spawn a subflow of two tasks A1 and A2</li><li>Lines 9-18 spawn another subflow of two tasks A2_1 and A2_2 out of its parent task A2</li><li>Lines 23 runs the defined taskflow graph</li></ul><aside class="m-note m-warning"><h4>Attention</h4>To properly visualize subflows, you must call <a href="classtf_1_1Subflow.html#ac585638d8ca8fb2f34c4826cb0d4f39f" class="m-doc">tf::Subflow::retain</a> on each subflow and execute the taskflow once to ensure all associated subflows are spawned.</aside></section>

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Taskflow handbook is part of the <a href="https://taskflow.github.io">Taskflow project</a>, copyright © <a href="https://tsung-wei-huang.github.io/">Dr. Tsung-Wei Huang</a>, 2018–2026. Generated by <a href="https://doxygen.org/">Doxygen</a> 1.9.1 and <a href="https://mcss.mosra.cz/">m.css</a>.

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