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<!DOCTYPE html>

<head>

<title>Cookbook » Dynamic 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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<li><a href="pages.html">Handbook</a></li>

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<h1>

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

Dynamic Tasking

</h1>

<h3>Contents</h3>

<ul>

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

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

<li><a href="#DetachASubflow">Detach a Subflow</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 dynamic tasking.<section id="CreateASubflow"><h2><a href="#CreateASubflow">Create a Subflow</a></h2>Dynamic 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. Taskflow has an unified interface for static and dynamic tasking. 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"); // dynamic task B1

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

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

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

22: taskflow.dump(std::cout); // dump the taskflow to a DOT format</pre><div><div class="m-graph"><svg style="width: 23.250rem; height: 15.188rem;" viewBox="0.00 0.00 372.00 243.00">

<title>Taskflow</title>

<title>cluster_p0x7ffee9781810</title>

<text text-anchor="middle" x="182" y="-210.2">Taskflow</text>

</g>

<title>cluster_p0x7f9866c01b70</title>

<text text-anchor="middle" x="141" y="-176.2">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><li>Line 22 dumps the entire task dependency graph</li></ul>Lines 8-14 are the main block to enable dynamic tasking. Taskflow uses a <a href="https://en.cppreference.com/w/cpp/utility/variant">std::variant</a> date type to unify the interface of static tasking and dynamic tasking. 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. Due to the property of dynamic tasking, we cannot dump its structure before execution. We will need to run the graph first to spawn the graph and then call <a href="classtf_1_1Taskflow.html#ac433018262e44b12c4cc9f0c4748d758" class="m-doc">tf::Taskflow::dump</a>.</section><section id="JoinASubflow"><h2><a href="#JoinASubflow">Join a Subflow</a></h2>By default, a subflow joins its parent task when the program leaves its execution context. All nodes of zero outgoing edges in the subflow precede its parent task. You can explicitly join a subflow within its execution context to carry out recursive patterns. A famous implementation is fibonacci recursion.<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: 41.875rem; height: 36.125rem;" viewBox="0.00 0.00 670.00 577.85">

<title>Taskflow</title>

<title>cluster_p0x7ffd972c0cd0</title>

<text text-anchor="middle" x="331" y="-545.0478">Taskflow: fibonacci</text>

</g>

<title>cluster_p0xa445c0</title>

<text text-anchor="middle" x="331" y="-511.0478">Subflow: 5</text>

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<title>cluster_p0x7fe918000b90</title>

<text text-anchor="middle" x="445" y="-477.0478">Subflow: 4</text>

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<title>cluster_p0x7fe910000b90</title>

<text text-anchor="middle" x="520" y="-443.0478">Subflow: 3</text>

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<title>cluster_p0x7fe918000fe0</title>

<text text-anchor="middle" x="551" y="-409.0478">Subflow: 2</text>

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<title>cluster_p0x7fe910000c48</title>

<text text-anchor="middle" x="331" y="-296.2782">Subflow: 2</text>

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<title>cluster_p0x7fe918000c48</title>

<text text-anchor="middle" x="134" y="-330.2782">Subflow: 3</text>

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<title>cluster_p0x7fe918000d00</title>

<text text-anchor="middle" x="165" y="-296.2782">Subflow: 2</text>

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</div>Our implementation to join subflows is recursive in order to preserve the thread context in each subflow task. Having a deep recursion of subflows may cause stack overflow.</section><section id="DetachASubflow"><h2><a href="#DetachASubflow">Detach a Subflow</a></h2>In contract to joined subflow, you can detach a subflow from its parent task, allowing its execution to flow independently.<pre class="m-code"> 1: tf::Taskflow taskflow;

2:

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

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

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

6:

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

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

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

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

11: B1.precede(B3); // B1 runs bofore B3

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

13: subflow.detach(); // detach this subflow

15:

20:

21: tf::Executor executor;

22: executor.run(taskflow).wait(); // execute the graph to spawn the subflow

22: taskflow.dump(std::cout); // dump the taskflow to DOT format</pre>The figure below demonstrates a detached subflow based on the previous example. A detached subflow will eventually join the topology of its parent task.<div class="m-graph"><svg style="width: 19.750rem; height: 17.625rem;" viewBox="0.00 0.00 316.00 282.31">

<title>Taskflow</title>

<title>cluster_p0x7ffeecc59810</title>

<text text-anchor="middle" x="154" y="-249.5087">Taskflow</text>

</g>

<title>cluster_p0x7fdc0dc02b60</title>

<text text-anchor="middle" x="221" y="-215.5087">Subflow: B</text>

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</div>Detached subflow becomes an independent graph attached to the top-most taskflow. Running a taskflow multiple times will accumulate all detached tasks in the graph. For example, running the above taskflow 5 times results in a total of 19 tasks.<pre class="m-code">executor.run_n(taskflow, 5).wait();

assert(taskflow.num_tasks() == 19);

taskflow.dump(std::cout);</pre>The dumped graph is shown as follows:<div class="m-graph"><svg style="width: 52.812rem; height: 11.875rem;" viewBox="0.00 0.00 845.00 190.31">

<title>Taskflow</title>

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</div></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& sbf){

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

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

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

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

8:

9: tf::Task A2 = sbf.emplace([] (tf::Subflow& sbf2){

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

11: tf::Task A2_1 = sbf2.emplace([] () {

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

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

14: tf::Task A2_2 = sbf2.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();

24: taskflow.dump(std::cout);</pre><div class="m-graph"><svg style="width: 27.250rem; height: 14.562rem;" viewBox="0.00 0.00 435.56 233.00">

<title>Taskflow</title>

<title>cluster_p0x7ffeeca03810</title>

<text text-anchor="middle" x="213.7817" y="-200.2">Taskflow</text>

</g>

<title>cluster_p0x7fbc40c02830</title>

<text text-anchor="middle" x="213.7817" y="-166.2">Subflow: A</text>

</g>

<title>cluster_p0x7fbc40d00240</title>

<text text-anchor="middle" x="172.7817" y="-132.2">Subflow: A2</text>

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</svg>

</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 23-24 runs the graph asynchronously and dump its structure when it finishes</li></ul>Similarly, you can detach a nested subflow from its parent subflow. A detached subflow will run independently and eventually join the topology of its parent subflow.</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>, 2018–2022. Generated by <a href="https://doxygen.org/">Doxygen</a> 1.8.14 and <a href="https://mcss.mosra.cz/">m.css</a>.

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