forked from taskflow/taskflow
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathDependentAsyncTasking.html
More file actions
200 lines (195 loc) · 28.7 KB
/
Copy pathDependentAsyncTasking.html
File metadata and controls
200 lines (195 loc) · 28.7 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<title>Cookbook » Asynchronous Tasking with Dependencies | Taskflow QuickStart</title>
<link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Source+Sans+Pro:400,400i,600,600i%7CSource+Code+Pro:400,400i,600" />
<link rel="stylesheet" href="m-dark+documentation.compiled.css" />
<link rel="icon" href="favicon.ico" type="image/vnd.microsoft.icon" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<meta name="theme-color" content="#22272e" />
</head>
<body>
<header><nav id="navigation">
<div class="m-container">
<div class="m-row">
<span id="m-navbar-brand" class="m-col-t-8 m-col-m-none m-left-m">
<a href="https://taskflow.github.io"><img src="taskflow_logo.png" alt="" />Taskflow</a> <span class="m-breadcrumb">|</span> <a href="index.html" class="m-thin">QuickStart</a>
</span>
<div class="m-col-t-4 m-hide-m m-text-right m-nopadr">
<a href="#search" class="m-doc-search-icon" title="Search" onclick="return showSearch()"><svg style="height: 0.9rem;" viewBox="0 0 16 16">
<path id="m-doc-search-icon-path" d="m6 0c-3.31 0-6 2.69-6 6 0 3.31 2.69 6 6 6 1.49 0 2.85-0.541 3.89-1.44-0.0164 0.338 0.147 0.759 0.5 1.15l3.22 3.79c0.552 0.614 1.45 0.665 2 0.115 0.55-0.55 0.499-1.45-0.115-2l-3.79-3.22c-0.392-0.353-0.812-0.515-1.15-0.5 0.895-1.05 1.44-2.41 1.44-3.89 0-3.31-2.69-6-6-6zm0 1.56a4.44 4.44 0 0 1 4.44 4.44 4.44 4.44 0 0 1-4.44 4.44 4.44 4.44 0 0 1-4.44-4.44 4.44 4.44 0 0 1 4.44-4.44z"/>
</svg></a>
<a id="m-navbar-show" href="#navigation" title="Show navigation"></a>
<a id="m-navbar-hide" href="#" title="Hide navigation"></a>
</div>
<div id="m-navbar-collapse" class="m-col-t-12 m-show-m m-col-m-none m-right-m">
<div class="m-row">
<ol class="m-col-t-6 m-col-m-none">
<li><a href="pages.html">Handbook</a></li>
<li><a href="namespaces.html">Namespaces</a></li>
</ol>
<ol class="m-col-t-6 m-col-m-none" start="3">
<li><a href="annotated.html">Classes</a></li>
<li><a href="files.html">Files</a></li>
<li class="m-show-m"><a href="#search" class="m-doc-search-icon" title="Search" onclick="return showSearch()"><svg style="height: 0.9rem;" viewBox="0 0 16 16">
<use href="#m-doc-search-icon-path" />
</svg></a></li>
</ol>
</div>
</div>
</div>
</div>
</nav></header>
<main><article>
<div class="m-container m-container-inflatable">
<div class="m-row">
<div class="m-col-l-10 m-push-l-1">
<h1>
<span class="m-breadcrumb"><a href="Cookbook.html">Cookbook</a> »</span>
Asynchronous Tasking with Dependencies
</h1>
<nav class="m-block m-default">
<h3>Contents</h3>
<ul>
<li><a href="#CreateADynamicTaskGraph">Create a Dynamic Task Graph</a></li>
<li><a href="#SpecifyARagneOfDependentTasks">Specify a Range of Dependent Tasks</a></li>
<li><a href="#UnderstandTheLifeTimeOfADependentAsyncTask">Understand the Lifetime of a Dependent Async Task</a></li>
<li><a href="#CreateADynamicTaskGraphByMultipleThreads">Create a Dynamic Task Graph by Multiple Threads</a></li>
</ul>
</nav>
<p>This chapters discusses how to create a task graph dynamically using asynchronous tasks, which is extremely beneficial for workloads that want to (1) explore task graph parallelism out of dynamic control flow or (2) overlap task graph creation time with individual task execution time. We recommend that you first read <a href="AsyncTasking.html" class="m-doc">Asynchronous Tasking</a> before digesting this chapter.</p><section id="CreateADynamicTaskGraph"><h2><a href="#CreateADynamicTaskGraph">Create a Dynamic Task Graph</a></h2><p>When the construct-and-run model of a task graph is not possible in your application, you can use <a href="classtf_1_1Executor.html#aee02b63d3a91ad5ca5a1c0e71f3e128f" class="m-doc">tf::<wbr />Executor::<wbr />dependent_async</a> and <a href="classtf_1_1Executor.html#a0e2d792f28136b8227b413d0c27d5c7f" class="m-doc">tf::<wbr />Executor::<wbr />silent_dependent_async</a> to create a task graph dynamically. This type of parallelism is also known as <em>on-the-fly</em> task graph parallelism, which offers great flexibility for expressing dynamic task graph parallelism. The example below dynamically creates a task graph of four dependent async tasks, <code>A</code>, <code>B</code>, <code>C</code>, and <code>D</code>, where <code>A</code> runs before <code>B</code> and <code>C</code> and <code>D</code> runs after <code>B</code> and <code>C:</code></p><div class="m-graph"><svg style="width: 24.200rem; height: 9.800rem;" viewBox="0.00 0.00 242.00 98.00">
<g transform="scale(1 1) rotate(0) translate(4 94)">
<title>G</title>
<g class="m-node m-flat">
<title>A</title>
<ellipse cx="27" cy="-45" rx="27" ry="18"/>
<text text-anchor="middle" x="27" y="-42.5" font-family="Helvetica,sans-Serif" font-size="10.00" fill="#000000">A</text>
</g>
<g class="m-node m-flat">
<title>B</title>
<ellipse cx="117" cy="-72" rx="27" ry="18"/>
<text text-anchor="middle" x="117" y="-69.5" font-family="Helvetica,sans-Serif" font-size="10.00" fill="#000000">B</text>
</g>
<g class="m-edge">
<title>A->B</title>
<path d="M52.07,-52.521C61.5102,-55.3531 72.4169,-58.6251 82.564,-61.6692"/>
<polygon points="81.7915,-65.0915 92.3755,-64.6126 83.803,-58.3867 81.7915,-65.0915"/>
</g>
<g class="m-node m-flat">
<title>C</title>
<ellipse cx="117" cy="-18" rx="27" ry="18"/>
<text text-anchor="middle" x="117" y="-15.5" font-family="Helvetica,sans-Serif" font-size="10.00" fill="#000000">C</text>
</g>
<g class="m-edge">
<title>A->C</title>
<path d="M52.07,-37.479C61.5102,-34.6469 72.4169,-31.3749 82.564,-28.3308"/>
<polygon points="83.803,-31.6133 92.3755,-25.3874 81.7915,-24.9085 83.803,-31.6133"/>
</g>
<g class="m-node m-flat">
<title>D</title>
<ellipse cx="207" cy="-45" rx="27" ry="18"/>
<text text-anchor="middle" x="207" y="-42.5" font-family="Helvetica,sans-Serif" font-size="10.00" fill="#000000">D</text>
</g>
<g class="m-edge">
<title>B->D</title>
<path d="M142.07,-64.479C151.5102,-61.6469 162.4169,-58.3749 172.564,-55.3308"/>
<polygon points="173.803,-58.6133 182.3755,-52.3874 171.7915,-51.9085 173.803,-58.6133"/>
</g>
<g class="m-edge">
<title>C->D</title>
<path d="M142.07,-25.521C151.5102,-28.3531 162.4169,-31.6251 172.564,-34.6692"/>
<polygon points="171.7915,-38.0915 182.3755,-37.6126 173.803,-31.3867 171.7915,-38.0915"/>
</g>
</g>
</svg>
</div><pre class="m-code"><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="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"A</span><span class="se">\n</span><span class="s">"</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">AsyncTask</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"B</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"C</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="k">auto</span><span class="w"> </span><span class="p">[</span><span class="n">D</span><span class="p">,</span><span class="w"> </span><span class="n">fuD</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"D</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">);</span><span class="w"></span>
<span class="n">fuD</span><span class="p">.</span><span class="n">get</span><span class="p">();</span><span class="w"> </span><span class="c1">// wait for D to finish, which in turns means A, B, C finish</span></pre><p>Both <a href="classtf_1_1Executor.html#aee02b63d3a91ad5ca5a1c0e71f3e128f" class="m-doc">tf::<wbr />Executor::<wbr />dependent_async</a> and <a href="classtf_1_1Executor.html#a0e2d792f28136b8227b413d0c27d5c7f" class="m-doc">tf::<wbr />Executor::<wbr />silent_dependent_async</a> create a task of type <a href="classtf_1_1AsyncTask.html" class="m-doc">tf::<wbr />AsyncTask</a> to run the given function asynchronously. Additionally, <a href="classtf_1_1Executor.html#aee02b63d3a91ad5ca5a1c0e71f3e128f" class="m-doc">tf::<wbr />Executor::<wbr />dependent_async</a> returns a <a href="https://en.cppreference.com/w/cpp/thread/future">std::<wbr />future</a> that eventually holds the result of the execution. When returning from both calls, the executor has scheduled a worker to run the task whenever its dependencies are met. That is, task execution happens <em>simultaneously</em> with the creation of the task graph, which is different from constructing a Taskflow and running it from an executor, illustrated in the figure below:</p><img class="m-image" src="dependent_async_execution_diagram.png" alt="Image" /><p>Since this model only allows relating a dependency from the current task to a previously created task, you need a correct topological order of graph expression. In our example, there are only two possible topological orderings, either <code>ABCD</code> or <code>ACBD</code>. The code below shows another feasible order of expressing this dynamic task graph parallelism:</p><pre class="m-code"><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="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"A</span><span class="se">\n</span><span class="s">"</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">AsyncTask</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"C</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"B</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="k">auto</span><span class="w"> </span><span class="p">[</span><span class="n">D</span><span class="p">,</span><span class="w"> </span><span class="n">fuD</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"D</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">);</span><span class="w"></span>
<span class="n">fuD</span><span class="p">.</span><span class="n">get</span><span class="p">();</span><span class="w"> </span><span class="c1">// wait for D to finish, which in turns means A, B, C finish</span></pre><p>In addition to using <a href="https://en.cppreference.com/w/cpp/thread/future">std::<wbr />future</a> to synchronize the execution, you can use <a href="classtf_1_1Executor.html#ab9aa252f70e9a40020a1e5a89d485b85" class="m-doc">tf::<wbr />Executor::<wbr />wait_for_all</a> to wait for all scheduled tasks to finish:</p><pre class="m-code"><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="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"A</span><span class="se">\n</span><span class="s">"</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">AsyncTask</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"B</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"C</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">D</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){</span><span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"D</span><span class="se">\n</span><span class="s">"</span><span class="p">);</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">);</span><span class="w"></span>
<span class="n">executor</span><span class="p">.</span><span class="n">wait_for_all</span><span class="p">();</span><span class="w"></span></pre></section><section id="SpecifyARagneOfDependentTasks"><h2><a href="#SpecifyARagneOfDependentTasks">Specify a Range of Dependent Tasks</a></h2><p>Both <a href="classtf_1_1Executor.html#aee02b63d3a91ad5ca5a1c0e71f3e128f" class="m-doc">tf::<wbr />Executor::<wbr />dependent_async</a>(F&& func, Tasks&&... tasks) and <a href="classtf_1_1Executor.html#a0e2d792f28136b8227b413d0c27d5c7f" class="m-doc">tf::<wbr />Executor::<wbr />silent_dependent_async</a>(F&& func, Tasks&&... tasks) accept an arbitrary number of tasks in the dependency list. If the number of dependent tasks is unknown at programming time, such as those relying on runtime variables, you can use the following two overloads to specify dependent tasks in an iterable range <code>[first, last)</code>:</p><ul><li><a href="classtf_1_1Executor.html#a01e51e564f5def845506bcf6b4bb1664" class="m-doc">tf::<wbr />Executor::<wbr />dependent_async(F&& func, I first, I last)</a></li><li><a href="classtf_1_1Executor.html#aa9b08e47e68ae1e568f18aa7104cb9b1" class="m-doc">tf::<wbr />Executor::<wbr />silent_dependent_async(F&& func, I first, I last)</a></li></ul><p>The code below creates an asynchronous task that depends on <code>N</code> previously created asynchronous tasks stored in a vector, where <code>N</code> is a runtime variable:</p><pre class="m-code"><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="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="o">></span><span class="w"> </span><span class="n">dependents</span><span class="p">;</span><span class="w"></span>
<span class="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="n">N</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// N is a runtime variable</span>
<span class="w"> </span><span class="n">dependents</span><span class="p">.</span><span class="n">push_back</span><span class="p">(</span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){}));</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){},</span><span class="w"> </span><span class="n">dependents</span><span class="p">.</span><span class="n">begin</span><span class="p">(),</span><span class="w"> </span><span class="n">dependents</span><span class="p">.</span><span class="n">end</span><span class="p">());</span><span class="w"></span>
<span class="n">executor</span><span class="p">.</span><span class="n">wait_for_all</span><span class="p">();</span><span class="w"></span></pre></section><section id="UnderstandTheLifeTimeOfADependentAsyncTask"><h2><a href="#UnderstandTheLifeTimeOfADependentAsyncTask">Understand the Lifetime of a Dependent Async Task</a></h2><p>A <a href="classtf_1_1AsyncTask.html" class="m-doc">tf::<wbr />AsyncTask</a> is a lightweight handle that retains <em>shared</em> ownership of a dependent async task created by an executor. This shared ownership ensures that the async task remains alive when adding it to the dependency list of another async task, thus avoiding the classical <a href="https://en.wikipedia.org/wiki/ABA_problem">ABA problem</a>.</p><pre class="m-code"><span class="c1">// main thread retains shared ownership of async task A</span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){});</span><span class="w"></span>
<span class="c1">// task A remains alive (i.e., at least one ref count by the main thread) </span>
<span class="c1">// when being added to the dependency list of async task B</span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span></pre><p>Currently, <a href="classtf_1_1AsyncTask.html" class="m-doc">tf::<wbr />AsyncTask</a> is implemented based on C++ smart pointer <a href="http://en.cppreference.com/w/cpp/memory/shared_ptr.html" class="m-doc-external">std::<wbr />shared_ptr</a> and is considered cheap to copy or move as long as only a handful of objects own it. When a worker completes an async task, it will remove the task from the executor, decrementing the number of shared owners by one. If that counter reaches zero, the task is destroyed.</p></section><section id="CreateADynamicTaskGraphByMultipleThreads"><h2><a href="#CreateADynamicTaskGraphByMultipleThreads">Create a Dynamic Task Graph by Multiple Threads</a></h2><p>You can use multiple threads to create a dynamic task graph as long as the order of simultaneously creating tasks is topologically correct. The example below uses creates a dynamic task graph using three threads (including the main thread), where task <code>A</code> runs before task <code>B</code> and task <code>C:</code></p><pre class="m-code"><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="c1">// main thread creates a dependent async task A</span>
<span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){});</span><span class="w"></span>
<span class="c1">// spawn a new thread to create an async task B that runs after A</span>
<span class="n">std</span><span class="o">::</span><span class="kr">thread</span><span class="w"> </span><span class="nf">t1</span><span class="p">([</span><span class="o">&</span><span class="p">](){</span><span class="w"></span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="p">});</span><span class="w"></span>
<span class="c1">// spawn a new thread to create an async task C that runs after A</span>
<span class="n">std</span><span class="o">::</span><span class="kr">thread</span><span class="w"> </span><span class="nf">t2</span><span class="p">([</span><span class="o">&</span><span class="p">](){</span><span class="w"></span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">AsyncTask</span><span class="w"> </span><span class="n">C</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">silent_dependent_async</span><span class="p">([](){},</span><span class="w"> </span><span class="n">A</span><span class="p">);</span><span class="w"></span>
<span class="p">});</span><span class="w"></span>
<span class="n">executor</span><span class="p">.</span><span class="n">wait_for_all</span><span class="p">();</span><span class="w"></span>
<span class="n">t1</span><span class="p">.</span><span class="n">join</span><span class="p">();</span><span class="w"></span>
<span class="n">t2</span><span class="p">.</span><span class="n">join</span><span class="p">();</span><span class="w"></span></pre><p>Regardless of <code>t1</code> runs before or after <code>t2</code>, the resulting topological order is always correct with the graph definition, either <code>ABC</code> or <code>ACB</code>.</p></section>
</div>
</div>
</div>
</article></main>
<div class="m-doc-search" id="search">
<a href="#!" onclick="return hideSearch()"></a>
<div class="m-container">
<div class="m-row">
<div class="m-col-m-8 m-push-m-2">
<div class="m-doc-search-header m-text m-small">
<div><span class="m-label m-default">Tab</span> / <span class="m-label m-default">T</span> to search, <span class="m-label m-default">Esc</span> to close</div>
<div id="search-symbolcount">…</div>
</div>
<div class="m-doc-search-content">
<form>
<input type="search" name="q" id="search-input" placeholder="Loading …" disabled="disabled" autofocus="autofocus" autocomplete="off" spellcheck="false" />
</form>
<noscript class="m-text m-danger m-text-center">Unlike everything else in the docs, the search functionality <em>requires</em> JavaScript.</noscript>
<div id="search-help" class="m-text m-dim m-text-center">
<p class="m-noindent">Search for symbols, directories, files, pages or
modules. You can omit any prefix from the symbol or file path; adding a
<code>:</code> or <code>/</code> suffix lists all members of given symbol or
directory.</p>
<p class="m-noindent">Use <span class="m-label m-dim">↓</span>
/ <span class="m-label m-dim">↑</span> to navigate through the list,
<span class="m-label m-dim">Enter</span> to go.
<span class="m-label m-dim">Tab</span> autocompletes common prefix, you can
copy a link to the result using <span class="m-label m-dim">⌘</span>
<span class="m-label m-dim">L</span> while <span class="m-label m-dim">⌘</span>
<span class="m-label m-dim">M</span> produces a Markdown link.</p>
</div>
<div id="search-notfound" class="m-text m-warning m-text-center">Sorry, nothing was found.</div>
<ul id="search-results"></ul>
</div>
</div>
</div>
</div>
</div>
<script src="search-v2.js"></script>
<script src="searchdata-v2.js" async="async"></script>
<footer><nav>
<div class="m-container">
<div class="m-row">
<div class="m-col-l-10 m-push-l-1">
<p>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>, 2018–2023.<br />Generated by <a href="https://doxygen.org/">Doxygen</a> 1.8.14 and <a href="https://mcss.mosra.cz/">m.css</a>.</p>
</div>
</div>
</div>
</nav></footer>
</body>
</html>