Skip to content
Navigation Menu
{{ message }}
forked from taskflow/taskflow
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathchapter2.html
More file actions
133 lines (131 loc) · 16.3 KB
/
Copy pathchapter2.html
File metadata and controls
133 lines (131 loc) · 16.3 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
<!-- HTML header for doxygen 1.8.13-->
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/xhtml;charset=UTF-8"/>
<meta http-equiv="X-UA-Compatible" content="IE=9"/>
<meta name="generator" content="Doxygen 1.8.13"/>
<meta name="viewport" content="width=device-width, initial-scale=1"/>
<title>Cpp-Taskflow</title>
<link href="tabs.css" rel="stylesheet" type="text/css"/>
<link rel="icon" type="image/x-icon" href="favicon.ico" />
<script type="text/javascript" src="jquery.js"></script>
<script type="text/javascript" src="dynsections.js"></script>
<link href="navtree.css" rel="stylesheet" type="text/css"/>
<script type="text/javascript" src="resize.js"></script>
<script type="text/javascript" src="navtreedata.js"></script>
<script type="text/javascript" src="navtree.js"></script>
<script type="text/javascript">
$(document).ready(initResizable);
</script>
<link href="search/search.css" rel="stylesheet" type="text/css"/>
<script type="text/javascript" src="search/searchdata.js"></script>
<script type="text/javascript" src="search/search.js"></script>
<script type="text/javascript">
$(document).ready(function() { init_search(); });
</script>
<link href="doxygen.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div id="top"><!-- do not remove this div, it is closed by doxygen! -->
<div id="titlearea">
<table cellspacing="0" cellpadding="0">
<tbody>
<tr style="height: 56px;">
<td id="projectalign" style="padding-left: 0.5em;">
<div id="projectname"><a href="https://github.com/cpp-taskflow/cpp-taskflow">Cpp-Taskflow</a>
 <span id="projectnumber">2.2.0</span>
</div>
</td>
<td> <div id="MSearchBox" class="MSearchBoxInactive">
<span class="left">
<img id="MSearchSelect" src="search/mag_sel.png"
onmouseover="return searchBox.OnSearchSelectShow()"
onmouseout="return searchBox.OnSearchSelectHide()"
alt=""/>
<input type="text" id="MSearchField" value="Search" accesskey="S"
onfocus="searchBox.OnSearchFieldFocus(true)"
onblur="searchBox.OnSearchFieldFocus(false)"
onkeyup="searchBox.OnSearchFieldChange(event)"/>
</span><span class="right">
<a id="MSearchClose" href="javascript:searchBox.CloseResultsWindow()"><img id="MSearchCloseImg" border="0" src="search/close.png" alt=""/></a>
</span>
</div>
</td>
</tr>
</tbody>
</table>
</div>
<!-- end header part -->
<!-- Generated by Doxygen 1.8.13 -->
<script type="text/javascript">
var searchBox = new SearchBox("searchBox", "search",false,'Search');
</script>
</div><!-- top -->
<div id="side-nav" class="ui-resizable side-nav-resizable">
<div id="nav-tree">
<div id="nav-tree-contents">
<div id="nav-sync" class="sync"></div>
</div>
</div>
<div id="splitbar" style="-moz-user-select:none;"
class="ui-resizable-handle">
</div>
</div>
<script type="text/javascript">
$(document).ready(function(){initNavTree('chapter2.html','');});
</script>
<div id="doc-content">
<!-- window showing the filter options -->
<div id="MSearchSelectWindow"
onmouseover="return searchBox.OnSearchSelectShow()"
onmouseout="return searchBox.OnSearchSelectHide()"
onkeydown="return searchBox.OnSearchSelectKey(event)">
</div>
<!-- iframe showing the search results (closed by default) -->
<div id="MSearchResultsWindow">
<iframe src="javascript:void(0)" frameborder="0"
name="MSearchResults" id="MSearchResults">
</iframe>
</div>
<div class="header">
<div class="headertitle">
<div class="title">C2: Execute a Taskflow </div> </div>
</div><!--header-->
<div class="contents">
<div class="textblock"><p>After you create a task dependency graph, you need to dispatch it to threads for execution. In this chapter, we will show you how to execute a task dependency graph.</p>
<h1><a class="anchor" id="GraphAndTopology"></a>
Graph and Topology</h1>
<p>Each taskflow object has exactly one graph at a time that represents a set of dependent tasks constructed so far. To execute a graph, you need to create an <em>executor</em> from <a class="el" href="classtf_1_1Executor.html" title="The executor class to run a taskflow graph. ">tf::Executor</a>. An executor manages a set of worker threads and schedules task execution through an efficient <em>work-stealing</em> algorithm. Issuing a call to run a taskflow creates a <em>topology</em>. A topology is a data structure wrapping around a running graph. Each executor object has a list of topologies to keep track of the execution status of running and finished graphs. Users can retrieve this information for graph inspection and debugging.</p>
<h1><a class="anchor" id="C2_CreateAnExecutor"></a>
Create an Executor</h1>
<p>The first step is to create an executor to run a taskflow. <a class="el" href="classtf_1_1Executor.html" title="The executor class to run a taskflow graph. ">tf::Executor</a> takes an unsigned integer to construct an executor of <code>N</code> worker threads. The default value is <a class="elRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency.html">std::thread::hardware_concurrency</a>.</p>
<div class="fragment"><div class="line"><a class="code" href="classtf_1_1Executor.html">tf::Executor</a> executor1; <span class="comment">// create an executor of std::thread::hardware_concurrency worker threads</span></div><div class="line"><a class="code" href="classtf_1_1Executor.html">tf::Executor</a> executor2(4); <span class="comment">// create an executor of 4 worker threads</span></div></div><!-- fragment --><p>An executor can be reused to execute multiple taskflows. In most workloads, you may need only one executor but multiple taskflows to represent parts of a parallel decomposition.</p>
<h1><a class="anchor" id="C2_ExecuteATaskflow"></a>
Execute a Taskflow</h1>
<p><a class="el" href="classtf_1_1Executor.html" title="The executor class to run a taskflow graph. ">tf::Executor</a> provides a set of <code>run_*</code> methods, <a class="el" href="classtf_1_1Executor.html#a93d3b5c6f5612c1b32033f2dcd27b2a5" title="runs the taskflow once ">tf::Executor::run</a>, <a class="el" href="classtf_1_1Executor.html#a81e5af6069fbae3069257ff63182b544" title="runs the taskflow for N times ">tf::Executor::run_n</a>, and <a class="el" href="classtf_1_1Executor.html#ad7867f1e6541555de96bdd8f6a6b56b2" title="runs the taskflow multiple times until the predicate becomes true and then invokes a callback ...">tf::Executor::run_until</a> to run a taskflow for one time, multiple times, or until a given predicate evaluates to true. All methods accept an optional callback to invoke after the execution completes, and return a <a class="elRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/doxygen/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> for users to access the execution status. The code below shows several ways to run a taskflow.</p>
<div class="fragment"><div class="line"> 1: <span class="comment">// Declare an executor and a taskflow</span></div><div class="line"> 2: <a class="code" href="classtf_1_1Executor.html">tf::Executor</a> executor;</div><div class="line"> 3: <a class="code" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"> 4:</div><div class="line"> 5: <span class="comment">// Add three tasks into the taskflow</span></div><div class="line"> 6: <a class="code" href="classtf_1_1Task.html">tf::Task</a> A = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([] () { <a class="codeRef" doxygen="/home/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">"This is TaskA\n"</span>; });</div><div class="line"> 7: <a class="code" href="classtf_1_1Task.html">tf::Task</a> B = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([] () { <a class="codeRef" doxygen="/home/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">"This is TaskB\n"</span>; });</div><div class="line"> 8: <a class="code" href="classtf_1_1Task.html">tf::Task</a> C = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([] () { <a class="codeRef" doxygen="/home/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">"This is TaskC\n"</span>; });</div><div class="line"> 9: </div><div class="line">10: <span class="comment">// Build precedence between tasks</span></div><div class="line">11: A.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(B, C); </div><div class="line">12: </div><div class="line">13: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/shared_future.html">std::shared_future<void></a> fu = executor.<a class="code" href="classtf_1_1Executor.html#a93d3b5c6f5612c1b32033f2dcd27b2a5">run</a>(taskflow);</div><div class="line">14: fu.get(); <span class="comment">// block until the execution completes</span></div><div class="line">15:</div><div class="line">16: executor.<a class="code" href="classtf_1_1Executor.html#a93d3b5c6f5612c1b32033f2dcd27b2a5">run</a>(taskflow, [](){ <a class="codeRef" doxygen="/home/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">"end of one execution\n"</span>; }).<span class="keyword">get</span>();</div><div class="line">17: executor.<a class="code" href="classtf_1_1Executor.html#a81e5af6069fbae3069257ff63182b544">run_n</a>(taskflow, 4);</div><div class="line">18: executor.<a class="code" href="classtf_1_1Executor.html#ab9aa252f70e9a40020a1e5a89d485b85">wait_for_all</a>(); <span class="comment">// block until all associated executions finish</span></div><div class="line">19: executor.<a class="code" href="classtf_1_1Executor.html#a81e5af6069fbae3069257ff63182b544">run_n</a>(taskflow, 4, [](){ <a class="codeRef" doxygen="/home/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">"end of four executions\n"</span>; }).<span class="keyword">get</span>();</div><div class="line">20: executor.<a class="code" href="classtf_1_1Executor.html#ad7867f1e6541555de96bdd8f6a6b56b2">run_until</a>(taskflow, [<span class="keywordtype">int</span> cnt=0] () <span class="keyword">mutable</span> { <span class="keywordflow">return</span> (++cnt == 10); });</div></div><!-- fragment --><p>Debrief:</p>
<ul>
<li>Line 6-8 creates a taskflow of three tasks A, B, and C </li>
<li>Line 13-14 runs the taskflow once and use <a class="elRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/shared_future/get.html">std::shared_future::get</a> to wait for completion </li>
<li>Line 16 runs the taskflow once with a callback to invoke when the execution finishes </li>
<li>Line 17-18 runs the taskflow four times and use tf::Taskflow::wait_for_all to wait for completion </li>
<li>Line 19 runs the taskflow four times and invokes a callback at the end of the forth execution </li>
<li>Line 20 keeps running the taskflow until the predicate returns true</li>
</ul>
<p>Issuing multiple runs on a same taskflow will automatically <em>synchronize</em> to a sequential chain of executions following the order of the run calls.</p>
<div class="fragment"><div class="line">executor.<a class="code" href="classtf_1_1Executor.html#a93d3b5c6f5612c1b32033f2dcd27b2a5">run</a>(taskflow); <span class="comment">// execution 1</span></div><div class="line">executor.<a class="code" href="classtf_1_1Executor.html#a81e5af6069fbae3069257ff63182b544">run_n</a>(taskflow, 10); <span class="comment">// execution 2</span></div><div class="line">executor.<a class="code" href="classtf_1_1Executor.html#a93d3b5c6f5612c1b32033f2dcd27b2a5">run</a>(taskflow); <span class="comment">// execution 3</span></div><div class="line">executor.<a class="code" href="classtf_1_1Executor.html#ab9aa252f70e9a40020a1e5a89d485b85">wait_for_all</a>(); <span class="comment">// execution 1 -> execution 2 -> execution 3</span></div></div><!-- fragment --><p>A key point to notice is a running taskflow must remain alive during its execution. It is your responsibility to ensure a taskflow not being destructed when it is running. For example, the code below can result undefined behavior.</p>
<div class="fragment"><div class="line"><a class="code" href="classtf_1_1Executor.html">tf::Executor</a> executor; <span class="comment">// create an executor</span></div><div class="line"></div><div class="line"><span class="comment">// create a taskflow whose lifetime is restricted by the scope</span></div><div class="line">{</div><div class="line"> <a class="code" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"> </div><div class="line"> <span class="comment">// add tasks to the taskflow</span></div><div class="line"> <span class="comment">// ... </span></div><div class="line"></div><div class="line"> <span class="comment">// run the taskflow</span></div><div class="line"> executor.<a class="code" href="classtf_1_1Executor.html#a93d3b5c6f5612c1b32033f2dcd27b2a5">run</a>(f);</div><div class="line"></div><div class="line">} <span class="comment">// at this point, taskflow might get destructed while it is running, resulting in defined behavior</span></div></div><!-- fragment --><p>Similarly, you should avoid touching a taskflow while it is running.</p>
<div class="fragment"><div class="line"><a class="code" href="classtf_1_1Taskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"></div><div class="line"><span class="comment">// Add tasks into the taskflow</span></div><div class="line"><span class="comment">// ...</span></div><div class="line"></div><div class="line"><span class="comment">// Declare an executor</span></div><div class="line"><a class="code" href="classtf_1_1Executor.html">tf::Executor</a> executor;</div><div class="line"></div><div class="line"><a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/doxygen/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/shared_future.html">std::shared_future<void></a> future = taskflow.run(f); <span class="comment">// non-blocking return</span></div><div class="line"></div><div class="line"><span class="comment">// alter the taskflow while running leads to undefined behavior </span></div><div class="line">f.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([](){ <a class="codeRef" doxygen="/home/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">"Add a new task\n"</span>; });</div></div><!-- fragment --><p>A rule of thumb is to always keep a taskflow alive in your function scope while it is participating in an execution. </p>
</div></div><!-- contents -->
</div><!-- doc-content -->
<!-- start footer part -->
<div id="nav-path" class="navpath"><!-- id is needed for treeview function! -->
<ul>
<li class="footer">Generated by
<a href="http://www.doxygen.org/index.html">
<img class="footer" src="doxygen.png" alt="doxygen"/></a> 1.8.13 </li>
</ul>
</div>
</body>
</html>
You can’t perform that action at this time.