docs/ExecuteTaskflow.html · uxsys/cpp-taskflow

代码拉取完成,页面将自动刷新

richgiteeai

Watch

不关注关注所有动态仅关注版本发行动态关注但不提醒动态

1 Star 0 Fork 37

uxsys/cpp-taskflow

forked from Gitee 极速下载/cpp-taskflow

代码 Wiki 统计流水线

服务

加入 Gitee

与超过 1400万开发者一起发现、参与优秀开源项目,私有仓库也完全免费 :)

免费加入

已有帐号? 立即登录

文件

master

分支 (3)

标签 (15)

管理

master

dev

cpp14

v3.4.0

v3.3.0

v3.2.0

v3.1.0

v3.0.0

v2.7.0

v2.6.0

v2.5.0

2.5.0

v2.4.0

v2.3.1

v2.3.0

v2.2.0

v2.1.0

v2.0.0

克隆/下载

HTTPS SSH SVN SVN+SSH 下载ZIP

提示

下载代码请复制以下命令到终端执行

为确保你提交的代码身份被 Gitee 正确识别,请执行以下命令完成配置

git config --global user.name userName 
git config --global user.email userEmail

初次使用 SSH 协议进行代码克隆、推送等操作时,需按下述提示完成 SSH 配置

1 生成 RSA 密钥

2 获取 RSA 公钥内容,并配置到 SSH公钥中

在 Gitee 上使用 SVN,请访问使用指南

使用 HTTPS 协议时,命令行会出现如下账号密码验证步骤。基于安全考虑,Gitee 建议配置并使用私人令牌替代登录密码进行克隆、推送等操作

Username for 'https://gitee.com': userName

Password for 'https://userName@gitee.com': # 私人令牌

分支 3

标签 15

cpp-taskflow

docs

ExecuteTaskflow.html

ExecuteTaskflow.html 70.03 KB

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8" />
 <title>Cookbook &raquo; Executor | 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">
 
 <a href="https://taskflow.github.io"><img src="taskflow_logo.png" alt="" />Taskflow</a> | <a href="index.html" class="m-thin">QuickStart</a>
 
 <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>
 <a href="Cookbook.html">Cookbook</a> &raquo;
 Executor
 </h1>
 <nav class="m-block m-default">
 <h3>Contents</h3>
 <ul>
 <li><a href="#CreateAnExecutor">Create an Executor</a></li>
 <li><a href="#ExecuteATaskflow">Execute a Taskflow</a></li>
 <li><a href="#ExecuteATaskflowWithTransferredOwnership">Execute a Taskflow with Transferred Ownership</a></li>
 <li><a href="#ExecuteATaskflowFromAnInternalWorker">Execute a Taskflow from an Internal Worker</a></li>
 <li><a href="#ThreadSafety">Touch an Executor from Multiple Threads</a></li>
 <li><a href="#QueryTheWorkerID">Query the Worker Identieir</a></li>
 <li><a href="#ObserveThreadActivities">Observe Thread Activities</a></li>
 <li><a href="#ModifyWorkerProperty">Modify Worker Property</a></li>
 </ul>
 </nav>
After you create a task dependency graph, you need to submit it to threads for execution. In this chapter, we will show you how to execute a task dependency graph.<section id="CreateAnExecutor"><h2><a href="#CreateAnExecutor">Create an Executor</a></h2>To execute a taskflow, you need to create an executor of type <a href="classtf_1_1Executor.html" class="m-doc">tf::Executor</a>. An executor is a thread-safe object that manages a set of worker threads and executes tasks through an efficient work-stealing algorithm. Issuing a call to run a taskflow creates a topology, a data structure to keep track of the execution status of a running graph. <a href="classtf_1_1Executor.html" class="m-doc">tf::Executor</a> takes an unsigned integer to construct with <code>N</code> worker threads. The default value is <a href="http://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency.html" class="m-doc-external">std::thread::hardware_concurrency</a>.<pre class="m-code">tf::Executor executor1; // create an executor with the number of workers
 // equal to std::thread::hardware_concurrency
tf::Executor executor2(4); // create an executor of 4 worker threads</pre>An executor can be reused to execute multiple taskflows. In most workloads, you may need only one executor to run multiple taskflows where each taskflow represents a part of a parallel decomposition.</section><section id="ExecuteATaskflow"><h2><a href="#ExecuteATaskflow">Execute a Taskflow</a></h2><a href="classtf_1_1Executor.html" class="m-doc">tf::Executor</a> provides a set of <code>run_*</code> methods, <a href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069" class="m-doc">tf::Executor::run</a>, <a href="classtf_1_1Executor.html#a6d0617eebc9421f1ba1f82ce6dd02c00" class="m-doc">tf::Executor::run_n</a>, and <a href="classtf_1_1Executor.html#a0f52e9dd64b65aba32ca0e13c1ed300a" class="m-doc">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 href="classtf_1_1Future.html" class="m-doc">tf::Future</a> for users to access the execution status. The code below shows several ways to run a taskflow.<pre class="m-code"> 1: // Declare an executor and a taskflow
 2: tf::Executor executor;
 3: tf::Taskflow taskflow;
 4:
 5: // Add three tasks into the taskflow
 6: tf::Task A = taskflow.emplace([] () { std::cout &lt;&lt; &quot;This is TaskA\n&quot;; });
 7: tf::Task B = taskflow.emplace([] () { std::cout &lt;&lt; &quot;This is TaskB\n&quot;; });
 8: tf::Task C = taskflow.emplace([] () { std::cout &lt;&lt; &quot;This is TaskC\n&quot;; });
 9: 
10: // Build precedence between tasks
11: A.precede(B, C); 
12: 
13: tf::Future&lt;void&gt; fu = executor.run(taskflow);
14: fu.wait(); // block until the execution completes
15:
16: executor.run(taskflow, [](){ std::cout &lt;&lt; &quot;end of 1 run&quot;; }).wait();
17: executor.run_n(taskflow, 4);
18: executor.wait_for_all(); // block until all associated executions finish
19: executor.run_n(taskflow, 4, [](){ std::cout &lt;&lt; &quot;end of 4 runs&quot;; }).wait();
20: executor.run_until(taskflow, [cnt=0] () mutable { return ++cnt == 10; });</pre>Debrief:<ul><li>Lines 6-8 create a taskflow of three tasks A, B, and C</li><li>Lines 13-14 run the taskflow once and wait for completion</li><li>Line 16 runs the taskflow once with a callback to invoke when the execution finishes</li><li>Lines 17-18 run the taskflow four times and use <a href="classtf_1_1Executor.html#ab9aa252f70e9a40020a1e5a89d485b85" class="m-doc">tf::Executor::wait_for_all</a> 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>Issuing multiple runs on the same taskflow will automatically synchronize to a sequential chain of executions in the order of run calls.<pre class="m-code">executor.run(taskflow); // execution 1
executor.run_n(taskflow, 10); // execution 2
executor.run(taskflow); // execution 3
executor.wait_for_all(); // execution 1 -&gt; execution 2 -&gt; execution 3</pre><aside class="m-note m-warning"><h4>Attention</h4>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.</aside><pre class="m-code">tf::Executor executor; // create an executor

// create a taskflow whose lifetime is restricted by the scope
{
 tf::Taskflow taskflow;
 
 // add tasks to the taskflow
 // ...

// run the taskflow
 executor.run(f);

} // leaving the scope will destroy taskflow while it is running, 
 // resulting in undefined behavior</pre>Similarly, you should avoid touching a taskflow while it is running.<pre class="m-code">tf::Taskflow taskflow;

// Add tasks into the taskflow
// ...

// Declare an executor
tf::Executor executor;

tf::Future&lt;void&gt; future = taskflow.run(f); // non-blocking return

// alter the taskflow while running leads to undefined behavior 
f.emplace([](){ std::cout &lt;&lt; &quot;Add a new task\n&quot;; });</pre>You must always keep a taskflow alive and must not modify it while it is running on an executor.</section><section id="ExecuteATaskflowWithTransferredOwnership"><h2><a href="#ExecuteATaskflowWithTransferredOwnership">Execute a Taskflow with Transferred Ownership</a></h2>You can transfer the ownership of a taskflow to an executor and run it without wrangling with the lifetime issue of that taskflow. Each <code>run_*</code> method discussed in the previous section comes with an overload that takes a moved taskflow object.<pre class="m-code">tf::Taskflow taskflow;
tf::Executor executor;

taskflow.emplace([](){});

// let the executor manage the lifetime of the submitted taskflow
executor.run(std::move(taskflow));

// now taskflow has no tasks
assert(taskflow.num_tasks() == 0);</pre>However, you should avoid moving a running taskflow which can result in undefined behavior.<pre class="m-code">tf::Taskflow taskflow;
tf::Executor executor;

taskflow.emplace([](){});

// executor does not manage the lifetime of taskflow
executor.run(taskflow);

// error! you cannot move a taskflow while it is running
executor.run(std::move(taskflow)); </pre>The correct way to submit a taskflow with moved ownership to an executor is to ensure all previous runs have completed. The executor will automatically release the resources of a moved taskflow right after its execution completes.<pre class="m-code">// submit the taskflow and wait until it completes
executor.run(taskflow).wait();

// now it&#39;s safe to move the taskflow to the executor and run it
executor.run(std::move(taskflow)); </pre>Likewise, you cannot move a taskflow that is running on an executor. You must wait until all the previous fires of runs on that taskflow complete before calling move.<pre class="m-code">// submit the taskflow and wait until it completes
executor.run(taskflow).wait();

// now it&#39;s safe to move the taskflow to another
tf::Taskflow moved_taskflow(std::move(taskflow)); </pre></section><section id="ExecuteATaskflowFromAnInternalWorker"><h2><a href="#ExecuteATaskflowFromAnInternalWorker">Execute a Taskflow from an Internal Worker</a></h2>The typical <code><a href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069" class="m-doc">tf::Executor::run</a></code> series returns a <a href="classtf_1_1Future.html" class="m-doc">tf::Future</a> object which allows you to wait for the result to complete. When calling <code>tf::Future::wait</code>, the caller blocks without doing anything until the associated state has indicated completion. This design, however, can introduce deadlock problem especially when you need to run multiple taskflows from the internal workers of an executor. To avoid this problem, the executor has a method, <a href="classtf_1_1Executor.html#ab05ad34c59cf11a7c4de82cf58bba91e" class="m-doc">tf::Executor::run_and_wait</a>, to execute a taskflow and wait until the execution finishes using an internal worker of the executor.<pre class="m-code">tf::Executor executor(2);
tf::Taskflow taskflow;
std::array&lt;tf::Taskflow, 1000&gt; others;

std::atomic&lt;size_t&gt; counter{0};

for(size_t n=0; n&lt;1000; n++) {
 for(size_t i=0; i&lt;1000; i++) {
 others[n].emplace([&amp;](){ counter++; });
 }
 taskflow.emplace([&amp;executor, &amp;tf=others[n]](){
 executor.run_and_wait(tf);
 //executor.run(tf).wait(); &lt;- blocking the worker without doing anything
 // will introduce deadlock
 });
}
executor.run(taskflow).wait();</pre>Similar to <a href="classtf_1_1Executor.html#ab05ad34c59cf11a7c4de82cf58bba91e" class="m-doc">tf::Executor::run_and_wait</a>, the method <a href="classtf_1_1Executor.html#afbff30c324c5887aace09e7fe9a0d607" class="m-doc">tf::Executor::loop_until</a> is another variant that keeps the calling worker in the work-stealing loop until the given predicate becomes true. You can use this method to prevent blocking a worker from doing useful things, such as being blocked when submitting an outstanding task (e.g., a GPU operation).<pre class="m-code">taskflow.emplace([&amp;](){
 auto fu = std::async([](){ std::sleep(100s); });
 executor.loop_until([](){
 return fu.wait_for(std::chrono::seconds(0)) == future_status::ready;
 });
});</pre><aside class="m-note m-warning"><h4>Attention</h4>You must call <a href="classtf_1_1Executor.html#afbff30c324c5887aace09e7fe9a0d607" class="m-doc">tf::Executor::loop_until</a> and <a href="classtf_1_1Executor.html" class="m-doc">tf::Executor</a>:run_and_wait from a worker of the calling executor or an exception will be thrown.</aside></section><section id="ThreadSafety"><h2><a href="#ThreadSafety">Touch an Executor from Multiple Threads</a></h2>All <code>run_*</code> methods are thread-safe. You can have multiple threads call these methods from an executor to run different taskflows. However, the order which taskflow runs first is non-deterministic and is up to the runtime.<pre class="m-code"> 1: tf::Executor executor;
 2:
 3: for(int i=0; i&lt;10; ++i) {
 4: std::thread([i, &amp;](){
 5: // ... modify my taskflow at i
 6: executor.run(taskflows[i]); // run my taskflow at i
 7: }).detach();
 8: }
 9:
10: executor.wait_for_all();</pre></section><section id="QueryTheWorkerID"><h2><a href="#QueryTheWorkerID">Query the Worker Identieir</a></h2>Each worker in an executor has an unique integer identifier in the range <code>[0, N)</code> that can be queried by the caller thread using <a href="classtf_1_1Executor.html#a6487d589cb1f6b078b69fd3bb1082345" class="m-doc">tf::Executor::this_worker_id</a>. If the caller thread is not a worker in the executor, <code>-1</code> is returned. This method is convenient for users to maintain a one-to-one mapping between a worker and its application data structure.<pre class="m-code">std::vector&lt;int&gt; worker_vectors[8]; // one vector per worker

tf::Taskflow taskflow;
tf::Executor executor(8); // an executor of eight workers

assert(executor.this_worker_id() == -1); // master thread is not a worker

taskflow.emplace([&amp;](){
 int id = executor.this_worker_id(); // in the range [0, 8)
 auto&amp; vec = worker_vectors[worker_id];
 // ...
});</pre></section><section id="ObserveThreadActivities"><h2><a href="#ObserveThreadActivities">Observe Thread Activities</a></h2>You can observe thread activities in an executor when a worker thread participates in executing a task and leaves the execution using <a href="classtf_1_1ObserverInterface.html" class="m-doc">tf::ObserverInterface</a> &ndash; an interface class that provides a set of methods for you to define what to do when a thread enters and leaves the execution context of a task.<pre class="m-code">class ObserverInterface {
 virtual ~ObserverInterface() = default;
 virtual void set_up(size_t num_workers) = 0;
 virtual void on_entry(tf::WorkerView worker_view, tf::TaskView task_view) = 0;
 virtual void on_exit(tf::WorkerView worker_view, tf::TaskView task_view) = 0;
};</pre>There are three methods you must define in your derived class, <a href="classtf_1_1ObserverInterface.html#a41e6e62f12bf9d9dc4fa74632f6825d9" class="m-doc">tf::ObserverInterface::set_up</a>, <a href="classtf_1_1ObserverInterface.html#a8225fcacb03089677a1efc4b16b734cc" class="m-doc">tf::ObserverInterface::on_entry</a>, and <a href="classtf_1_1ObserverInterface.html#aa22f5378154653f08d9a58326bda4754" class="m-doc">tf::ObserverInterface::on_exit</a>. The method, <a href="classtf_1_1ObserverInterface.html#a41e6e62f12bf9d9dc4fa74632f6825d9" class="m-doc">tf::ObserverInterface::set_up</a>, is a constructor-like method that will be called by the executor when the observer is constructed. It passes an argument of the number of workers to observer in the executor. You may use it to preallocate or initialize data storage, e.g., an independent vector for each worker. The methods, <a href="classtf_1_1ObserverInterface.html#a8225fcacb03089677a1efc4b16b734cc" class="m-doc">tf::ObserverInterface::on_entry</a> and <a href="classtf_1_1ObserverInterface.html#aa22f5378154653f08d9a58326bda4754" class="m-doc">tf::ObserverInterface::on_exit</a>, are called by a worker thread before and after the execution context of a task, respectively. Both methods provide immutable access to the underlying worker and the running task using <a href="classtf_1_1WorkerView.html" class="m-doc">tf::WorkerView</a> and <a href="classtf_1_1TaskView.html" class="m-doc">tf::TaskView</a>. You may use them to record timepoints and calculate the elapsed time of a task.You can associate an executor with one or multiple observers (though one is common) using <a href="classtf_1_1Executor.html#aff77def96ae740d648dd84e571237c83" class="m-doc">tf::Executor::make_observer</a>. We use <a href="http://en.cppreference.com/w/cpp/memory/shared_ptr.html" class="m-doc-external">std::shared_ptr</a> to manage the ownership of an observer. The executor loops through each observer and invoke the corresponding methods accordingly.<pre class="m-code">#include &lt;taskflow/taskflow.hpp&gt;

struct MyObserver : public tf::ObserverInterface {

MyObserver(const std::string&amp; name) {
 std::cout &lt;&lt; &quot;constructing observer &quot; &lt;&lt; name &lt;&lt; &#39;\n&#39;;
 }

void set_up(size_t num_workers) override final {
 std::cout &lt;&lt; &quot;setting up observer with &quot; &lt;&lt; num_workers &lt;&lt; &quot; workers\n&quot;;
 }

void on_entry(tf::WorkerView w, tf::TaskView tv) override final {
 std::ostringstream oss;
 oss &lt;&lt; &quot;worker &quot; &lt;&lt; w.id() &lt;&lt; &quot; ready to run &quot; &lt;&lt; tv.name() &lt;&lt; &#39;\n&#39;;
 std::cout &lt;&lt; oss.str();
 }

void on_exit(tf::WorkerView w, tf::TaskView tv) override final {
 std::ostringstream oss;
 oss &lt;&lt; &quot;worker &quot; &lt;&lt; w.id() &lt;&lt; &quot; finished running &quot; &lt;&lt; tv.name() &lt;&lt; &#39;\n&#39;;
 std::cout &lt;&lt; oss.str();
 }

};

int main(){

tf::Executor executor(4);

// Create a taskflow of eight tasks
 tf::Taskflow taskflow;

auto A = taskflow.emplace([] () { std::cout &lt;&lt; &quot;1\n&quot;; }).name(&quot;A&quot;);
 auto B = taskflow.emplace([] () { std::cout &lt;&lt; &quot;2\n&quot;; }).name(&quot;B&quot;);
 auto C = taskflow.emplace([] () { std::cout &lt;&lt; &quot;3\n&quot;; }).name(&quot;C&quot;);
 auto D = taskflow.emplace([] () { std::cout &lt;&lt; &quot;4\n&quot;; }).name(&quot;D&quot;);
 auto E = taskflow.emplace([] () { std::cout &lt;&lt; &quot;5\n&quot;; }).name(&quot;E&quot;);
 auto F = taskflow.emplace([] () { std::cout &lt;&lt; &quot;6\n&quot;; }).name(&quot;F&quot;);
 auto G = taskflow.emplace([] () { std::cout &lt;&lt; &quot;7\n&quot;; }).name(&quot;G&quot;);
 auto H = taskflow.emplace([] () { std::cout &lt;&lt; &quot;8\n&quot;; }).name(&quot;H&quot;);

// create an observer
 std::shared_ptr&lt;MyObserver&gt; observer = executor.make_observer&lt;MyObserver&gt;(
 &quot;MyObserver&quot;
 );

// run the taskflow
 executor.run(taskflow).get();

// remove the observer (optional)
 executor.remove_observer(std::move(observer));

return 0;
}</pre>The above code produces the following output:<pre class="m-code">constructing observer MyObserver
setting up observer with 4 workers
worker 2 ready to run A
1
worker 2 finished running A
worker 2 ready to run B
2
worker 1 ready to run C
worker 2 finished running B
3
worker 2 ready to run D
worker 3 ready to run E
worker 1 finished running C
4
5
worker 1 ready to run F
worker 2 finished running D
worker 3 finished running E
6
worker 2 ready to run G
worker 3 ready to run H
worker 1 finished running F
7
8
worker 2 finished running G
worker 3 finished running H</pre>It is expected each line of <a href="http://en.cppreference.com/w/cpp/io/basic_ostream.html" class="m-doc-external">std::cout</a> interleaves with each other as there are four workers participating in task scheduling. However, the ready message always appears before the corresponding task message (e.g., numbers) and then the finished message.</section><section id="ModifyWorkerProperty"><h2><a href="#ModifyWorkerProperty">Modify Worker Property</a></h2>You can change the property of each worker thread from its executor, such as assigning thread-processor affinity before the worker enters the scheduler loop and post-processing additional information after the worker leaves the scheduler loop, by passing an instance derived from <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::WorkerInterface</a> to the executor. The example below prints the worker identifier when it enters and leaves the scheduler loop, respectively:<pre class="m-code">class CustomWorkerBehavior : public tf::WorkerInterface {
 public:
 // to call before the worker enters the scheduling loop
 void scheduler_prologue(tf::Worker&amp; w) override {
 std::cout &lt;&lt; tf::stringify(&quot;worker &quot;, w.id(), &quot; enters the scheduler loop\n&quot;); 
 }
 // to call after the worker leaves the scheduling loop
 void scheduler_epilogue(tf::Worker&amp; w, std::exception_ptr) override {
 std::cout &lt;&lt; tf::stringify(&quot;worker &quot;, w.id(), &quot; leaves the scheduler loop\n&quot;); 
 }
};

int main() {
 tf::Executor executor(4, std::make_shared&lt;CustomWorkerBehavior&gt;());
 return 0;
}</pre>When running the program, we see the following one possible output:<pre class="m-console">worker 1 enters the scheduler loop
worker 0 enters the scheduler loop
worker 2 enters the scheduler loop
worker 3 enters the scheduler loop
worker 3 leaves the scheduler loop
worker 0 leaves the scheduler loop
worker 2 leaves the scheduler loop
worker 1 leaves the scheduler loop</pre>When you create an executor, it spawns a set of worker threads to run tasks using a work-stealing algorithm. &lt;&lt;&lt;&lt;&lt;&lt;&lt; HEAD <a href="classtf_1_1Executor.html" class="m-doc">Executor</a> logic of the scheduler and its interaction with each spawned worker<h2>via </h2>The interaction with each spawned worker using <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::WorkerInterface</a> is given below:<blockquote><blockquote><blockquote><blockquote><blockquote><blockquote><blockquote>0dc0291de3cf926fe81561f8073ed43718146077</blockquote></blockquote></blockquote></blockquote></blockquote></blockquote></blockquote><pre class="m-code">for(size_t n=0; n&lt;num_workers; n++) {
 create_thread([](Worker&amp; worker)

// pre-processing executor-specific worker information
 // ...
 
 // enter the scheduling loop
 // Here, WorkerInterface::scheduler_prologue is invoked, if any
 
 while(1) {
 perform_work_stealing_algorithm();
 if(stop) {
 break; 
 }
 }
 
 // leaves the scheduling loop and joins this worker thread
 // Here, WorkerInterface::scheduler_epilogue is invoked, if any
 );
}</pre><aside class="m-note m-info"><h4>Note</h4>Methods defined in <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::WorkerInterface</a> are not thread-safe and may be be invoked by multiple workers concurrently.</aside></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>Tab / T to search, Esc to close</div>
 <div id="search-symbolcount">&hellip;</div>
 </div>
 <div class="m-doc-search-content">
 <form>
 <input type="search" name="q" id="search-input" placeholder="Loading &hellip;" 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 requires JavaScript.</noscript>
 <div id="search-help" class="m-text m-dim m-text-center">
 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.
 Use &darr;
 / &uarr; to navigate through the list,
 Enter to go.
 Tab autocompletes common prefix, you can
 copy a link to the result using ⌘
 L while ⌘
 M produces a Markdown link.
 </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">
 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&ndash;2022. Generated by <a href="https://doxygen.org/">Doxygen</a> 1.8.14 and <a href="https://mcss.mosra.cz/">m.css</a>.
 </div>
 </div>
 </div>
</nav></footer>
</body>
</html>

一键复制编辑原始数据按行查看历史

twhuang 提交于 2022年08月16日 04:55 +08:00 . Merge branch 'dev' of into dev

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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8" />
 <title>Cookbook &raquo; Executor | 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> &raquo;</span>
 Executor
 </h1>
 <nav class="m-block m-default">
 <h3>Contents</h3>
 <ul>
 <li><a href="#CreateAnExecutor">Create an Executor</a></li>
 <li><a href="#ExecuteATaskflow">Execute a Taskflow</a></li>
 <li><a href="#ExecuteATaskflowWithTransferredOwnership">Execute a Taskflow with Transferred Ownership</a></li>
 <li><a href="#ExecuteATaskflowFromAnInternalWorker">Execute a Taskflow from an Internal Worker</a></li>
 <li><a href="#ThreadSafety">Touch an Executor from Multiple Threads</a></li>
 <li><a href="#QueryTheWorkerID">Query the Worker Identieir</a></li>
 <li><a href="#ObserveThreadActivities">Observe Thread Activities</a></li>
 <li><a href="#ModifyWorkerProperty">Modify Worker Property</a></li>
 </ul>
 </nav>
<p>After you create a task dependency graph, you need to submit it to threads for execution. In this chapter, we will show you how to execute a task dependency graph.</p><section id="CreateAnExecutor"><h2><a href="#CreateAnExecutor">Create an Executor</a></h2><p>To execute a taskflow, you need to create an <em>executor</em> of type <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a>. An executor is a <em>thread-safe</em> object that manages a set of worker threads and executes tasks through an efficient <em>work-stealing</em> algorithm. Issuing a call to run a taskflow creates a <em>topology</em>, a data structure to keep track of the execution status of a running graph. <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> takes an unsigned integer to construct with <code>N</code> worker threads. The default value is <a href="http://en.cppreference.com/w/cpp/thread/thread/hardware_concurrency.html" class="m-doc-external">std::<wbr />thread::<wbr />hardware_concurrency</a>.</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">executor1</span><span class="p">;</span><span class="w"> </span><span class="c1">// create an executor with the number of workers</span>
<span class="w"> </span><span class="c1">// equal to std::thread::hardware_concurrency</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="nf">executor2</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span><span class="w"> </span><span class="c1">// create an executor of 4 worker threads</span></pre><p>An executor can be reused to execute multiple taskflows. In most workloads, you may need only one executor to run multiple taskflows where each taskflow represents a part of a parallel decomposition.</p></section><section id="ExecuteATaskflow"><h2><a href="#ExecuteATaskflow">Execute a Taskflow</a></h2><p><a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> provides a set of <code>run_*</code> methods, <a href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069" class="m-doc">tf::<wbr />Executor::<wbr />run</a>, <a href="classtf_1_1Executor.html#a6d0617eebc9421f1ba1f82ce6dd02c00" class="m-doc">tf::<wbr />Executor::<wbr />run_n</a>, and <a href="classtf_1_1Executor.html#a0f52e9dd64b65aba32ca0e13c1ed300a" class="m-doc">tf::<wbr />Executor::<wbr />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 href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> for users to access the execution status. The code below shows several ways to run a taskflow.</p><pre class="m-code"><span class="w"> </span><span class="mi">1</span><span class="o">:</span><span class="w"> </span><span class="c1">// Declare an executor and a taskflow</span>
<span class="w"> </span><span class="mi">2</span><span class="o">:</span><span class="w"> </span><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="w"> </span><span class="mi">3</span><span class="o">:</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="mi">4</span><span class="o">:</span><span class="w"></span>
<span class="w"> </span><span class="mi">5</span><span class="o">:</span><span class="w"> </span><span class="c1">// Add three tasks into the taskflow</span>
<span class="w"> </span><span class="mi">6</span><span class="o">:</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Task</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;This is TaskA</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="mi">7</span><span class="o">:</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Task</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;This is TaskB</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="mi">8</span><span class="o">:</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Task</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;This is TaskC</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="mi">9</span><span class="o">:</span><span class="w"> </span>
<span class="mi">10</span><span class="o">:</span><span class="w"> </span><span class="c1">// Build precedence between tasks</span>
<span class="mi">11</span><span class="o">:</span><span class="w"> </span><span class="n">A</span><span class="p">.</span><span class="n">precede</span><span class="p">(</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="mi">12</span><span class="o">:</span><span class="w"> </span>
<span class="mi">13</span><span class="o">:</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Future</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">&gt;</span><span class="w"> </span><span class="n">fu</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">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">);</span><span class="w"></span>
<span class="mi">14</span><span class="o">:</span><span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">wait</span><span class="p">();</span><span class="w"> </span><span class="c1">// block until the execution completes</span>
<span class="mi">15</span><span class="o">:</span><span class="w"></span>
<span class="mi">16</span><span class="o">:</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">,</span><span class="w"> </span><span class="p">[](){</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;end of 1 run&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">wait</span><span class="p">();</span><span class="w"></span>
<span class="mi">17</span><span class="o">:</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run_n</span><span class="p">(</span><span class="n">taskflow</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">);</span><span class="w"></span>
<span class="mi">18</span><span class="o">:</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="c1">// block until all associated executions finish</span>
<span class="mi">19</span><span class="o">:</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run_n</span><span class="p">(</span><span class="n">taskflow</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="p">[](){</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;end of 4 runs&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">wait</span><span class="p">();</span><span class="w"></span>
<span class="mi">20</span><span class="o">:</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run_until</span><span class="p">(</span><span class="n">taskflow</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="n">cnt</span><span class="o">=</span><span class="mi">0</span><span class="p">]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="k">mutable</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="o">++</span><span class="n">cnt</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span></pre><p>Debrief:</p><ul><li>Lines 6-8 create a taskflow of three tasks A, B, and C</li><li>Lines 13-14 run the taskflow once and wait for completion</li><li>Line 16 runs the taskflow once with a callback to invoke when the execution finishes</li><li>Lines 17-18 run the taskflow four times and use <a href="classtf_1_1Executor.html#ab9aa252f70e9a40020a1e5a89d485b85" class="m-doc">tf::<wbr />Executor::<wbr />wait_for_all</a> 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 the same taskflow will automatically <em>synchronize</em> to a sequential chain of executions in the order of run calls.</p><pre class="m-code"><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">);</span><span class="w"> </span><span class="c1">// execution 1</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run_n</span><span class="p">(</span><span class="n">taskflow</span><span class="p">,</span><span class="w"> </span><span class="mi">10</span><span class="p">);</span><span class="w"> </span><span class="c1">// execution 2</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">);</span><span class="w"> </span><span class="c1">// execution 3</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="c1">// execution 1 -&gt; execution 2 -&gt; execution 3</span></pre><aside class="m-note m-warning"><h4>Attention</h4><p>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></aside><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">// create an executor</span>

<span class="c1">// create a taskflow whose lifetime is restricted by the scope</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">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span>
<span class="w"> </span><span class="c1">// add tasks to the taskflow</span>
<span class="w"> </span><span class="c1">// ... </span>

<span class="w"> </span><span class="c1">// run the taskflow</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">f</span><span class="p">);</span><span class="w"></span>

<span class="p">}</span><span class="w"> </span><span class="c1">// leaving the scope will destroy taskflow while it is running, </span>
<span class="w"> </span><span class="c1">// resulting in undefined behavior</span></pre><p>Similarly, you should avoid touching a taskflow while it is running.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>

<span class="c1">// Add tasks into the taskflow</span>
<span class="c1">// ...</span>

<span class="c1">// Declare an executor</span>
<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">Future</span><span class="o">&lt;</span><span class="kt">void</span><span class="o">&gt;</span><span class="w"> </span><span class="n">future</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">f</span><span class="p">);</span><span class="w"> </span><span class="c1">// non-blocking return</span>

<span class="c1">// alter the taskflow while running leads to undefined behavior </span>
<span class="n">f</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;Add a new task</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span></pre><p>You must always keep a taskflow alive and must not modify it while it is running on an executor.</p></section><section id="ExecuteATaskflowWithTransferredOwnership"><h2><a href="#ExecuteATaskflowWithTransferredOwnership">Execute a Taskflow with Transferred Ownership</a></h2><p>You can transfer the ownership of a taskflow to an executor and run it without wrangling with the lifetime issue of that taskflow. Each <code>run_*</code> method discussed in the previous section comes with an overload that takes a <em>moved</em> taskflow object.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>
<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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){});</span><span class="w"></span>

<span class="c1">// let the executor manage the lifetime of the submitted taskflow</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">taskflow</span><span class="p">));</span><span class="w"></span>

<span class="c1">// now taskflow has no tasks</span>
<span class="n">assert</span><span class="p">(</span><span class="n">taskflow</span><span class="p">.</span><span class="n">num_tasks</span><span class="p">()</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span><span class="w"></span></pre><p>However, you should avoid moving a <em>running</em> taskflow which can result in undefined behavior.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>
<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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){});</span><span class="w"></span>

<span class="c1">// executor does not manage the lifetime of taskflow</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">);</span><span class="w"></span>

<span class="c1">// error! you cannot move a taskflow while it is running</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">taskflow</span><span class="p">));</span><span class="w"> </span></pre><p>The correct way to submit a taskflow with moved ownership to an executor is to ensure all previous runs have completed. The executor will automatically release the resources of a moved taskflow right <em>after</em> its execution completes.</p><pre class="m-code"><span class="c1">// submit the taskflow and wait until it completes</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span><span class="w"></span>

<span class="c1">// now it&#39;s safe to move the taskflow to the executor and run it</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">taskflow</span><span class="p">));</span><span class="w"> </span></pre><p>Likewise, you cannot move a taskflow that is running on an executor. You must wait until all the previous fires of runs on that taskflow complete before calling move.</p><pre class="m-code"><span class="c1">// submit the taskflow and wait until it completes</span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span><span class="w"></span>

<span class="c1">// now it&#39;s safe to move the taskflow to another</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="nf">moved_taskflow</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">taskflow</span><span class="p">));</span><span class="w"> </span></pre></section><section id="ExecuteATaskflowFromAnInternalWorker"><h2><a href="#ExecuteATaskflowFromAnInternalWorker">Execute a Taskflow from an Internal Worker</a></h2><p>The typical <code><a href="classtf_1_1Executor.html#a519777f5783981d534e9e53b99712069" class="m-doc">tf::<wbr />Executor::<wbr />run</a></code> series returns a <a href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> object which allows you to wait for the result to complete. When calling <code>tf::Future::wait</code>, the caller blocks without doing anything until the associated state has indicated completion. This design, however, can introduce deadlock problem especially when you need to run multiple taskflows from the internal workers of an executor. To avoid this problem, the executor has a method, <a href="classtf_1_1Executor.html#ab05ad34c59cf11a7c4de82cf58bba91e" class="m-doc">tf::<wbr />Executor::<wbr />run_and_wait</a>, to execute a taskflow and wait until the execution finishes using an internal worker of the executor.</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="nf">executor</span><span class="p">(</span><span class="mi">2</span><span class="p">);</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>
<span class="n">std</span><span class="o">::</span><span class="n">array</span><span class="o">&lt;</span><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="p">,</span><span class="w"> </span><span class="mi">1000</span><span class="o">&gt;</span><span class="w"> </span><span class="n">others</span><span class="p">;</span><span class="w"></span>

<span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o">&lt;</span><span class="kt">size_t</span><span class="o">&gt;</span><span class="w"> </span><span class="n">counter</span><span class="p">{</span><span class="mi">0</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">n</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">n</span><span class="o">&lt;</span><span class="mi">1000</span><span class="p">;</span><span class="w"> </span><span class="n">n</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></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">&lt;</span><span class="mi">1000</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="w"> </span><span class="n">others</span><span class="p">[</span><span class="n">n</span><span class="p">].</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](){</span><span class="w"> </span><span class="n">counter</span><span class="o">++</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="n">executor</span><span class="p">,</span><span class="w"> </span><span class="o">&amp;</span><span class="n">tf</span><span class="o">=</span><span class="n">others</span><span class="p">[</span><span class="n">n</span><span class="p">]](){</span><span class="w"></span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run_and_wait</span><span class="p">(</span><span class="n">tf</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="c1">//executor.run(tf).wait(); &lt;- blocking the worker without doing anything</span>
<span class="w"> </span><span class="c1">// will introduce deadlock</span>
<span class="w"> </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">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span><span class="w"></span></pre><p>Similar to <a href="classtf_1_1Executor.html#ab05ad34c59cf11a7c4de82cf58bba91e" class="m-doc">tf::<wbr />Executor::<wbr />run_and_wait</a>, the method <a href="classtf_1_1Executor.html#afbff30c324c5887aace09e7fe9a0d607" class="m-doc">tf::<wbr />Executor::<wbr />loop_until</a> is another variant that keeps the calling worker in the work-stealing loop until the given predicate becomes true. You can use this method to prevent blocking a worker from doing useful things, such as being blocked when submitting an outstanding task (e.g., a GPU operation).</p><pre class="m-code"><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](){</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">fu</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">async</span><span class="p">([](){</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">sleep</span><span class="p">(</span><span class="mi">100</span><span class="n">s</span><span class="p">);</span><span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">loop_until</span><span class="p">([](){</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">wait_for</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">chrono</span><span class="o">::</span><span class="n">seconds</span><span class="p">(</span><span class="mi">0</span><span class="p">))</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">future_status</span><span class="o">::</span><span class="n">ready</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="p">});</span><span class="w"></span></pre><aside class="m-note m-warning"><h4>Attention</h4><p>You must call <a href="classtf_1_1Executor.html#afbff30c324c5887aace09e7fe9a0d607" class="m-doc">tf::<wbr />Executor::<wbr />loop_until</a> and <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a>:run_and_wait from a worker of the calling executor or an exception will be thrown.</p></aside></section><section id="ThreadSafety"><h2><a href="#ThreadSafety">Touch an Executor from Multiple Threads</a></h2><p>All <code>run_*</code> methods are <em>thread-safe</em>. You can have multiple threads call these methods from an executor to run different taskflows. However, the order which taskflow runs first is non-deterministic and is up to the runtime.</p><pre class="m-code"><span class="w"> </span><span class="mi">1</span><span class="o">:</span><span class="w"> </span><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="w"> </span><span class="mi">2</span><span class="o">:</span><span class="w"></span>
<span class="w"> </span><span class="mi">3</span><span class="o">:</span><span class="w"> </span><span class="k">for</span><span class="p">(</span><span class="kt">int</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">&lt;</span><span class="mi">10</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">i</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="mi">4</span><span class="o">:</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="kr">thread</span><span class="p">([</span><span class="n">i</span><span class="p">,</span><span class="w"> </span><span class="o">&amp;</span><span class="p">](){</span><span class="w"></span>
<span class="w"> </span><span class="mi">5</span><span class="o">:</span><span class="w"> </span><span class="c1">// ... modify my taskflow at i</span>
<span class="w"> </span><span class="mi">6</span><span class="o">:</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflows</span><span class="p">[</span><span class="n">i</span><span class="p">]);</span><span class="w"> </span><span class="c1">// run my taskflow at i</span>
<span class="w"> </span><span class="mi">7</span><span class="o">:</span><span class="w"> </span><span class="p">}).</span><span class="n">detach</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="mi">8</span><span class="o">:</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="mi">9</span><span class="o">:</span><span class="w"></span>
<span class="mi">10</span><span class="o">:</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="QueryTheWorkerID"><h2><a href="#QueryTheWorkerID">Query the Worker Identieir</a></h2><p>Each worker in an executor has an unique integer identifier in the range <code>[0, N)</code> that can be queried by the caller thread using <a href="classtf_1_1Executor.html#a6487d589cb1f6b078b69fd3bb1082345" class="m-doc">tf::<wbr />Executor::<wbr />this_worker_id</a>. If the caller thread is not a worker in the executor, <code>-1</code> is returned. This method is convenient for users to maintain a one-to-one mapping between a worker and its application data structure.</p><pre class="m-code"><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="w"> </span><span class="n">worker_vectors</span><span class="p">[</span><span class="mi">8</span><span class="p">];</span><span class="w"> </span><span class="c1">// one vector per worker</span>

<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="nf">executor</span><span class="p">(</span><span class="mi">8</span><span class="p">);</span><span class="w"> </span><span class="c1">// an executor of eight workers</span>

<span class="n">assert</span><span class="p">(</span><span class="n">executor</span><span class="p">.</span><span class="n">this_worker_id</span><span class="p">()</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">-1</span><span class="p">);</span><span class="w"> </span><span class="c1">// master thread is not a worker</span>

<span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](){</span><span class="w"></span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">id</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">this_worker_id</span><span class="p">();</span><span class="w"> </span><span class="c1">// in the range [0, 8)</span>
<span class="w"> </span><span class="k">auto</span><span class="o">&amp;</span><span class="w"> </span><span class="n">vec</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">worker_vectors</span><span class="p">[</span><span class="n">worker_id</span><span class="p">];</span><span class="w"></span>
<span class="w"> </span><span class="c1">// ...</span>
<span class="p">});</span><span class="w"></span></pre></section><section id="ObserveThreadActivities"><h2><a href="#ObserveThreadActivities">Observe Thread Activities</a></h2><p>You can observe thread activities in an executor when a worker thread participates in executing a task and leaves the execution using <a href="classtf_1_1ObserverInterface.html" class="m-doc">tf::<wbr />ObserverInterface</a> &ndash; an <em>interface</em> class that provides a set of methods for you to define what to do when a thread enters and leaves the execution context of a task.</p><pre class="m-code"><span class="k">class</span><span class="w"> </span><span class="nc">ObserverInterface</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="o">~</span><span class="n">ObserverInterface</span><span class="p">()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">default</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">set_up</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">num_workers</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">on_entry</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">WorkerView</span><span class="w"> </span><span class="n">worker_view</span><span class="p">,</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">TaskView</span><span class="w"> </span><span class="n">task_view</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="k">virtual</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">on_exit</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">WorkerView</span><span class="w"> </span><span class="n">worker_view</span><span class="p">,</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">TaskView</span><span class="w"> </span><span class="n">task_view</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="p">};</span><span class="w"></span></pre><p>There are three methods you must define in your derived class, <a href="classtf_1_1ObserverInterface.html#a41e6e62f12bf9d9dc4fa74632f6825d9" class="m-doc">tf::<wbr />ObserverInterface::<wbr />set_up</a>, <a href="classtf_1_1ObserverInterface.html#a8225fcacb03089677a1efc4b16b734cc" class="m-doc">tf::<wbr />ObserverInterface::<wbr />on_entry</a>, and <a href="classtf_1_1ObserverInterface.html#aa22f5378154653f08d9a58326bda4754" class="m-doc">tf::<wbr />ObserverInterface::<wbr />on_exit</a>. The method, <a href="classtf_1_1ObserverInterface.html#a41e6e62f12bf9d9dc4fa74632f6825d9" class="m-doc">tf::<wbr />ObserverInterface::<wbr />set_up</a>, is a constructor-like method that will be called by the executor when the observer is constructed. It passes an argument of the number of workers to observer in the executor. You may use it to preallocate or initialize data storage, e.g., an independent vector for each worker. The methods, <a href="classtf_1_1ObserverInterface.html#a8225fcacb03089677a1efc4b16b734cc" class="m-doc">tf::<wbr />ObserverInterface::<wbr />on_entry</a> and <a href="classtf_1_1ObserverInterface.html#aa22f5378154653f08d9a58326bda4754" class="m-doc">tf::<wbr />ObserverInterface::<wbr />on_exit</a>, are called by a worker thread before and after the execution context of a task, respectively. Both methods provide immutable access to the underlying worker and the running task using <a href="classtf_1_1WorkerView.html" class="m-doc">tf::<wbr />WorkerView</a> and <a href="classtf_1_1TaskView.html" class="m-doc">tf::<wbr />TaskView</a>. You may use them to record timepoints and calculate the elapsed time of a task.</p><p>You can associate an executor with one or multiple observers (though one is common) using <a href="classtf_1_1Executor.html#aff77def96ae740d648dd84e571237c83" class="m-doc">tf::<wbr />Executor::<wbr />make_observer</a>. We use <a href="http://en.cppreference.com/w/cpp/memory/shared_ptr.html" class="m-doc-external">std::<wbr />shared_ptr</a> to manage the ownership of an observer. The executor loops through each observer and invoke the corresponding methods accordingly.</p><pre class="m-code"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/taskflow.hpp&gt;</span><span class="cp"></span>

<span class="k">struct</span><span class="w"> </span><span class="nc">MyObserver</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">ObserverInterface</span><span class="w"> </span><span class="p">{</span><span class="w"></span>

<span class="w"> </span><span class="n">MyObserver</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">string</span><span class="o">&amp;</span><span class="w"> </span><span class="n">name</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;constructing observer &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>

<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">set_up</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">num_workers</span><span class="p">)</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="k">final</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;setting up observer with &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">num_workers</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; workers</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>

<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">on_entry</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">WorkerView</span><span class="w"> </span><span class="n">w</span><span class="p">,</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">TaskView</span><span class="w"> </span><span class="n">tv</span><span class="p">)</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="k">final</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">ostringstream</span><span class="w"> </span><span class="n">oss</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">oss</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;worker &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">w</span><span class="p">.</span><span class="n">id</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; ready to run &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">tv</span><span class="p">.</span><span class="n">name</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">oss</span><span class="p">.</span><span class="n">str</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>

<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">on_exit</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">WorkerView</span><span class="w"> </span><span class="n">w</span><span class="p">,</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">TaskView</span><span class="w"> </span><span class="n">tv</span><span class="p">)</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="k">final</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">ostringstream</span><span class="w"> </span><span class="n">oss</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">oss</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;worker &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">w</span><span class="p">.</span><span class="n">id</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot; finished running &quot;</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">tv</span><span class="p">.</span><span class="n">name</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="sc">&#39;\n&#39;</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">oss</span><span class="p">.</span><span class="n">str</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>

<span class="p">};</span><span class="w"></span>

<span class="kt">int</span><span class="w"> </span><span class="nf">main</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">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">(</span><span class="mi">4</span><span class="p">);</span><span class="w"></span>

<span class="w"> </span><span class="c1">// Create a taskflow of eight tasks</span>
<span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span><span class="w"></span>

<span class="w"> </span><span class="k">auto</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;1</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;A&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;2</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;B&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;3</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;C&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</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">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;4</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;D&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">E</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;5</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;E&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">F</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;6</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;F&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">G</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;7</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;G&quot;</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">H</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;8</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">name</span><span class="p">(</span><span class="s">&quot;H&quot;</span><span class="p">);</span><span class="w"></span>

<span class="w"> </span><span class="c1">// create an observer</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">shared_ptr</span><span class="o">&lt;</span><span class="n">MyObserver</span><span class="o">&gt;</span><span class="w"> </span><span class="n">observer</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">make_observer</span><span class="o">&lt;</span><span class="n">MyObserver</span><span class="o">&gt;</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="s">&quot;MyObserver&quot;</span><span class="w"></span>
<span class="w"> </span><span class="p">);</span><span class="w"></span>

<span class="w"> </span><span class="c1">// run the taskflow</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">get</span><span class="p">();</span><span class="w"></span>

<span class="w"> </span><span class="c1">// remove the observer (optional)</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">remove_observer</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">observer</span><span class="p">));</span><span class="w"></span>

<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><p>The above code produces the following output:</p><pre class="m-code">constructing observer MyObserver
setting up observer with <span class="m">4</span> workers
worker <span class="m">2</span> ready to run A
<span class="m">1</span>
worker <span class="m">2</span> finished running A
worker <span class="m">2</span> ready to run B
<span class="m">2</span>
worker <span class="m">1</span> ready to run C
worker <span class="m">2</span> finished running B
<span class="m">3</span>
worker <span class="m">2</span> ready to run D
worker <span class="m">3</span> ready to run E
worker <span class="m">1</span> finished running C
<span class="m">4</span>
<span class="m">5</span>
worker <span class="m">1</span> ready to run F
worker <span class="m">2</span> finished running D
worker <span class="m">3</span> finished running E
<span class="m">6</span>
worker <span class="m">2</span> ready to run G
worker <span class="m">3</span> ready to run H
worker <span class="m">1</span> finished running F
<span class="m">7</span>
<span class="m">8</span>
worker <span class="m">2</span> finished running G
worker <span class="m">3</span> finished running H</pre><p>It is expected each line of <a href="http://en.cppreference.com/w/cpp/io/basic_ostream.html" class="m-doc-external">std::<wbr />cout</a> interleaves with each other as there are four workers participating in task scheduling. However, the <em>ready</em> message always appears before the corresponding task message (e.g., numbers) and then the <em>finished</em> message.</p></section><section id="ModifyWorkerProperty"><h2><a href="#ModifyWorkerProperty">Modify Worker Property</a></h2><p>You can change the property of each worker thread from its executor, such as assigning thread-processor affinity before the worker enters the scheduler loop and post-processing additional information after the worker leaves the scheduler loop, by passing an instance derived from <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::<wbr />WorkerInterface</a> to the executor. The example below prints the worker identifier when it enters and leaves the scheduler loop, respectively:</p><pre class="m-code"><span class="k">class</span><span class="w"> </span><span class="nc">CustomWorkerBehavior</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">WorkerInterface</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">public</span><span class="o">:</span><span class="w"></span>
<span class="w"> </span><span class="c1">// to call before the worker enters the scheduling loop</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">scheduler_prologue</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">Worker</span><span class="o">&amp;</span><span class="w"> </span><span class="n">w</span><span class="p">)</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">stringify</span><span class="p">(</span><span class="s">&quot;worker &quot;</span><span class="p">,</span><span class="w"> </span><span class="n">w</span><span class="p">.</span><span class="n">id</span><span class="p">(),</span><span class="w"> </span><span class="s">&quot; enters the scheduler loop</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span><span class="w"> </span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="c1">// to call after the worker leaves the scheduling loop</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">scheduler_epilogue</span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">Worker</span><span class="o">&amp;</span><span class="w"> </span><span class="n">w</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">exception_ptr</span><span class="p">)</span><span class="w"> </span><span class="k">override</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">stringify</span><span class="p">(</span><span class="s">&quot;worker &quot;</span><span class="p">,</span><span class="w"> </span><span class="n">w</span><span class="p">.</span><span class="n">id</span><span class="p">(),</span><span class="w"> </span><span class="s">&quot; leaves the scheduler loop</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">);</span><span class="w"> </span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">};</span><span class="w"></span>

<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </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">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">(</span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">make_shared</span><span class="o">&lt;</span><span class="n">CustomWorkerBehavior</span><span class="o">&gt;</span><span class="p">());</span><span class="w"></span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><p>When running the program, we see the following one possible output:</p><pre class="m-console"><span class="go">worker 1 enters the scheduler loop</span>
<span class="go">worker 0 enters the scheduler loop</span>
<span class="go">worker 2 enters the scheduler loop</span>
<span class="go">worker 3 enters the scheduler loop</span>
<span class="go">worker 3 leaves the scheduler loop</span>
<span class="go">worker 0 leaves the scheduler loop</span>
<span class="go">worker 2 leaves the scheduler loop</span>
<span class="go">worker 1 leaves the scheduler loop</span></pre><p>When you create an executor, it spawns a set of worker threads to run tasks using a work-stealing algorithm. &lt;&lt;&lt;&lt;&lt;&lt;&lt; HEAD <a href="classtf_1_1Executor.html" class="m-doc">Executor</a> logic of the scheduler and its interaction with each spawned worker</p><h2>via </h2><p>The interaction with each spawned worker using <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::<wbr />WorkerInterface</a> is given below:</p><blockquote><blockquote><blockquote><blockquote><blockquote><blockquote><blockquote><p>0dc0291de3cf926fe81561f8073ed43718146077</p></blockquote></blockquote></blockquote></blockquote></blockquote></blockquote></blockquote><pre class="m-code"><span class="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">n</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">n</span><span class="o">&lt;</span><span class="n">num_workers</span><span class="p">;</span><span class="w"> </span><span class="n">n</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">create_thread</span><span class="p">([](</span><span class="n">Worker</span><span class="o">&amp;</span><span class="w"> </span><span class="n">worker</span><span class="p">)</span><span class="w"></span>

<span class="w"> </span><span class="c1">// pre-processing executor-specific worker information</span>
<span class="w"> </span><span class="c1">// ...</span>
<span class="w"> </span>
<span class="w"> </span><span class="c1">// enter the scheduling loop</span>
<span class="w"> </span><span class="c1">// Here, WorkerInterface::scheduler_prologue is invoked, if any</span>
<span class="w"> </span>
<span class="w"> </span><span class="k">while</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">perform_work_stealing_algorithm</span><span class="p">();</span><span class="w"></span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">stop</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">break</span><span class="p">;</span><span class="w"> </span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span>
<span class="w"> </span><span class="c1">// leaves the scheduling loop and joins this worker thread</span>
<span class="w"> </span><span class="c1">// Here, WorkerInterface::scheduler_epilogue is invoked, if any</span>
<span class="w"> </span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span></pre><aside class="m-note m-info"><h4>Note</h4><p>Methods defined in <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::<wbr />WorkerInterface</a> are not thread-safe and may be be invoked by multiple workers concurrently.</p></aside></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">&hellip;</div>
 </div>
 <div class="m-doc-search-content">
 <form>
 <input type="search" name="q" id="search-input" placeholder="Loading &hellip;" 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">&darr;</span>
 / <span class="m-label m-dim">&uarr;</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&ndash;2022.<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>