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<span class="m-breadcrumb"><a href="Cookbook.html">Cookbook</a> »</span>
Executor
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<h3>Contents</h3>
<ul>
<li><a href="#CreateAnExecutor">Create an Executor</a></li>
<li><a href="#UnderstandWorkStealingInExecutor">Understand Work Stealing in 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 Cooperatively</a></li>
<li><a href="#ThreadSafetyOfExecution">Thread Safety of Executor</a></li>
<li><a href="#QueryTheWorkerID">Query the Worker ID</a></li>
<li><a href="#ObserveThreadActivities">Observe Thread Activities</a></li>
<li><a href="#ModifyWorkerProperty">Modify Worker Property</a></li>
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<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><aside class="m-note m-warning"><h4>Attention</h4><p>Creating a <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> has non-negligible overhead. Unless your application requires multiple executors, we recommend creating a single <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> and reusing it to run multiple taskflows.</p></aside></section><section id="UnderstandWorkStealingInExecutor"><h2><a href="#UnderstandWorkStealingInExecutor">Understand Work Stealing in Executor</a></h2><p>Taskflow designs a highly efficient <em>work-stealing</em> algorithm to schedule and run tasks in an executor. Work-stealing is a dynamic scheduling algorithm widely used in parallel computing to distribute and balance workload among multiple threads or cores. Specifically, within an executor, each worker maintains its own local queue of tasks. When a worker finishes its own tasks, instead of becoming idle or going sleep, it (thief) tries to <em>steal</em> a task from the queue another worker (victim). The figure below illustrates the idea of work-stealing:</p><img class="m-image" src="work-stealing.png" alt="Image" /><p>The key advantage of work-stealing lies in its <em>decentralized</em> nature and efficiency. Most of the time, worker threads work on their local queues without contention. Stealing only occurs when a worker becomes idle, minimizing overhead associated with synchronization and task distribution. This decentralized strategy effectively balances the workload, ensuring that idle workers are put to work and that the overall computation progresses efficiently. That being said, the internal scheduling mechanisms in <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> are not trivial, and it's not easy to explain every detail in just a few sentences. If you're interested in learning more about the technical details, please refer to our paper published in 2022 <em>IEEE Transactions on Parallel and Distributed Systems (TPDS)</em>:</p><ul><li>Tsung-Wei Huang, Dian-Lun Lin, Chun-Xun Lin, and Yibo Lin, "<a href="https://tsung-wei-huang.github.io/papers/tpds21-taskflow.pdf">Taskflow: A Lightweight Parallel and Heterogeneous Task Graph Computing System</a>," <em>IEEE Transactions on Parallel and Distributed Systems (TPDS)</em>, vol. 33, no. 6, pp. 1303-1320, June 2022</li></ul></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="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="mi">4</span><span class="o">:</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"><<</span><span class="w"> </span><span class="s">"This is TaskA</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span><span class="p">});</span>
<span class="w"> </span><span class="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"><<</span><span class="w"> </span><span class="s">"This is TaskB</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span><span class="p">});</span>
<span class="w"> </span><span class="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"><<</span><span class="w"> </span><span class="s">"This is TaskC</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span><span class="p">});</span>
<span class="w"> </span><span class="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"><</span><span class="kt">void</span><span class="o">></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="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="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"><<</span><span class="w"> </span><span class="s">"end of 1 run"</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">wait</span><span class="p">();</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="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"><<</span><span class="w"> </span><span class="s">"end of 4 runs"</span><span class="p">;</span><span class="w"> </span><span class="p">}).</span><span class="n">wait</span><span class="p">();</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></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 fourth 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 -> execution 2 -> 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="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="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">taskflow</span><span class="p">);</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="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="n">tf</span><span class="o">::</span><span class="n">Future</span><span class="o"><</span><span class="kt">void</span><span class="o">></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">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">// alter the taskflow while running leads to undefined behavior </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="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"Add a new task</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span><span class="p">});</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="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="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){});</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="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></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="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="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){});</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="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="c1">// now it'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="c1">// now it'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 Cooperatively</a></h2><p>Each run variant of <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> returns a <a href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> object that allows you to wait for the associated execution to complete. When <code>tf::Future::wait</code> is called, the caller blocks without making any progress until the underlying state becomes ready. However, this blocking design can lead to potential deadlocks, particularly when multiple taskflows are launched from within the internal workers of the same executor. For example, the following code creates a taskflow of 1,000 tasks, where each task runs another taskflow of 500 tasks in a blocking manner, leading to a potential deadlock problem:</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="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="n">std</span><span class="o">::</span><span class="n">array</span><span class="o"><</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">></span><span class="w"> </span><span class="n">others</span><span class="p">;</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"><</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="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="mi">500</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="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">&</span><span class="p">](){});</span>
<span class="w"> </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="o">&</span><span class="n">executor</span><span class="p">,</span><span class="w"> </span><span class="o">&</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="c1">// Blocking a worker can cause deadlock if all workers are waiting</span>
<span class="w"> </span><span class="c1">// for their taskflows to complete without making any progress</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">tf</span><span class="p">).</span><span class="n">wait</span><span class="p">();</span>
<span class="w"> </span><span class="p">});</span>
<span class="p">}</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></pre><p>To avoid this deadlock issue, <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> provides a cooperative execution method, <a href="classtf_1_1Executor.html#a8fcd9e0557922bb8194999f0cd433ea8" class="m-doc">tf::<wbr />Executor::<wbr />corun</a>, which allows a worker to execute a taskflow <em>cooperatively</em> with other workers within the same executor. Specifically, although the worker is blocking until corun finishes, it is <em>cooperatively</em> blocked and continues to execute the taskflow alongside other tasks in the executor's work-stealing loop. For instance, the following code with <a href="classtf_1_1Executor.html#a8fcd9e0557922bb8194999f0cd433ea8" class="m-doc">tf::<wbr />Executor::<wbr />corun</a> is deadlock-free:</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="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="n">std</span><span class="o">::</span><span class="n">array</span><span class="o"><</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">></span><span class="w"> </span><span class="n">others</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">atomic</span><span class="o"><</span><span class="kt">size_t</span><span class="o">></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="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"><</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="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o"><</span><span class="mi">500</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="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">&</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="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="o">&</span><span class="n">executor</span><span class="p">,</span><span class="w"> </span><span class="o">&</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="c1">// calling worker coruns the taskflow cooperatively with other workers</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">corun</span><span class="p">(</span><span class="n">tf</span><span class="p">);</span>
<span class="w"> </span><span class="p">});</span>
<span class="p">}</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></pre><p>Similar to <a href="classtf_1_1Executor.html#a8fcd9e0557922bb8194999f0cd433ea8" class="m-doc">tf::<wbr />Executor::<wbr />corun</a>, <a href="classtf_1_1Executor.html#a0fc6eb19f168dc4a9cd0a7c6187c1d2d" class="m-doc">tf::<wbr />Executor::<wbr />corun_until</a> provides a more general mechanism for cooperative execution. It allows the calling worker to execute tasks alongside other workers until the specified condition becomes true. This function is particularly useful when implementing custom synchronization or pooling patterns, where the worker should continue making progress on other available tasks while periodically checking for a termination condition.</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">&</span><span class="p">](){</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="n">executor</span><span class="p">.</span><span class="n">corun_until</span><span class="p">([](){</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="p">});</span>
<span class="p">});</span></pre><aside class="m-note m-warning"><h4>Attention</h4><p>You must call <a href="classtf_1_1Executor.html#a0fc6eb19f168dc4a9cd0a7c6187c1d2d" class="m-doc">tf::<wbr />Executor::<wbr />corun_until</a> and <a href="classtf_1_1Executor.html#a8fcd9e0557922bb8194999f0cd433ea8" class="m-doc">tf::<wbr />Executor::<wbr />corun</a> from a worker of the calling executor or an exception will be thrown.</p></aside></section><section id="ThreadSafetyOfExecution"><h2><a href="#ThreadSafetyOfExecution">Thread Safety of Executor</a></h2><p>All <code>run_*</code> methods of <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> are <em>thread-safe</em>. You can safely call these methods from multiple threads to execute different taskflows concurrently. However, the execution order of the submitted taskflows is non-deterministic and determined by the runtime scheduler.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">std</span><span class="o">::</span><span class="n">array</span><span class="o"><</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">10</span><span class="o">></span><span class="w"> </span><span class="n">taskflows</span><span class="p">;</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"><</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="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">&</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">taskflows</span><span class="p">[</span><span class="n">i</span><span class="p">]);</span><span class="w"> </span><span class="c1">// thread i runs taskflow i</span>
<span class="w"> </span><span class="p">}).</span><span class="n">detach</span><span class="p">();</span>
<span class="p">}</span>
<span class="n">executor</span><span class="p">.</span><span class="n">wait_for_all</span><span class="p">();</span></pre></section><section id="QueryTheWorkerID"><h2><a href="#QueryTheWorkerID">Query the Worker ID</a></h2><p>Each worker thread in a <a href="classtf_1_1Executor.html" class="m-doc">tf::<wbr />Executor</a> is assigned a <em>unique</em> integer identifier in the range <code>[0, N)</code>, where <code>N</code> is the number of worker threads in the executor. You can query the identifier of the calling thread using <a href="classtf_1_1Executor.html#a6487d589cb1f6b078b69fd3bb1082345" class="m-doc">tf::<wbr />Executor::<wbr />this_worker_id</a>. If the calling thread is not a worker of the executor, the method returns -1. This functionality is particularly useful for establishing a one-to-one mapping between worker threads and application-specific data structures.</p><pre class="m-code"><span class="n">std</span><span class="o">::</span><span class="n">vector</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">data</span><span class="p">[</span><span class="mi">8</span><span class="p">];</span><span class="w"> </span><span class="c1">// worker-specific data vector</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="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">// calling thread is not a worker of the executor</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&</span><span class="p">](){</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">&</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">data</span><span class="p">[</span><span class="n">id</span><span class="p">];</span><span class="w"> </span><span class="c1">// worker id process data[id]</span>
<span class="p">});</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> – 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="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="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="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="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="p">};</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"><taskflow/taskflow.hpp></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="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">&</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="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"constructing observer "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="sc">'\n'</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</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="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"setting up observer with "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">num_workers</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" workers</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="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="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="n">oss</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"worker "</span><span class="w"> </span><span class="o"><<</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"><<</span><span class="w"> </span><span class="s">" ready to run "</span><span class="w"> </span><span class="o"><<</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"><<</span><span class="w"> </span><span class="sc">'\n'</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"><<</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="p">}</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="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="n">oss</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"worker "</span><span class="w"> </span><span class="o"><<</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"><<</span><span class="w"> </span><span class="s">" finished running "</span><span class="w"> </span><span class="o"><<</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"><<</span><span class="w"> </span><span class="sc">'\n'</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"><<</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="p">}</span>
<span class="p">};</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="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="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="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"><<</span><span class="w"> </span><span class="s">"1</span><span class="se">\n</span><span class="s">"</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">"A"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"2</span><span class="se">\n</span><span class="s">"</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">"B"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"3</span><span class="se">\n</span><span class="s">"</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">"C"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"4</span><span class="se">\n</span><span class="s">"</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">"D"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"5</span><span class="se">\n</span><span class="s">"</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">"E"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"6</span><span class="se">\n</span><span class="s">"</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">"F"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"7</span><span class="se">\n</span><span class="s">"</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">"G"</span><span class="p">);</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"><<</span><span class="w"> </span><span class="s">"8</span><span class="se">\n</span><span class="s">"</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">"H"</span><span class="p">);</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"><</span><span class="n">MyObserver</span><span class="o">></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"><</span><span class="n">MyObserver</span><span class="o">></span><span class="p">(</span>
<span class="w"> </span><span class="s">"MyObserver"</span>
<span class="w"> </span><span class="p">);</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="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="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span></pre><p>The above code produces the following output:</p><pre class="m-code">constructing<span class="w"> </span>observer<span class="w"> </span>MyObserver
setting<span class="w"> </span>up<span class="w"> </span>observer<span class="w"> </span>with<span class="w"> </span><span class="m">4</span><span class="w"> </span>workers
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>A
<span class="m">1</span>
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>A
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>B
<span class="m">2</span>
worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>C
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>B
<span class="m">3</span>
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>D
worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>E
worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>C
<span class="m">4</span>
<span class="m">5</span>
worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>F
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>D
worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>E
<span class="m">6</span>
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>G
worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>ready<span class="w"> </span>to<span class="w"> </span>run<span class="w"> </span>H
worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>F
<span class="m">7</span>
<span class="m">8</span>
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>G
worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>finished<span class="w"> </span>running<span class="w"> </span>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 demonstrates the usage of <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::<wbr />WorkerInterface</a> to affine a worker to a specific CPU core equal to its id on a Linux platform:</p><pre class="m-code"><span class="c1">// affine the given thread to the given core index (linux-specific)</span>
<span class="kt">bool</span><span class="w"> </span><span class="nf">affine</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="kr">thread</span><span class="o">&</span><span class="w"> </span><span class="kr">thread</span><span class="p">,</span><span class="w"> </span><span class="kt">unsigned</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">core_id</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">cpu_set_t</span><span class="w"> </span><span class="n">cpuset</span><span class="p">;</span>
<span class="w"> </span><span class="n">CPU_ZERO</span><span class="p">(</span><span class="o">&</span><span class="n">cpuset</span><span class="p">);</span>
<span class="w"> </span><span class="n">CPU_SET</span><span class="p">(</span><span class="n">core_id</span><span class="p">,</span><span class="w"> </span><span class="o">&</span><span class="n">cpuset</span><span class="p">);</span>
<span class="w"> </span><span class="n">pthread_t</span><span class="w"> </span><span class="n">native_handle</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="kr">thread</span><span class="p">.</span><span class="n">native_handle</span><span class="p">();</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">pthread_setaffinity_np</span><span class="p">(</span><span class="n">native_handle</span><span class="p">,</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="kt">cpu_set_t</span><span class="p">),</span><span class="w"> </span><span class="o">&</span><span class="n">cpuset</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="p">}</span>
<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="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">&</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="n">printf</span><span class="p">(</span><span class="s">"worker %lu prepares to enter the work-stealing loop</span><span class="se">\n</span><span class="s">"</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="w"> </span><span class="c1">// now affine the worker to a particular CPU core equal to its id</span>
<span class="w"> </span><span class="k">if</span><span class="p">(</span><span class="n">affine</span><span class="p">(</span><span class="n">w</span><span class="p">.</span><span class="kr">thread</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="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"successfully affines worker %lu to CPU core %lu</span><span class="se">\n</span><span class="s">"</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="n">w</span><span class="p">.</span><span class="n">id</span><span class="p">());</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">else</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"failed to affine worker %lu to CPU core %lu</span><span class="se">\n</span><span class="s">"</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="n">w</span><span class="p">.</span><span class="n">id</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="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">&</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="n">printf</span><span class="p">(</span><span class="s">"worker %lu left the work-stealing loop</span><span class="se">\n</span><span class="s">"</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="p">}</span>
<span class="p">};</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="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">tf</span><span class="o">::</span><span class="n">make_worker_interface</span><span class="o"><</span><span class="n">CustomWorkerBehavior</span><span class="o">></span><span class="p">());</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="p">}</span></pre><p>When running the program, we see the following one possible output:</p><pre class="m-code">worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>prepares<span class="w"> </span>to<span class="w"> </span>enter<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
successfully<span class="w"> </span>affines<span class="w"> </span>worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>to<span class="w"> </span>CPU<span class="w"> </span>core<span class="w"> </span><span class="m">3</span>
worker<span class="w"> </span><span class="m">3</span><span class="w"> </span>left<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
worker<span class="w"> </span><span class="m">0</span><span class="w"> </span>prepares<span class="w"> </span>to<span class="w"> </span>enter<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
successfully<span class="w"> </span>affines<span class="w"> </span>worker<span class="w"> </span><span class="m">0</span><span class="w"> </span>to<span class="w"> </span>CPU<span class="w"> </span>core<span class="w"> </span><span class="m">0</span>
worker<span class="w"> </span><span class="m">0</span><span class="w"> </span>left<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>prepares<span class="w"> </span>to<span class="w"> </span>enter<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>prepares<span class="w"> </span>to<span class="w"> </span>enter<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
successfully<span class="w"> </span>affines<span class="w"> </span>worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>to<span class="w"> </span>CPU<span class="w"> </span>core<span class="w"> </span><span class="m">1</span>
worker<span class="w"> </span><span class="m">1</span><span class="w"> </span>left<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop
successfully<span class="w"> </span>affines<span class="w"> </span>worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>to<span class="w"> </span>CPU<span class="w"> </span>core<span class="w"> </span><span class="m">2</span>
worker<span class="w"> </span><span class="m">2</span><span class="w"> </span>left<span class="w"> </span>the<span class="w"> </span>work-stealing<span class="w"> </span>loop</pre><p>When you create an executor, it spawns a set of worker threads to run tasks using a work-stealing scheduling algorithm. The execution logic of the scheduler and its interaction with each spawned worker via <a href="classtf_1_1WorkerInterface.html" class="m-doc">tf::<wbr />WorkerInterface</a> is given below:</p><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"><</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="n">create_thread</span><span class="p">([](</span><span class="n">Worker</span><span class="o">&</span><span class="w"> </span><span class="n">worker</span><span class="p">)</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="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="n">worker_interface</span><span class="o">-></span><span class="n">scheduler_prologue</span><span class="p">(</span><span class="n">worker</span><span class="p">);</span>
<span class="w"> </span>
<span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">{</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="n">perform_work_stealing_algorithm</span><span class="p">();</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="k">break</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="p">}</span><span class="w"> </span><span class="k">catch</span><span class="p">(...)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">exception_ptr</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">current_exception</span><span class="p">();</span>
<span class="w"> </span><span class="p">}</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="n">worker_interface</span><span class="o">-></span><span class="n">scheduler_epilogue</span><span class="p">(</span><span class="n">worker</span><span class="p">,</span><span class="w"> </span><span class="n">exception_ptr</span><span class="p">);</span>
<span class="w"> </span><span class="p">);</span>
<span class="p">}</span></pre><aside class="m-note m-warning"><h4>Attention</h4><p><a href="classtf_1_1WorkerInterface.html#a41c3b931a36bde8eff4aa8d375e8888a" class="m-doc">tf::<wbr />WorkerInterface::<wbr />scheduler_prologue</a> and <a href="classtf_1_1WorkerInterface.html#a3e6d68fd4041f433d1b7ca9e5786b57c" class="m-doc">tf::<wbr />WorkerInterface::<wbr />scheduler_epilogue</a> are invoked by each worker simultaneously. It is your responsibility to ensure no data race can occur during their invocation.</p></aside></section>
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