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<h1>
<span class="m-breadcrumb"><a href="Cookbook.html">Cookbook</a> »</span>
Interact with the Runtime
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<h3>Contents</h3>
<ul>
<li><a href="#CreateARuntimeTask">Create a Runtime Object</a></li>
<li><a href="#AcquireTheRunningExecutor">Acquire the Running Executor</a></li>
<li><a href="#RuntimeTaskingRunATaskGraphSynchronously">Run a Task Graph Synchronously</a></li>
<li><a href="#LearnMoreAboutRuntime">Learn More About Runtime</a></li>
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<p>Taskflow allows you to interact with the scheduling runtime by taking a <em>runtime object</em> as an argument of a task. This is mostly useful for designing specialized parallel algorithms extended from the existing facility of Taskflow.</p><section id="CreateARuntimeTask"><h2><a href="#CreateARuntimeTask">Create a Runtime Object</a></h2><p>Taskflow allows a static task and a condition task to take a referenced <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a> object that provides a set of methods to interact with the scheduling runtime. The following example creates a static task that leverages <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a> to explicitly schedule a conditioned task which would never run under the normal scheduling circumstance:</p><pre class="m-code"><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="p">,</span><span class="w"> </span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">,</span><span class="w"> </span><span class="n">D</span><span class="p">;</span><span class="w"></span>
<span class="n">std</span><span class="o">::</span><span class="n">tie</span><span class="p">(</span><span class="n">A</span><span class="p">,</span><span class="w"> </span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">,</span><span class="w"> </span><span class="n">D</span><span class="p">)</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="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">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>
<span class="w"> </span><span class="p">[</span><span class="o">&</span><span class="n">C</span><span class="p">]</span><span class="w"> </span><span class="p">(</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&</span><span class="w"> </span><span class="n">rt</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// C must be captured by reference</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">"B</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">schedule</span><span class="p">(</span><span class="n">C</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="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">"C</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"></span>
<span class="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="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">"D</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">);</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="n">D</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><div class="m-graph"><svg style="width: 20.600rem; height: 12.700rem;" viewBox="0.00 0.00 206.00 127.00">
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</div><p>When the condition task <code>A</code> completes and returns <code>0</code>, the scheduler moves on to task <code>B</code>. Under the normal circumstance, tasks <code>C</code> and <code>D</code> will not run because their conditional dependencies never happen. This can be broken by forcefully scheduling <code>C</code> or/and <code>D</code> via a runtime object of a task that resides in the same graph. Here, task <code>B</code> call <a href="classtf_1_1Runtime.html#aa7e72cc0f298475195b252c8f1793343" class="m-doc">tf::<wbr />Runtime::<wbr />schedule</a> to forcefully run task <code>C</code> even though the weak dependency between <code>A</code> and <code>C</code> will never happen based on the graph structure itself. As a result, we will see both <code>B</code> and <code>C</code> in the output:</p><pre class="m-console"><span class="go">B # B leverages a runtime object to schedule C out of its dependency constraint</span>
<span class="go">C</span></pre><aside class="m-note m-warning"><h4>Attention</h4><p>You should only schedule an <em>active</em> task from a runtime object. An active task is a task in a running taskflow. The task may or may not be running, and scheduling that task will immediately put it into the task queue of the worker that is running the runtime object.</p></aside></section><section id="AcquireTheRunningExecutor"><h2><a href="#AcquireTheRunningExecutor">Acquire the Running Executor</a></h2><p>You can acquire the reference to the running executor using <a href="classtf_1_1Runtime.html#a4ee48a82df1f9758a999d18e6015cec4" class="m-doc">tf::<wbr />Runtime::<wbr />executor()</a>. The executor associated with a runtime object is the executor that runs the parent task of that runtime object.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span><span class="w"></span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</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="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span><span class="w"></span>
<span class="w"> </span><span class="n">assert</span><span class="p">(</span><span class="o">&</span><span class="p">(</span><span class="n">rt</span><span class="p">.</span><span class="n">executor</span><span class="p">())</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="o">&</span><span class="n">executor</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></section><section id="RuntimeTaskingRunATaskGraphSynchronously"><h2><a href="#RuntimeTaskingRunATaskGraphSynchronously">Run a Task Graph Synchronously</a></h2><p>A runtime object can spawn and run a task graph synchronously using <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a>. This model allows you to leverage dynamic tasking to execute a parallel workload within a runtime object. The following code creates a subflow of two independent tasks and executes it synchronously via the given runtime object:</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="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span><span class="w"></span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">corun</span><span class="p">([](</span><span class="n">tf</span><span class="o">::</span><span class="n">Subflow</span><span class="o">&</span><span class="w"> </span><span class="n">sf</span><span class="p">){</span><span class="w"></span>
<span class="w"> </span><span class="n">sf</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">"independent task 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="w"></span>
<span class="w"> </span><span class="n">sf</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">"independent task 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="w"></span>
<span class="w"> </span><span class="c1">// subflow joins upon corun returns</span>
<span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="p">});</span><span class="w"></span></pre><p>You can also create a task graph yourself and execute it through a runtime object. This organization avoids repetitive creation of a subflow with the same topology, such as running a runtime object repetitively. The following code performs the same execution logic as the above example but using the given task graph to avoid repetitive creations of a subflow:</p><pre class="m-code"><span class="c1">// create a custom graph</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">graph</span><span class="p">;</span><span class="w"></span>
<span class="n">graph</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">"independent task 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="w"></span>
<span class="n">graph</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">"independent task 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="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="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span><span class="w"> </span>
<span class="w"> </span><span class="c1">// this worker coruns the graph through its work-stealing loop</span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">corun</span><span class="p">(</span><span class="n">graph</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_n</span><span class="p">(</span><span class="n">taskflow</span><span class="p">,</span><span class="w"> </span><span class="mi">10000</span><span class="p">);</span><span class="w"></span></pre><p>Although <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a> blocks until the operation completes, the caller thread (worker) is not preempted (e.g., sleep or holding any lock). Instead, the caller thread joins the work-stealing loop of the executor and leaves whenever the spawned task graph completes. This is different from waiting for a submitted taskflow using tf::Future<T>::wait which blocks the caller thread until the submitted taskflow completes. When multiple submitted taskflows are being waited, their executions can potentially lead to deadlock. For example, the code below creates a taskflow of 1000 tasks with each task running a taskflow of 500 tasks in a blocking fashion:</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"><</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="w"></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="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"><</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"><</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="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="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">&</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="w"> </span><span class="c1">// blocking the worker can introduce deadlock where</span>
<span class="w"> </span><span class="c1">// all workers are waiting for their taskflows to finish</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="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>Using <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a> allows each worker to corun these taskflows through its work-stealing loop, thus avoiding deadlock problem caused by blocking wait.</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"><</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="w"></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="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"><</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"><</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="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="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">&</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="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span><span class="w"></span>
<span class="w"> </span><span class="c1">// the caller worker will not block but corun these</span>
<span class="w"> </span><span class="c1">// taskflows through its work-stealing loop</span>
<span class="w"> </span><span class="n">rt</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="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></section><section id="LearnMoreAboutRuntime"><h2><a href="#LearnMoreAboutRuntime">Learn More About Runtime</a></h2><p>t the following pages to learn more about <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a>:</p><ul><li><a href="AsyncTasking.html#LaunchAsynchronousTasksFromARuntime" class="m-doc">Launch Asynchronous Tasks from a Runtime</a></li></ul></section>
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