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RuntimeTasking.html

RuntimeTasking.html 21.51 KB

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 Runtime Tasking
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 <h3>Contents</h3>
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 <li><a href="#CreateARuntimeTask">Create a Runtime Task</a></li>
 <li><a href="#AcquireTheRunningExecutor">Acquire the Running Executor</a></li>
 <li><a href="#RuntimeTaskingRunATaskGraphSynchronously">Run a Task Graph Synchronously</a></li>
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Taskflow allows you to interact with the scheduling runtime from the execution context of a runtime task. Runtime tasking is mostly used for designing specialized parallel algorithms that go beyond the default scheduling rules of taskflows.<section id="CreateARuntimeTask"><h2><a href="#CreateARuntimeTask">Create a Runtime Task</a></h2>A runtime task is a callable that takes a reference to a <a href="classtf_1_1Runtime.html" class="m-doc">tf::Runtime</a> object in its argument. A <a href="classtf_1_1Runtime.html" class="m-doc">tf::Runtime</a> object is created by the running executor and contains several methods for users to interact with the scheduling runtime. For instance, the following code creates a runtime task to forcefully schedule a conditioned task that would never happens:<pre class="m-code">tf::Task A, B, C, D;
std::tie(A, B, C, D) = taskflow.emplace(
 [] () { return 0; },
 [&amp;C] (tf::Runtime&amp; rt) { // C must be captured by reference
 std::cout &lt;&lt; &quot;B\n&quot;; 
 rt.schedule(C);
 },
 [] () { std::cout &lt;&lt; &quot;C\n&quot;; },
 [] () { std::cout &lt;&lt; &quot;D\n&quot;; }
);
A.precede(B, C, D);
executor.run(taskflow).wait();</pre><div class="m-graph"><svg style="width: 12.875rem; height: 9.438rem;" viewBox="0.00 0.00 206.00 150.77">
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</div>When the condition task <code>A</code> completes and returns <code>0</code>, the scheduler moves on to the runtime 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 task that resides in the same graph. Here, the runtime task <code>B</code> call <a href="classtf_1_1Runtime.html#aa7e72cc0f298475195b252c8f1793343" class="m-doc">tf::Runtime::schedule(tf::Task)</a> to 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:<pre class="m-console">B # B is a runtime task to schedule C out of its dependency constraint
C</pre><aside class="m-note m-warning"><h4>Attention</h4>You should only schedule an active task from a runtime task. 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 the task into the task queue of the worker that is running the runtime task.</aside></section><section id="AcquireTheRunningExecutor"><h2><a href="#AcquireTheRunningExecutor">Acquire the Running Executor</a></h2>You can acquire the reference to the running executor using <a href="classtf_1_1Runtime.html#a4ee48a82df1f9758a999d18e6015cec4" class="m-doc">tf::Runtime::executor()</a>. The running executor of a runtime task is the executor that runs the parent taskflow of that runtime task.<pre class="m-code">tf::Executor executor;
tf::Taskflow taskflow;
taskflow.emplace([&amp;](tf::Runtime&amp; rt){
 assert(&amp;(rt.executor()) == &amp;executor);
});
executor.run(taskflow).wait();</pre></section><section id="RuntimeTaskingRunATaskGraphSynchronously"><h2><a href="#RuntimeTaskingRunATaskGraphSynchronously">Run a Task Graph Synchronously</a></h2>A runtime task can spawn and run a task graph synchronously using <a href="classtf_1_1Runtime.html#a479dc5dbf2b988bd166a2b6da089bb17" class="m-doc">tf::Runtime::run_and_wait</a>. This model allows you to leverage dynamic tasking to execute a parallel workload within a runtime task. The following code creates a subflow of two independent tasks and executes it synchronously via the given runtime task:<pre class="m-code">taskflow.emplace([](tf::Runtime&amp; rt){
 rt.run_and_wait([](tf::Subflow&amp; sf){
 sf.emplace([](){ std::cout &lt;&lt; &quot;independent task 1\n&quot;; });
 sf.emplace([](){ std::cout &lt;&lt; &quot;independent task 2\n&quot;; });
 });
 // subflow joins upon run_and_wait returns
});</pre>You can also create a task graph yourself and execute it through a runtime task. This organization avoids repetitive creation of a subflow with the same topology, such as running a runtime task 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:<pre class="m-code">// create a custom graph
tf::Taskflow graph;
graph.emplace([](){ std::cout &lt;&lt; &quot;independent task 1\n&quot;; });
graph.emplace([](){ std::cout &lt;&lt; &quot;independent task 2\n&quot;; });

taskflow.emplace([&amp;](tf::Runtime&amp; rt){ 
 rt.run_and_wait(graph); // this worker thread continues the work-stealing loop
});
executor.run_n(taskflow, 10000);</pre>Although <a href="classtf_1_1Runtime.html#a479dc5dbf2b988bd166a2b6da089bb17" class="m-doc">tf::Runtime::run_and_wait</a> blocks until the operation completes, the caller thread (worker) is not blocked (e.g., sleep or holding any lock). Instead, the caller thread joins the work-stealing loop of the executor and returns whenever the spawned task graph completes. This is different from waiting for a submitted taskflow to finish (<code>executor.run(taskflow).wait()</code>) which blocks the caller thread until the submitted taskflow completes. When multiple submitted taskflows are being waited, their executions can potentially lead to deadlock. Using <a href="classtf_1_1Runtime.html#a479dc5dbf2b988bd166a2b6da089bb17" class="m-doc">tf::Runtime::run_and_wait</a> avoids the deadlock problem.</section>
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twhuang 提交于 2022年08月16日 02:24 +08:00 . revised doc

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<p>Taskflow allows you to interact with the scheduling runtime from the execution context of a runtime task. Runtime tasking is mostly used for designing specialized parallel algorithms that go beyond the default scheduling rules of taskflows.</p><section id="CreateARuntimeTask"><h2><a href="#CreateARuntimeTask">Create a Runtime Task</a></h2><p>A runtime task is a callable that takes a reference to a <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a> object in its argument. A <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a> object is created by the running executor and contains several methods for users to interact with the scheduling runtime. For instance, the following code creates a runtime task to <em>forcefully</em> schedule a conditioned task that would never happens:</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">&amp;</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">&amp;</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">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;B</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span>
<span class="w"> </span><span class="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">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;C</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"></span>
<span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;D</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="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: 12.875rem; height: 9.438rem;" viewBox="0.00 0.00 206.00 150.77">
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</div><p>When the condition task <code>A</code> completes and returns <code>0</code>, the scheduler moves on to the runtime 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 task that resides in the same graph. Here, the runtime task <code>B</code> call <a href="classtf_1_1Runtime.html#aa7e72cc0f298475195b252c8f1793343" class="m-doc">tf::<wbr />Runtime::<wbr />schedule(tf::<wbr />Task)</a> to 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 is a runtime task 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 task. 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 the task into the task queue of the worker that is running the runtime task.</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 running executor of a runtime task is the executor that runs the parent taskflow of that runtime task.</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">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&amp;</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">&amp;</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">&amp;</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 task can spawn and run a task graph synchronously using <a href="classtf_1_1Runtime.html#a479dc5dbf2b988bd166a2b6da089bb17" class="m-doc">tf::<wbr />Runtime::<wbr />run_and_wait</a>. This model allows you to leverage dynamic tasking to execute a parallel workload within a runtime task. The following code creates a subflow of two independent tasks and executes it synchronously via the given runtime task:</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">&amp;</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">run_and_wait</span><span class="p">([](</span><span class="n">tf</span><span class="o">::</span><span class="n">Subflow</span><span class="o">&amp;</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">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;independent task 1</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="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">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;independent task 2</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="p">});</span><span class="w"></span>
<span class="w"> </span><span class="c1">// subflow joins upon run_and_wait returns</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 task. This organization avoids repetitive creation of a subflow with the same topology, such as running a runtime task 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">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;independent task 1</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="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">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;independent task 2</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span><span class="w"></span>

<span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&amp;</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">run_and_wait</span><span class="p">(</span><span class="n">graph</span><span class="p">);</span><span class="w"> </span><span class="c1">// this worker thread continues the work-stealing loop</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#a479dc5dbf2b988bd166a2b6da089bb17" class="m-doc">tf::<wbr />Runtime::<wbr />run_and_wait</a> blocks until the operation completes, the caller thread (worker) is not blocked (e.g., sleep or holding any lock). Instead, the caller thread joins the work-stealing loop of the executor and returns whenever the spawned task graph completes. This is different from waiting for a submitted taskflow to finish (<code>executor.run(taskflow).wait()</code>) which blocks the caller thread until the submitted taskflow completes. When multiple submitted taskflows are being waited, their executions can potentially lead to deadlock. Using <a href="classtf_1_1Runtime.html#a479dc5dbf2b988bd166a2b6da089bb17" class="m-doc">tf::<wbr />Runtime::<wbr />run_and_wait</a> avoids the deadlock problem.</p></section>
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