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 <compounddef id="RuntimeTasking" kind="page">
 <compoundname>RuntimeTasking</compoundname>
 <title>Runtime Tasking</title>
 <tableofcontents/>
 <briefdescription>
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 <detaileddescription>
<para>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.</para><sect1 id="RuntimeTasking_1CreateARuntimeTask">
<title>Create a Runtime Task</title>
<para>A runtime task is a callable that takes a reference to a <ref refid="classtf_1_1Runtime" kindref="compound">tf::Runtime</ref> object in its argument. A <ref refid="classtf_1_1Runtime" kindref="compound">tf::Runtime</ref> 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 <emphasis>forcefully</emphasis> schedule a conditioned task that would never happens:</para><para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>A,<sp/>B,<sp/>C,<sp/>D;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="cpp/utility/tuple/tie" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::tie</ref>(A,<sp/>B,<sp/>C,<sp/>D)<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>(</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>[]<sp/>()<sp/>{<sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>0;<sp/>},</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>[&amp;C]<sp/>(<ref refid="classtf_1_1Runtime" kindref="compound">tf::Runtime</ref>&amp;<sp/>rt)<sp/>{<sp/><sp/></highlight><highlight class="comment">//<sp/>C<sp/>must<sp/>be<sp/>captured<sp/>by<sp/>reference</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;B\n&quot;</highlight><highlight class="normal">;<sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>rt.<ref refid="classtf_1_1Runtime_1aa7e72cc0f298475195b252c8f1793343" kindref="member">schedule</ref>(C);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>},</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>[]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;C\n&quot;</highlight><highlight class="normal">;<sp/>},</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>[]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;D\n&quot;</highlight><highlight class="normal">;<sp/>}</highlight></codeline>
<codeline><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal">A.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(B,<sp/>C,<sp/>D);</highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow).wait();</highlight></codeline>
</programlisting></para><para><dotfile name="/home/twhuang/Code/taskflow/doxygen/images/runtime_task_1.dot"></dotfile>
</para><para>When the condition task <computeroutput>A</computeroutput> completes and returns <computeroutput>0</computeroutput>, the scheduler moves on to the runtime task <computeroutput>B</computeroutput>. Under the normal circumstance, tasks <computeroutput>C</computeroutput> and <computeroutput>D</computeroutput> will not run because their conditional dependencies never happen. This can be broken by forcefully scheduling <computeroutput>C</computeroutput> or/and <computeroutput>D</computeroutput> via a runtime task that resides in the same graph. Here, the runtime task <computeroutput>B</computeroutput> call <ref refid="classtf_1_1Runtime_1aa7e72cc0f298475195b252c8f1793343" kindref="member">tf::Runtime::schedule(tf::Task)</ref> to run task <computeroutput>C</computeroutput> even though the weak dependency between <computeroutput>A</computeroutput> and <computeroutput>C</computeroutput> will never happen based on the graph structure itself. As a result, we will see both <computeroutput>B</computeroutput> and <computeroutput>C</computeroutput> in the output:</para><para><programlisting filename=".shell-session"><codeline><highlight class="normal">B<sp/><sp/><sp/><sp/>#<sp/>B<sp/>is<sp/>a<sp/>runtime<sp/>task<sp/>to<sp/>schedule<sp/>C<sp/>out<sp/>of<sp/>its<sp/>dependency<sp/>constraint</highlight></codeline>
<codeline><highlight class="normal">C</highlight></codeline>
</programlisting></para><para><simplesect kind="attention"><para>You should only schedule an <emphasis>active</emphasis> 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.</para></simplesect>
</para></sect1>
<sect1 id="RuntimeTasking_1AcquireTheRunningExecutor">
<title>Acquire the Running Executor</title>
<para>You can acquire the reference to the running executor using <ref refid="classtf_1_1Runtime_1a4ee48a82df1f9758a999d18e6015cec4" kindref="member">tf::Runtime::executor()</ref>. The running executor of a runtime task is the executor that runs the parent taskflow of that runtime task.</para><para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](<ref refid="classtf_1_1Runtime" kindref="compound">tf::Runtime</ref>&amp;<sp/>rt){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>assert(&amp;(rt.<ref refid="classtf_1_1Runtime_1a4ee48a82df1f9758a999d18e6015cec4" kindref="member">executor</ref>())<sp/>==<sp/>&amp;executor);</highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow).wait();</highlight></codeline>
</programlisting></para></sect1>
<sect1 id="RuntimeTasking_1RuntimeTaskingRunATaskGraphSynchronously">
<title>Run a Task Graph Synchronously</title>
<para>A runtime task can spawn and run a task graph synchronously using <ref refid="classtf_1_1Runtime_1a479dc5dbf2b988bd166a2b6da089bb17" kindref="member">tf::Runtime::run_and_wait</ref>. 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:</para><para><programlisting filename=".cpp"><codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](<ref refid="classtf_1_1Runtime" kindref="compound">tf::Runtime</ref>&amp;<sp/>rt){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>rt.<ref refid="classtf_1_1Runtime_1a479dc5dbf2b988bd166a2b6da089bb17" kindref="member">run_and_wait</ref>([](<ref refid="classtf_1_1Subflow" kindref="compound">tf::Subflow</ref>&amp;<sp/>sf){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>sf.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;independent<sp/>task<sp/>1\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>sf.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;independent<sp/>task<sp/>2\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>subflow<sp/>joins<sp/>upon<sp/>run_and_wait<sp/>returns</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
</programlisting></para><para>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:</para><para><programlisting filename=".cpp"><codeline><highlight class="comment">//<sp/>create<sp/>a<sp/>custom<sp/>graph</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>graph;</highlight></codeline>
<codeline><highlight class="normal">graph.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;independent<sp/>task<sp/>1\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal">graph.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/home/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/>&lt;&lt;<sp/></highlight><highlight class="stringliteral">&quot;independent<sp/>task<sp/>2\n&quot;</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&amp;](<ref refid="classtf_1_1Runtime" kindref="compound">tf::Runtime</ref>&amp;<sp/>rt){<sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>rt.<ref refid="classtf_1_1Runtime_1a479dc5dbf2b988bd166a2b6da089bb17" kindref="member">run_and_wait</ref>(graph);<sp/><sp/></highlight><highlight class="comment">//<sp/>this<sp/>worker<sp/>thread<sp/>continues<sp/>the<sp/>work-stealing<sp/>loop</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a6d0617eebc9421f1ba1f82ce6dd02c00" kindref="member">run_n</ref>(taskflow,<sp/>10000);</highlight></codeline>
</programlisting></para><para>Although <ref refid="classtf_1_1Runtime_1a479dc5dbf2b988bd166a2b6da089bb17" kindref="member">tf::Runtime::run_and_wait</ref> 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 (<computeroutput>executor.run(taskflow).wait()</computeroutput>) 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 <ref refid="classtf_1_1Runtime_1a479dc5dbf2b988bd166a2b6da089bb17" kindref="member">tf::Runtime::run_and_wait</ref> avoids the deadlock problem. </para></sect1>
 </detaileddescription>
 </compounddef>
</doxygen>