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<?xml version='1.0' encoding='UTF-8' standalone='no'?>
<doxygen xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="compound.xsd" version="1.12.0" xml:lang="en-US">
<compounddef id="ExecuteTaskflow" kind="page">
<compoundname>ExecuteTaskflow</compoundname>
<title>Executor</title>
<tableofcontents>
<tocsect>
<name>Create an Executor</name>
<reference>ExecuteTaskflow_1CreateAnExecutor</reference>
</tocsect>
<tocsect>
<name>Understand Work Stealing in Executor</name>
<reference>ExecuteTaskflow_1UnderstandWorkStealingInExecutor</reference>
</tocsect>
<tocsect>
<name>Execute a Taskflow</name>
<reference>ExecuteTaskflow_1ExecuteATaskflow</reference>
</tocsect>
<tocsect>
<name>Execute a Taskflow with Transferred Ownership</name>
<reference>ExecuteTaskflow_1ExecuteATaskflowWithTransferredOwnership</reference>
</tocsect>
<tocsect>
<name>Execute a Taskflow from an Internal Worker Cooperatively</name>
<reference>ExecuteTaskflow_1ExecuteATaskflowFromAnInternalWorker</reference>
</tocsect>
<tocsect>
<name>Thread Safety of Executor</name>
<reference>ExecuteTaskflow_1ThreadSafetyOfExecution</reference>
</tocsect>
<tocsect>
<name>Query the Worker ID</name>
<reference>ExecuteTaskflow_1QueryTheWorkerID</reference>
</tocsect>
<tocsect>
<name>Observe Thread Activities</name>
<reference>ExecuteTaskflow_1ObserveThreadActivities</reference>
</tocsect>
<tocsect>
<name>Modify Worker Property</name>
<reference>ExecuteTaskflow_1ModifyWorkerProperty</reference>
</tocsect>
</tableofcontents>
<briefdescription>
</briefdescription>
<detaileddescription>
<para>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.</para>
<sect1 id="ExecuteTaskflow_1CreateAnExecutor">
<title>Create an Executor</title><para>To execute a taskflow, you need to create an <emphasis>executor</emphasis> of type <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref>. An executor is a <emphasis>thread-safe</emphasis> object that manages a set of worker threads and executes tasks through an efficient <emphasis>work-stealing</emphasis> algorithm. Issuing a call to run a taskflow creates a <emphasis>topology</emphasis>, a data structure to keep track of the execution status of a running graph. <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> takes an unsigned integer to construct with <computeroutput>N</computeroutput> worker threads. The default value is <ref refid="cpp/thread/thread/hardware_concurrency" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::thread::hardware_concurrency</ref>.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor1;<sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>create<sp/>an<sp/>executor<sp/>with<sp/>the<sp/>number<sp/>of<sp/>workers</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>equal<sp/>to<sp/>std::thread::hardware_concurrency</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor2(4);<sp/><sp/></highlight><highlight class="comment">//<sp/>create<sp/>an<sp/>executor<sp/>of<sp/>4<sp/>worker<sp/>threads</highlight></codeline>
</programlisting></para>
<para><simplesect kind="attention"><para>Creating a <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> has non-negligible overhead. Unless your application requires multiple executors, we recommend creating a single <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> and reusing it to run multiple taskflows.</para>
</simplesect>
</para>
</sect1>
<sect1 id="ExecuteTaskflow_1UnderstandWorkStealingInExecutor">
<title>Understand Work Stealing in Executor</title><para>Taskflow designs a highly efficient <emphasis>work-stealing</emphasis> 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 <emphasis>steal</emphasis> a task from the queue another worker (victim). The figure below illustrates the idea of work-stealing:</para>
<para><image type="html" name="work-stealing.png"></image>
</para>
<para>The key advantage of work-stealing lies in its <emphasis>decentralized</emphasis> 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 <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> 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 <emphasis>IEEE Transactions on Parallel and Distributed Systems (TPDS)</emphasis>:</para>
<para><itemizedlist>
<listitem><para>Tsung-Wei Huang, Dian-Lun Lin, Chun-Xun Lin, and Yibo Lin, "<ulink url="https://tsung-wei-huang.github.io/papers/tpds21-taskflow.pdf">Taskflow: A Lightweight Parallel and Heterogeneous Task Graph Computing System</ulink>," <emphasis>IEEE Transactions on Parallel and Distributed Systems (TPDS)</emphasis>, vol. 33, no. 6, pp. 1303-1320, June 2022</para>
</listitem></itemizedlist>
</para>
</sect1>
<sect1 id="ExecuteTaskflow_1ExecuteATaskflow">
<title>Execute a Taskflow</title><para><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> provides a set of <computeroutput>run_*</computeroutput> methods, <ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">tf::Executor::run</ref>, <ref refid="classtf_1_1Executor_1a6d0617eebc9421f1ba1f82ce6dd02c00" kindref="member">tf::Executor::run_n</ref>, and <ref refid="classtf_1_1Executor_1a0f52e9dd64b65aba32ca0e13c1ed300a" kindref="member">tf::Executor::run_until</ref> 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 <ref refid="classtf_1_1Future" kindref="compound">tf::Future</ref> for users to access the execution status. The code below shows several ways to run a taskflow.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><sp/>1:<sp/></highlight><highlight class="comment">//<sp/>Declare<sp/>an<sp/>executor<sp/>and<sp/>a<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/>2:<sp/><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor;</highlight></codeline>
<codeline><highlight class="normal"><sp/>3:<sp/><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><sp/>4:</highlight></codeline>
<codeline><highlight class="normal"><sp/>5:<sp/></highlight><highlight class="comment">//<sp/>Add<sp/>three<sp/>tasks<sp/>into<sp/>the<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/>6:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>A<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"This<sp/>is<sp/>TaskA\n"</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>7:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>B<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"This<sp/>is<sp/>TaskB\n"</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>8:<sp/><ref refid="classtf_1_1Task" kindref="compound">tf::Task</ref><sp/>C<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"This<sp/>is<sp/>TaskC\n"</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/>9:<sp/></highlight></codeline>
<codeline><highlight class="normal">10:<sp/></highlight><highlight class="comment">//<sp/>Build<sp/>precedence<sp/>between<sp/>tasks</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">11:<sp/>A.<ref refid="classtf_1_1Task_1a8c78c453295a553c1c016e4062da8588" kindref="member">precede</ref>(B,<sp/>C);<sp/></highlight></codeline>
<codeline><highlight class="normal">12:<sp/></highlight></codeline>
<codeline><highlight class="normal">13:<sp/><ref refid="classtf_1_1Future" kindref="compound">tf::Future<void></ref><sp/>fu<sp/>=<sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow);</highlight></codeline>
<codeline><highlight class="normal">14:<sp/>fu.wait();<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>block<sp/>until<sp/>the<sp/>execution<sp/>completes</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">15:</highlight></codeline>
<codeline><highlight class="normal">16:<sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow,<sp/>[](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"end<sp/>of<sp/>1<sp/>run"</highlight><highlight class="normal">;<sp/>}).wait();</highlight></codeline>
<codeline><highlight class="normal">17:<sp/>executor.<ref refid="classtf_1_1Executor_1a6d0617eebc9421f1ba1f82ce6dd02c00" kindref="member">run_n</ref>(taskflow,<sp/>4);</highlight></codeline>
<codeline><highlight class="normal">18:<sp/>executor.<ref refid="classtf_1_1Executor_1ab9aa252f70e9a40020a1e5a89d485b85" kindref="member">wait_for_all</ref>();<sp/><sp/></highlight><highlight class="comment">//<sp/>block<sp/>until<sp/>all<sp/>associated<sp/>executions<sp/>finish</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">19:<sp/>executor.<ref refid="classtf_1_1Executor_1a6d0617eebc9421f1ba1f82ce6dd02c00" kindref="member">run_n</ref>(taskflow,<sp/>4,<sp/>[](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"end<sp/>of<sp/>4<sp/>runs"</highlight><highlight class="normal">;<sp/>}).wait();</highlight></codeline>
<codeline><highlight class="normal">20:<sp/>executor.<ref refid="classtf_1_1Executor_1a0f52e9dd64b65aba32ca0e13c1ed300a" kindref="member">run_until</ref>(taskflow,<sp/>[cnt=0]<sp/>()<sp/></highlight><highlight class="keyword">mutable</highlight><highlight class="normal"><sp/>{<sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>++cnt<sp/>==<sp/>10;<sp/>});</highlight></codeline>
</programlisting></para>
<para>Debrief:</para>
<para><itemizedlist>
<listitem><para>Lines 6-8 create a taskflow of three tasks A, B, and C </para>
</listitem>
<listitem><para>Lines 13-14 run the taskflow once and wait for completion </para>
</listitem>
<listitem><para>Line 16 runs the taskflow once with a callback to invoke when the execution finishes </para>
</listitem>
<listitem><para>Lines 17-18 run the taskflow four times and use <ref refid="classtf_1_1Executor_1ab9aa252f70e9a40020a1e5a89d485b85" kindref="member">tf::Executor::wait_for_all</ref> to wait for completion </para>
</listitem>
<listitem><para>Line 19 runs the taskflow four times and invokes a callback at the end of the fourth execution </para>
</listitem>
<listitem><para>Line 20 keeps running the taskflow until the predicate returns true</para>
</listitem>
</itemizedlist>
Issuing multiple runs on the same taskflow will automatically <emphasis>synchronize</emphasis> to a sequential chain of executions in the order of run calls.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow);<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>execution<sp/>1</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a6d0617eebc9421f1ba1f82ce6dd02c00" kindref="member">run_n</ref>(taskflow,<sp/>10);<sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>execution<sp/>2</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow);<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>execution<sp/>3</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1ab9aa252f70e9a40020a1e5a89d485b85" kindref="member">wait_for_all</ref>();<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>execution<sp/>1<sp/>-><sp/>execution<sp/>2<sp/>-><sp/>execution<sp/>3</highlight></codeline>
</programlisting></para>
<para><simplesect kind="attention"><para>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.</para>
</simplesect>
<programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor;<sp/><sp/></highlight><highlight class="comment">//<sp/>create<sp/>an<sp/>executor</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>create<sp/>a<sp/>taskflow<sp/>whose<sp/>lifetime<sp/>is<sp/>restricted<sp/>by<sp/>the<sp/>scope</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>add<sp/>tasks<sp/>to<sp/>the<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>...<sp/></highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>run<sp/>the<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">}<sp/></highlight><highlight class="comment">//<sp/>leaving<sp/>the<sp/>scope<sp/>will<sp/>destroy<sp/>taskflow<sp/>while<sp/>it<sp/>is<sp/>running,<sp/></highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>resulting<sp/>in<sp/>undefined<sp/>behavior</highlight></codeline>
</programlisting></para>
<para>Similarly, you should avoid touching a taskflow while it is running:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>Add<sp/>tasks<sp/>into<sp/>the<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>...</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>Declare<sp/>an<sp/>executor</highlight><highlight class="normal"></highlight></codeline>
<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_1Future" kindref="compound">tf::Future<void></ref><sp/>future<sp/>=<sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow);<sp/></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>alter<sp/>the<sp/>taskflow<sp/>while<sp/>running<sp/>leads<sp/>to<sp/>undefined<sp/>behavior<sp/></highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"Add<sp/>a<sp/>new<sp/>task\n"</highlight><highlight class="normal">;<sp/>});</highlight></codeline>
</programlisting></para>
<para>You must always keep a taskflow alive and must not modify it while it is running on an executor.</para>
</sect1>
<sect1 id="ExecuteTaskflow_1ExecuteATaskflowWithTransferredOwnership">
<title>Execute a Taskflow with Transferred Ownership</title><para>You can transfer the ownership of a taskflow to an executor and run it without wrangling with the lifetime issue of that taskflow. Each <computeroutput>run_*</computeroutput> method discussed in the previous section comes with an overload that takes a <emphasis>moved</emphasis> taskflow object.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>let<sp/>the<sp/>executor<sp/>manage<sp/>the<sp/>lifetime<sp/>of<sp/>the<sp/>submitted<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(<ref refid="cpp/utility/move" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::move</ref>(taskflow));</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>now<sp/>taskflow<sp/>has<sp/>no<sp/>tasks</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">assert(taskflow.<ref refid="classtf_1_1Taskflow_1af4f03bca084deb5c2228ac8936d33649" kindref="member">num_tasks</ref>()<sp/>==<sp/>0);</highlight></codeline>
</programlisting></para>
<para>However, you should avoid moving a <emphasis>running</emphasis> taskflow which can result in undefined behavior.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([](){});</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>executor<sp/>does<sp/>not<sp/>manage<sp/>the<sp/>lifetime<sp/>of<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>error!<sp/>you<sp/>cannot<sp/>move<sp/>a<sp/>taskflow<sp/>while<sp/>it<sp/>is<sp/>running</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(<ref refid="cpp/utility/move" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::move</ref>(taskflow));<sp/><sp/></highlight></codeline>
</programlisting></para>
<para>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 <emphasis>after</emphasis> its execution completes.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="comment">//<sp/>submit<sp/>the<sp/>taskflow<sp/>and<sp/>wait<sp/>until<sp/>it<sp/>completes</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow).wait();</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>now<sp/>it's<sp/>safe<sp/>to<sp/>move<sp/>the<sp/>taskflow<sp/>to<sp/>the<sp/>executor<sp/>and<sp/>run<sp/>it</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(<ref refid="cpp/utility/move" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::move</ref>(taskflow));<sp/><sp/></highlight></codeline>
</programlisting></para>
<para>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.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="comment">//<sp/>submit<sp/>the<sp/>taskflow<sp/>and<sp/>wait<sp/>until<sp/>it<sp/>completes</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow).wait();</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="comment">//<sp/>now<sp/>it's<sp/>safe<sp/>to<sp/>move<sp/>the<sp/>taskflow<sp/>to<sp/>another</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>moved_taskflow(<ref refid="cpp/utility/move" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::move</ref>(taskflow));<sp/><sp/></highlight></codeline>
</programlisting></para>
</sect1>
<sect1 id="ExecuteTaskflow_1ExecuteATaskflowFromAnInternalWorker">
<title>Execute a Taskflow from an Internal Worker Cooperatively</title><para>Each run variant of <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> returns a <ref refid="classtf_1_1Future" kindref="compound">tf::Future</ref> object that allows you to wait for the associated execution to complete. When <computeroutput>tf::Future::wait</computeroutput> 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:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor(2);</highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="cpp/container/array" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::array<tf::Taskflow, 1000></ref><sp/>others;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>n=0;<sp/>n<1000;<sp/>n++)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>i=0;<sp/>i<500;<sp/>i++)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>others[n].emplace([&](){});</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&executor,<sp/>&<ref refid="namespacetf" kindref="compound">tf</ref>=others[n]](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>Blocking<sp/>a<sp/>worker<sp/>can<sp/>cause<sp/>deadlock<sp/>if<sp/>all<sp/>workers<sp/>are<sp/>waiting</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>for<sp/>their<sp/>taskflows<sp/>to<sp/>complete<sp/>without<sp/>making<sp/>any<sp/>progress</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(<ref refid="namespacetf" kindref="compound">tf</ref>).wait();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>});</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>
<para>To avoid this deadlock issue, <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> provides a cooperative execution method, <ref refid="classtf_1_1Executor_1a8fcd9e0557922bb8194999f0cd433ea8" kindref="member">tf::Executor::corun</ref>, which allows a worker to execute a taskflow <emphasis>cooperatively</emphasis> with other workers within the same executor. Specifically, although the worker is blocking until corun finishes, it is <emphasis>cooperatively</emphasis> blocked and continues to execute the taskflow alongside other tasks in the executor's work-stealing loop. For instance, the following code with <ref refid="classtf_1_1Executor_1a8fcd9e0557922bb8194999f0cd433ea8" kindref="member">tf::Executor::corun</ref> is deadlock-free:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor(2);</highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="cpp/container/array" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::array<tf::Taskflow, 1000></ref><sp/>others;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="cpp/atomic/atomic" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::atomic<size_t></ref><sp/>counter{0};</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>n=0;<sp/>n<1000;<sp/>n++)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>i=0;<sp/>i<500;<sp/>i++)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>others[n].emplace([&](){<sp/>counter++;<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&executor,<sp/>&<ref refid="namespacetf" kindref="compound">tf</ref>=others[n]](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>calling<sp/>worker<sp/>coruns<sp/>the<sp/>taskflow<sp/>cooperatively<sp/>with<sp/>other<sp/>workers</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a8fcd9e0557922bb8194999f0cd433ea8" kindref="member">corun</ref>(<ref refid="namespacetf" kindref="compound">tf</ref>);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>});</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>
<para>Similar to <ref refid="classtf_1_1Executor_1a8fcd9e0557922bb8194999f0cd433ea8" kindref="member">tf::Executor::corun</ref>, <ref refid="classtf_1_1Executor_1a0fc6eb19f168dc4a9cd0a7c6187c1d2d" kindref="member">tf::Executor::corun_until</ref> 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.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>fu<sp/>=<sp/><ref refid="cpp/thread/async" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::async</ref>([](){<sp/>std::sleep(100s);<sp/>});</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a0fc6eb19f168dc4a9cd0a7c6187c1d2d" kindref="member">corun_until</ref>([](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>fu.wait_for(<ref refid="cpp/chrono/duration" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::chrono::seconds</ref>(0))<sp/>==<sp/>future_status::ready;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>});</highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
</programlisting></para>
<para><simplesect kind="attention"><para>You must call <ref refid="classtf_1_1Executor_1a0fc6eb19f168dc4a9cd0a7c6187c1d2d" kindref="member">tf::Executor::corun_until</ref> and <ref refid="classtf_1_1Executor_1a8fcd9e0557922bb8194999f0cd433ea8" kindref="member">tf::Executor::corun</ref> from a worker of the calling executor or an exception will be thrown.</para>
</simplesect>
</para>
</sect1>
<sect1 id="ExecuteTaskflow_1ThreadSafetyOfExecution">
<title>Thread Safety of Executor</title><para>All <computeroutput>run_*</computeroutput> methods of <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> are <emphasis>thread-safe</emphasis>. 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.</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="cpp/container/array" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::array<tf::Taskflow, 10></ref><sp/>taskflows;</highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>i=0;<sp/>i<10;<sp/>++i)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="cpp/thread/thread" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::thread</ref>([i,<sp/>&](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflows[i]);<sp/><sp/></highlight><highlight class="comment">//<sp/>thread<sp/>i<sp/>runs<sp/>taskflow<sp/>i</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}).detach();</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
<codeline><highlight class="normal">executor.<ref refid="classtf_1_1Executor_1ab9aa252f70e9a40020a1e5a89d485b85" kindref="member">wait_for_all</ref>();</highlight></codeline>
</programlisting></para>
</sect1>
<sect1 id="ExecuteTaskflow_1QueryTheWorkerID">
<title>Query the Worker ID</title><para>Each worker thread in a <ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref> is assigned a <emphasis>unique</emphasis> integer identifier in the range <computeroutput>[0, N)</computeroutput>, where <computeroutput>N</computeroutput> is the number of worker threads in the executor. You can query the identifier of the calling thread using <ref refid="classtf_1_1Executor_1a6487d589cb1f6b078b69fd3bb1082345" kindref="member">tf::Executor::this_worker_id</ref>. 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.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="normal"><ref refid="cpp/container/vector" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::vector<int></ref><sp/>data[8];<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>worker-specific<sp/>data<sp/>vector</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor(8);<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>an<sp/>executor<sp/>of<sp/>eight<sp/>workers</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">assert(executor.<ref refid="classtf_1_1Executor_1a6487d589cb1f6b078b69fd3bb1082345" kindref="member">this_worker_id</ref>()<sp/>==<sp/>-1);<sp/><sp/></highlight><highlight class="comment">//<sp/>calling<sp/>thread<sp/>is<sp/>not<sp/>a<sp/>worker<sp/>of<sp/>the<sp/>executor</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([&](){</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/></highlight><highlight class="keywordtype">id</highlight><highlight class="normal"><sp/>=<sp/>executor.<ref refid="classtf_1_1Executor_1a6487d589cb1f6b078b69fd3bb1082345" kindref="member">this_worker_id</ref>();<sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>in<sp/>the<sp/>range<sp/>[0,<sp/>8)</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal">&<sp/>vec<sp/>=<sp/>data[id];<sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>worker<sp/>id<sp/>process<sp/>data[id]</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">});</highlight></codeline>
</programlisting></para>
</sect1>
<sect1 id="ExecuteTaskflow_1ObserveThreadActivities">
<title>Observe Thread Activities</title><para>You can observe thread activities in an executor when a worker thread participates in executing a task and leaves the execution using <ref refid="classtf_1_1ObserverInterface" kindref="compound">tf::ObserverInterface</ref> <ndash/> an <emphasis>interface</emphasis> 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.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="keyword">class<sp/></highlight><highlight class="normal">ObserverInterface<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">virtual</highlight><highlight class="normal"><sp/>~ObserverInterface()<sp/>=<sp/></highlight><highlight class="keywordflow">default</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">virtual</highlight><highlight class="normal"><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/>set_up(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>num_workers)<sp/>=<sp/>0;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">virtual</highlight><highlight class="normal"><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/>on_entry(<ref refid="classtf_1_1WorkerView" kindref="compound">tf::WorkerView</ref><sp/>worker_view,<sp/><ref refid="classtf_1_1TaskView" kindref="compound">tf::TaskView</ref><sp/>task_view)<sp/>=<sp/>0;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">virtual</highlight><highlight class="normal"><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/>on_exit(<ref refid="classtf_1_1WorkerView" kindref="compound">tf::WorkerView</ref><sp/>worker_view,<sp/><ref refid="classtf_1_1TaskView" kindref="compound">tf::TaskView</ref><sp/>task_view)<sp/>=<sp/>0;</highlight></codeline>
<codeline><highlight class="normal">};</highlight></codeline>
</programlisting></para>
<para>There are three methods you must define in your derived class, <ref refid="classtf_1_1ObserverInterface_1a41e6e62f12bf9d9dc4fa74632f6825d9" kindref="member">tf::ObserverInterface::set_up</ref>, <ref refid="classtf_1_1ObserverInterface_1a8225fcacb03089677a1efc4b16b734cc" kindref="member">tf::ObserverInterface::on_entry</ref>, and <ref refid="classtf_1_1ObserverInterface_1aa22f5378154653f08d9a58326bda4754" kindref="member">tf::ObserverInterface::on_exit</ref>. The method, <ref refid="classtf_1_1ObserverInterface_1a41e6e62f12bf9d9dc4fa74632f6825d9" kindref="member">tf::ObserverInterface::set_up</ref>, 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, <ref refid="classtf_1_1ObserverInterface_1a8225fcacb03089677a1efc4b16b734cc" kindref="member">tf::ObserverInterface::on_entry</ref> and <ref refid="classtf_1_1ObserverInterface_1aa22f5378154653f08d9a58326bda4754" kindref="member">tf::ObserverInterface::on_exit</ref>, 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 <ref refid="classtf_1_1WorkerView" kindref="compound">tf::WorkerView</ref> and <ref refid="classtf_1_1TaskView" kindref="compound">tf::TaskView</ref>. You may use them to record timepoints and calculate the elapsed time of a task.</para>
<para>You can associate an executor with one or multiple observers (though one is common) using <ref refid="classtf_1_1Executor_1aff77def96ae740d648dd84e571237c83" kindref="member">tf::Executor::make_observer</ref>. We use <ref refid="cpp/memory/shared_ptr" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::shared_ptr</ref> to manage the ownership of an observer. The executor loops through each observer and invoke the corresponding methods accordingly.</para>
<para><programlisting filename=".cpp"><codeline><highlight class="preprocessor">#include<sp/><<ref refid="taskflow_8hpp" kindref="compound">taskflow/taskflow.hpp</ref>></highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keyword">struct<sp/></highlight><highlight class="normal">MyObserver<sp/>:<sp/></highlight><highlight class="keyword">public</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1ObserverInterface" kindref="compound">tf::ObserverInterface</ref><sp/>{</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>MyObserver(</highlight><highlight class="keyword">const</highlight><highlight class="normal"><sp/><ref refid="cpp/string/basic_string" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::string</ref>&<sp/>name)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"constructing<sp/>observer<sp/>"</highlight><highlight class="normal"><sp/><<<sp/>name<sp/><<<sp/></highlight><highlight class="charliteral">'\n'</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1ObserverInterface_1a41e6e62f12bf9d9dc4fa74632f6825d9" kindref="member">set_up</ref>(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>num_workers)<sp/></highlight><highlight class="keyword">override</highlight><highlight class="normal"><sp/></highlight><highlight class="keyword">final</highlight><highlight class="normal"><sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"setting<sp/>up<sp/>observer<sp/>with<sp/>"</highlight><highlight class="normal"><sp/><<<sp/>num_workers<sp/><<<sp/></highlight><highlight class="stringliteral">"<sp/>workers\n"</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1ObserverInterface_1a8225fcacb03089677a1efc4b16b734cc" kindref="member">on_entry</ref>(<ref refid="classtf_1_1WorkerView" kindref="compound">tf::WorkerView</ref><sp/>w,<sp/><ref refid="classtf_1_1TaskView" kindref="compound">tf::TaskView</ref><sp/>tv)<sp/></highlight><highlight class="keyword">override</highlight><highlight class="normal"><sp/></highlight><highlight class="keyword">final</highlight><highlight class="normal"><sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostringstream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::ostringstream</ref><sp/>oss;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>oss<sp/><<<sp/></highlight><highlight class="stringliteral">"worker<sp/>"</highlight><highlight class="normal"><sp/><<<sp/>w.id()<sp/><<<sp/></highlight><highlight class="stringliteral">"<sp/>ready<sp/>to<sp/>run<sp/>"</highlight><highlight class="normal"><sp/><<<sp/>tv.name()<sp/><<<sp/></highlight><highlight class="charliteral">'\n'</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/>oss.str();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1ObserverInterface_1aa22f5378154653f08d9a58326bda4754" kindref="member">on_exit</ref>(<ref refid="classtf_1_1WorkerView" kindref="compound">tf::WorkerView</ref><sp/>w,<sp/><ref refid="classtf_1_1TaskView" kindref="compound">tf::TaskView</ref><sp/>tv)<sp/></highlight><highlight class="keyword">override</highlight><highlight class="normal"><sp/></highlight><highlight class="keyword">final</highlight><highlight class="normal"><sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostringstream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::ostringstream</ref><sp/>oss;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>oss<sp/><<<sp/></highlight><highlight class="stringliteral">"worker<sp/>"</highlight><highlight class="normal"><sp/><<<sp/>w.id()<sp/><<<sp/></highlight><highlight class="stringliteral">"<sp/>finished<sp/>running<sp/>"</highlight><highlight class="normal"><sp/><<<sp/>tv.name()<sp/><<<sp/></highlight><highlight class="charliteral">'\n'</highlight><highlight class="normal">;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/>oss.str();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal">};</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>main(){</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor(4);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>Create<sp/>a<sp/>taskflow<sp/>of<sp/>eight<sp/>tasks</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="classtf_1_1Taskflow" kindref="compound">tf::Taskflow</ref><sp/>taskflow;</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>A<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"1\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"A"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>B<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"2\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"B"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>C<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"3\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"C"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>D<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"4\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"D"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>E<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"5\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"E"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>F<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"6\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"F"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>G<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"7\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"G"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">auto</highlight><highlight class="normal"><sp/>H<sp/>=<sp/>taskflow.<ref refid="classtf_1_1FlowBuilder_1a60d7a666cab71ecfa3010b2efb0d6b57" kindref="member">emplace</ref>([]<sp/>()<sp/>{<sp/><ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref><sp/><<<sp/></highlight><highlight class="stringliteral">"8\n"</highlight><highlight class="normal">;<sp/>}).name(</highlight><highlight class="stringliteral">"H"</highlight><highlight class="normal">);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>create<sp/>an<sp/>observer</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="cpp/memory/shared_ptr" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::shared_ptr<MyObserver></ref><sp/>observer<sp/>=<sp/>executor.<ref refid="classtf_1_1Executor_1aff77def96ae740d648dd84e571237c83" kindref="member">make_observer</ref><MyObserver>(</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="stringliteral">"MyObserver"</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>);</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>run<sp/>the<sp/>taskflow</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a519777f5783981d534e9e53b99712069" kindref="member">run</ref>(taskflow).get();</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>remove<sp/>the<sp/>observer<sp/>(optional)</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>executor.<ref refid="classtf_1_1Executor_1a31081f492c376f7b798de0e430534531" kindref="member">remove_observer</ref>(<ref refid="cpp/utility/move" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::move</ref>(observer));</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>0;</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
</programlisting></para>
<para>The above code produces the following output:</para>
<para><programlisting filename=".bash"><codeline><highlight class="normal">constructing<sp/>observer<sp/>MyObserver</highlight></codeline>
<codeline><highlight class="normal">setting<sp/>up<sp/>observer<sp/>with<sp/>4<sp/>workers</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>ready<sp/>to<sp/>run<sp/>A</highlight></codeline>
<codeline><highlight class="normal">1</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>finished<sp/>running<sp/>A</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>ready<sp/>to<sp/>run<sp/>B</highlight></codeline>
<codeline><highlight class="normal">2</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>1<sp/>ready<sp/>to<sp/>run<sp/>C</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>finished<sp/>running<sp/>B</highlight></codeline>
<codeline><highlight class="normal">3</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>ready<sp/>to<sp/>run<sp/>D</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>3<sp/>ready<sp/>to<sp/>run<sp/>E</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>1<sp/>finished<sp/>running<sp/>C</highlight></codeline>
<codeline><highlight class="normal">4</highlight></codeline>
<codeline><highlight class="normal">5</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>1<sp/>ready<sp/>to<sp/>run<sp/>F</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>finished<sp/>running<sp/>D</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>3<sp/>finished<sp/>running<sp/>E</highlight></codeline>
<codeline><highlight class="normal">6</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>ready<sp/>to<sp/>run<sp/>G</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>3<sp/>ready<sp/>to<sp/>run<sp/>H</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>1<sp/>finished<sp/>running<sp/>F</highlight></codeline>
<codeline><highlight class="normal">7</highlight></codeline>
<codeline><highlight class="normal">8</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>finished<sp/>running<sp/>G</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>3<sp/>finished<sp/>running<sp/>H</highlight></codeline>
</programlisting></para>
<para>It is expected each line of <ref refid="cpp/io/basic_ostream" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::cout</ref> interleaves with each other as there are four workers participating in task scheduling. However, the <emphasis>ready</emphasis> message always appears before the corresponding task message (e.g., numbers) and then the <emphasis>finished</emphasis> message.</para>
</sect1>
<sect1 id="ExecuteTaskflow_1ModifyWorkerProperty">
<title>Modify Worker Property</title><para>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 <ref refid="classtf_1_1WorkerInterface" kindref="compound">tf::WorkerInterface</ref> to the executor. The example demonstrates the usage of <ref refid="classtf_1_1WorkerInterface" kindref="compound">tf::WorkerInterface</ref> to affine a worker to a specific CPU core equal to its id on a Linux platform:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="comment">//<sp/>affine<sp/>the<sp/>given<sp/>thread<sp/>to<sp/>the<sp/>given<sp/>core<sp/>index<sp/>(linux-specific)</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordtype">bool</highlight><highlight class="normal"><sp/>affine(<ref refid="cpp/thread/thread" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::thread</ref>&<sp/>thread,<sp/></highlight><highlight class="keywordtype">unsigned</highlight><highlight class="normal"><sp/></highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>core_id)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>cpu_set_t<sp/>cpuset;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>CPU_ZERO(&cpuset);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>CPU_SET(core_id,<sp/>&cpuset);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>pthread_t<sp/>native_handle<sp/>=<sp/>thread.native_handle();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>pthread_setaffinity_np(native_handle,<sp/></highlight><highlight class="keyword">sizeof</highlight><highlight class="normal">(cpu_set_t),<sp/>&cpuset)<sp/>==<sp/>0;</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keyword">class<sp/></highlight><highlight class="normal">CustomWorkerBehavior<sp/>:<sp/></highlight><highlight class="keyword">public</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1WorkerInterface" kindref="compound">tf::WorkerInterface</ref><sp/>{</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keyword">public</highlight><highlight class="normal">:</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>to<sp/>call<sp/>before<sp/>the<sp/>worker<sp/>enters<sp/>the<sp/>scheduling<sp/>loop</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1WorkerInterface_1a41c3b931a36bde8eff4aa8d375e8888a" kindref="member">scheduler_prologue</ref>(<ref refid="classtf_1_1Worker" kindref="compound">tf::Worker</ref>&<sp/>w)</highlight><highlight class="keyword"><sp/>override<sp/></highlight><highlight class="normal">{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/c/fprintf" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">printf</ref>(</highlight><highlight class="stringliteral">"worker<sp/>%lu<sp/>prepares<sp/>to<sp/>enter<sp/>the<sp/>work-stealing<sp/>loop\n"</highlight><highlight class="normal">,<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>());</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>now<sp/>affine<sp/>the<sp/>worker<sp/>to<sp/>a<sp/>particular<sp/>CPU<sp/>core<sp/>equal<sp/>to<sp/>its<sp/>id</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="keywordflow">if</highlight><highlight class="normal">(affine(w.<ref refid="classtf_1_1Worker_1a6158f91db3b980e3072cc0329cbe3c14" kindref="member">thread</ref>(),<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>()))<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><ref refid="cpp/io/c/fprintf" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">printf</ref>(</highlight><highlight class="stringliteral">"successfully<sp/>affines<sp/>worker<sp/>%lu<sp/>to<sp/>CPU<sp/>core<sp/>%lu\n"</highlight><highlight class="normal">,<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>(),<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>());</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="keywordflow">else</highlight><highlight class="normal"><sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><ref refid="cpp/io/c/fprintf" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">printf</ref>(</highlight><highlight class="stringliteral">"failed<sp/>to<sp/>affine<sp/>worker<sp/>%lu<sp/>to<sp/>CPU<sp/>core<sp/>%lu\n"</highlight><highlight class="normal">,<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>(),<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>());</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="comment">//<sp/>to<sp/>call<sp/>after<sp/>the<sp/>worker<sp/>leaves<sp/>the<sp/>scheduling<sp/>loop</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordtype">void</highlight><highlight class="normal"><sp/><ref refid="classtf_1_1WorkerInterface_1a3e6d68fd4041f433d1b7ca9e5786b57c" kindref="member">scheduler_epilogue</ref>(<ref refid="classtf_1_1Worker" kindref="compound">tf::Worker</ref>&<sp/>w,<sp/><ref refid="cpp/error/exception_ptr" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::exception_ptr</ref>)</highlight><highlight class="keyword"><sp/>override<sp/></highlight><highlight class="normal">{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><ref refid="cpp/io/c/fprintf" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">printf</ref>(</highlight><highlight class="stringliteral">"worker<sp/>%lu<sp/>left<sp/>the<sp/>work-stealing<sp/>loop\n"</highlight><highlight class="normal">,<sp/>w.<ref refid="classtf_1_1Worker_1a0180ea51cc46551157eaae451b50c7d8" kindref="member">id</ref>());</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal">};</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight><highlight class="keywordtype">int</highlight><highlight class="normal"><sp/>main()<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><ref refid="classtf_1_1Executor" kindref="compound">tf::Executor</ref><sp/>executor(4,<sp/><ref refid="namespacetf_1aa10195f7d5f2f1dd32bb852a9aa560f4" kindref="member">tf::make_worker_interface<CustomWorkerBehavior></ref>());</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/></highlight><highlight class="keywordflow">return</highlight><highlight class="normal"><sp/>0;</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
</programlisting></para>
<para>When running the program, we see the following one possible output:</para>
<para><programlisting filename=".bash"><codeline><highlight class="normal">worker<sp/>3<sp/>prepares<sp/>to<sp/>enter<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">successfully<sp/>affines<sp/>worker<sp/>3<sp/>to<sp/>CPU<sp/>core<sp/>3</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>3<sp/>left<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>0<sp/>prepares<sp/>to<sp/>enter<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">successfully<sp/>affines<sp/>worker<sp/>0<sp/>to<sp/>CPU<sp/>core<sp/>0</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>0<sp/>left<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>1<sp/>prepares<sp/>to<sp/>enter<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>prepares<sp/>to<sp/>enter<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">successfully<sp/>affines<sp/>worker<sp/>1<sp/>to<sp/>CPU<sp/>core<sp/>1</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>1<sp/>left<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
<codeline><highlight class="normal">successfully<sp/>affines<sp/>worker<sp/>2<sp/>to<sp/>CPU<sp/>core<sp/>2</highlight></codeline>
<codeline><highlight class="normal">worker<sp/>2<sp/>left<sp/>the<sp/>work-stealing<sp/>loop</highlight></codeline>
</programlisting></para>
<para>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 <ref refid="classtf_1_1WorkerInterface" kindref="compound">tf::WorkerInterface</ref> is given below:</para>
<para><programlisting filename=".cpp"><codeline><highlight class="keywordflow">for</highlight><highlight class="normal">(</highlight><highlight class="keywordtype">size_t</highlight><highlight class="normal"><sp/>n=0;<sp/>n<num_workers;<sp/>n++)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>create_thread([](Worker&<sp/>worker)</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>pre-processing<sp/>executor-specific<sp/>worker<sp/>information</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>...</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>enter<sp/>the<sp/>scheduling<sp/>loop</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>Here,<sp/>WorkerInterface::scheduler_prologue<sp/>is<sp/>invoked,<sp/>if<sp/>any</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>worker_interface->scheduler_prologue(worker);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="keywordflow">try</highlight><highlight class="normal"><sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/>while(1)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/>perform_work_stealing_algorithm();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/>if(stop)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/>break;</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>}<sp/></highlight><highlight class="keywordflow">catch</highlight><highlight class="normal">(...)<sp/>{</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/><sp/><sp/>exception_ptr<sp/>=<sp/><ref refid="cpp/error/current_exception" kindref="compound" external="/Users/twhuang/Code/taskflow/doxygen/cppreference-doxygen-web.tag.xml">std::current_exception</ref>();</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>}</highlight></codeline>
<codeline><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>leaves<sp/>the<sp/>scheduling<sp/>loop<sp/>and<sp/>joins<sp/>this<sp/>worker<sp/>thread</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/></highlight><highlight class="comment">//<sp/>Here,<sp/>WorkerInterface::scheduler_epilogue<sp/>is<sp/>invoked,<sp/>if<sp/>any</highlight><highlight class="normal"></highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/><sp/><sp/>worker_interface->scheduler_epilogue(worker,<sp/>exception_ptr);</highlight></codeline>
<codeline><highlight class="normal"><sp/><sp/>);</highlight></codeline>
<codeline><highlight class="normal">}</highlight></codeline>
</programlisting></para>
<para><simplesect kind="attention"><para><ref refid="classtf_1_1WorkerInterface_1a41c3b931a36bde8eff4aa8d375e8888a" kindref="member">tf::WorkerInterface::scheduler_prologue</ref> and <ref refid="classtf_1_1WorkerInterface_1a3e6d68fd4041f433d1b7ca9e5786b57c" kindref="member">tf::WorkerInterface::scheduler_epilogue</ref> are invoked by each worker simultaneously. It is your responsibility to ensure no data race can occur during their invocation. </para>
</simplesect>
</para>
</sect1>
</detaileddescription>
<location file="doxygen/cookbook/executor.dox"/>
</compounddef>
</doxygen>