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  <title>System execution context</title>
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<h1 class="title" style="text-align:center">System execution context</h1>

<table style="border:none;float:right">
  <tr>
    <td>Document #:</td>
    <td>P2079R2</td>
  </tr>
  <tr>
    <td>Date:</td>
    <td>2022-01-14</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Project:</td>
    <td>Programming Language C++</td>
  </tr>
  <tr>
    <td style="vertical-align:top">Audience:</td>
    <td>
      SG1, LEWG<br>
    </td>
  </tr>
  <tr>
    <td style="vertical-align:top">Reply-to:</td>
    <td>
      Lee Howes<br>&lt;<a href="mailto:lwh@fb.com" class="email">lwh@fb.com</a>&gt;<br>
      Ruslan Arutyunyan<br>&lt;<a href="mailto:ruslan.arutyunyan@intel.com" class="email">ruslan.arutyunyan@intel.com</a>&gt;<br>
      Michael Voss<br>&lt;<a href="mailto:michaelj.voss@intel.com" class="email">michaelj.voss@intel.com</a>&gt;<br>
    </td>
  </tr>
</table>

</header>
<div style="clear:both">
<h1 data-number="1" id="abtract"><span class="header-section-number">1</span> Abtract<a href="#abtract" class="self-link"></a></h1>
<p>A <code class="sourceCode default">system_context</code> and <code class="sourceCode default">system_scheduler</code> that expose a simple parallel-forward-progress thread pool that may share and expose an underlying system thread pool and is intended to be the basic execution context and scheduler that we recommend to be used in combination with <span class="citation" data-cites="P2300">[<a href="#ref-P2300" role="doc-biblioref">P2300R4</a>]</span>.</p>
<h1 data-number="2" id="changes"><span class="header-section-number">2</span> Changes<a href="#changes" class="self-link"></a></h1>
<h2 data-number="2.1" id="r2"><span class="header-section-number">2.1</span> R2<a href="#r2" class="self-link"></a></h2>
<ul>
<li>Significant redesign to fit in P2300 model.</li>
<li>Strictly limit to parallel progress without control over the level of parallelism.</li>
<li>Remove direct support for task groups, delegating that to <code class="sourceCode default">async_scope</code>.</li>
</ul>
<h2 data-number="2.2" id="r1"><span class="header-section-number">2.2</span> R1<a href="#r1" class="self-link"></a></h2>
<ul>
<li>Minor modifications</li>
</ul>
<h2 data-number="2.3" id="r0"><span class="header-section-number">2.3</span> R0<a href="#r0" class="self-link"></a></h2>
<ul>
<li>first revision</li>
</ul>
<h1 data-number="3" id="introduction"><span class="header-section-number">3</span> Introduction<a href="#introduction" class="self-link"></a></h1>
<p><span class="citation" data-cites="P2300">[<a href="#ref-P2300" role="doc-biblioref">P2300R4</a>]</span> describes a rounded set of primitives for asynchronous and parallel execution that give a firm grounding for the future. However, the paper lacks a standard execution context and scheduler. It has been broadly accepted that we need some sort of standard scheduler.</p>
<p>As noted in <span class="citation" data-cites="P2079R1">[<a href="#ref-P2079R1" role="doc-biblioref">P2079R1</a>]</span>, an earlier revision of this paper, the earlier <code class="sourceCode default">static_thread_pool</code> had many shortcomings. This was removed from <span class="citation" data-cites="P2300">[<a href="#ref-P2300" role="doc-biblioref">P2300R4</a>]</span> based on that and other input.</p>
<p>This revision updates <span class="citation" data-cites="P2079R1">[<a href="#ref-P2079R1" role="doc-biblioref">P2079R1</a>]</span> to match the structure of <span class="citation" data-cites="P2300">[<a href="#ref-P2300" role="doc-biblioref">P2300R4</a>]</span>. It aims to provide a simple, flexible, standard execution context that should be used as the basis for examples. It is a minimal design, with few constraints, and as such should be efficient to implement on top of something like a static thread pool, but also on top of system thread pools where fixing the number of threads diverges from efficient implementation goals.</p>
<p>Lifetime management and other functionality is delegated to other papers, primarily to the <code class="sourceCode default">async_scope</code> defined in <span class="citation" data-cites="P2519">[<a href="#ref-P2519" role="doc-biblioref">P2519R0</a>]</span>. Unlike in earlier verisons of this paper, we do not provide support for waiting on groups of tasks, delegating that to the separate <code class="sourceCode default">async_scope</code> design in <span class="citation" data-cites="P2519">[<a href="#ref-P2519" role="doc-biblioref">P2519R0</a>]</span>, because that is not functionality specific to a system context.</p>
<p>The system context is of undefined size, supporting explicitly <em>parallel forward progress</em>. By requiring only parallel forward progress, any created parallel context is able to be a view onto the underlying shared global context. All instances of the <code class="sourceCode default">system_context</code> share the same underlying execution context. If the underlying context is a static thread pool, then all <code class="sourceCode default">system_context</code>s should reference that same static thread pool. This is important to ensure that applications can rely on constructing <code class="sourceCode default">system_context</code>s as necessary, without spawning an ever increasing number of threads. It also means that there is no isolation between <code class="sourceCode default">system_context</code> instances, which people should be aware of when they use this functionality. Note that if they rely strictly on parallel forward progress, this is not a problem, and is generally a safe way to develop applications.</p>
<p>The minimal extensions to basic parallel forward progress are to support fundamental functionality that is necessary to make parallel algorithms work:</p>
<ul>
<li>Cancellation: work submitted through the parallel context must be cancellable.</li>
<li>Forward progress delegation: we must be able to implement a blocking operation that ensures forward progress of a complex parallel algorithm without special cases.</li>
</ul>
<p>In addition, early feedback on the paper from Sean Parent suggested a need to allow the system context to carry no threads of its own, and take over the main thread. This led us to add the <code class="sourceCode default">execute_chunk</code> and <code class="sourceCode default">execute_all</code> capability that makes forward progress delegation explicit such that in addition to the system context being able to delegate work when it needs to, we can build code that directly requests delegation of work such that an event loop can be constructed around this.</p>
<p>An implementation of <code class="sourceCode default">system_context</code> <em>should</em> allow link-time replacement of the implementation such that the context may be replaced with an implementation that compiles and runs in a single-threaded process or that can be replaced with an appropriately configured system thread pool by an end-user. We do not attempt to specify here the mechanism by which this should be implemented.</p>
<h1 data-number="4" id="design"><span class="header-section-number">4</span> Design<a href="#design" class="self-link"></a></h1>
<h2 data-number="4.1" id="system_context"><span class="header-section-number">4.1</span> system_context<a href="#system_context" class="self-link"></a></h2>
<p>The <code class="sourceCode default">system_context</code> creates a view on some underlying execution context supporting <em>parallel forward progress</em>. A <code class="sourceCode default">system_context</code> must outlive any work launched on it.</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb1-1"><a href="#cb1-1"></a><span class="kw">class</span> system_context <span class="op">{</span></span>
<span id="cb1-2"><a href="#cb1-2"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb1-3"><a href="#cb1-3"></a>  system_context<span class="op">()</span>;</span>
<span id="cb1-4"><a href="#cb1-4"></a>   <span class="op">~</span>system_context<span class="op">()</span>;</span>
<span id="cb1-5"><a href="#cb1-5"></a></span>
<span id="cb1-6"><a href="#cb1-6"></a>  system_context<span class="op">(</span><span class="kw">const</span> system_context<span class="op">&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb1-7"><a href="#cb1-7"></a>  system_context<span class="op">(</span>system_context<span class="op">&amp;&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb1-8"><a href="#cb1-8"></a>  system_context<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span><span class="kw">const</span> system_context<span class="op">&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb1-9"><a href="#cb1-9"></a>  system_context<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span>system_context<span class="op">&amp;&amp;)</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb1-10"><a href="#cb1-10"></a></span>
<span id="cb1-11"><a href="#cb1-11"></a>  system_scheduler get_scheduler<span class="op">()</span>;</span>
<span id="cb1-12"><a href="#cb1-12"></a>  implementation<span class="op">-</span>defined_delegation_sender execute_chunk<span class="op">()</span> <span class="kw">noexcept</span>;</span>
<span id="cb1-13"><a href="#cb1-13"></a>  implementation<span class="op">-</span>defined_delegation_sender execute_all<span class="op">()</span> <span class="kw">noexcept</span>;</span>
<span id="cb1-14"><a href="#cb1-14"></a>  <span class="dt">size_t</span> max_concurrency<span class="op">()</span> <span class="kw">noexcept</span>;</span>
<span id="cb1-15"><a href="#cb1-15"></a><span class="op">}</span>;</span></code></pre></div>
<ul>
<li>On construction, the <code class="sourceCode default">system_context</code> may initialize a shared system context, if it has not been previously initialized.</li>
<li>To support sharing of an underlying system context, two <code class="sourceCode default">system_context</code> objects do not guarantee task isolation. If work submitted by one can consume the thread pool, that can block progress of another.</li>
<li>The <code class="sourceCode default">system_context</code> is non-copyable and non-moveable.</li>
<li>The <code class="sourceCode default">system_context</code> must outlive work launched on it. If there is outstanding work at the point of destruction, <code class="sourceCode default">std::terminate</code> will be called.</li>
<li>The <code class="sourceCode default">system_context</code> must outlive schedulers obtained from it. If there are outstanding schedulers at destruction time, this is undefined behavior.</li>
<li><code class="sourceCode default">get_scheduler</code> returns a <code class="sourceCode default">system_scheduler</code> instance that holds a reference to the <code class="sourceCode default">system_context</code>.</li>
<li><code class="sourceCode default">execute_chunk</code> returns a sender that will:
<ul>
<li>If connected to a receiver that provides a scheduler in response to <code class="sourceCode default">get_delegatee_scheduler</code> report non-blocking if submitting work to that scheduler is non-blocking.
<ul>
<li>When <code class="sourceCode default">start()</code> an implementation-defined number of tasks will be moved from the <code class="sourceCode default">system_context</code>’s internal queue and scheduled on the delegatee scheduler.</li>
</ul></li>
<li>Otherwise will report blocking.
<ul>
<li>When <code class="sourceCode default">start()</code> is called on its operation state it will execute a chunk of work of implementation-defined size provided by the system_context.</li>
</ul></li>
<li>If connected to a receiver that provides a <code class="sourceCode default">stop_token</code>, will respond to stop requests by not executing or delegating further tasks from its queue and completing with <code class="sourceCode default">set_done</code>.</li>
<li>On success completes with the number of tasks that were executed or delegated.</li>
<li>Completes immediately with the value 0 if no tasks are available to run.</li>
</ul></li>
<li><code class="sourceCode default">execute_all</code> behaves like <code class="sourceCode default">execute_chunk</code> except that it will run or delegate all tasks in the context. After the sender returned by <code class="sourceCode default">execute_all</code> completes, and no tasks were added after that completion, a subsequent attempt must complete with <code class="sourceCode default">0</code></li>
<li><code class="sourceCode default">max_concurrency</code> will return a value representing the maximum number of threads the context may support. This is not a snapshot of the current number of threads, and may return <code class="sourceCode default">numeric_limits&lt;size_t&gt;::max</code>. If the return value is 0, then <code class="sourceCode default">execute_chunk</code> must be used by at least 1 thread to drive the context.</li>
</ul>
<h2 data-number="4.2" id="system_scheduler"><span class="header-section-number">4.2</span> system_scheduler<a href="#system_scheduler" class="self-link"></a></h2>
<p>A <code class="sourceCode default">system_scheduler</code> is a copyable handle to a <code class="sourceCode default">system_context</code>. It is the means through which agents are launched on a <code class="sourceCode default">system_context</code>. The <code class="sourceCode default">system_scheduler</code> instance does not have to outlive work submitted to it.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb2-1"><a href="#cb2-1"></a><span class="kw">class</span> system_scheduler <span class="op">{</span></span>
<span id="cb2-2"><a href="#cb2-2"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb2-3"><a href="#cb2-3"></a>  system_scheduler<span class="op">()</span> <span class="op">=</span> <span class="kw">delete</span>;</span>
<span id="cb2-4"><a href="#cb2-4"></a>  <span class="op">~</span>system_scheduler<span class="op">()</span>;</span>
<span id="cb2-5"><a href="#cb2-5"></a></span>
<span id="cb2-6"><a href="#cb2-6"></a>  system_scheduler<span class="op">(</span><span class="kw">const</span> system_scheduler<span class="op">&amp;)</span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a>  system_scheduler<span class="op">(</span>system_scheduler<span class="op">&amp;&amp;)</span>;</span>
<span id="cb2-8"><a href="#cb2-8"></a>  system_scheduler<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span><span class="kw">const</span> system_scheduler<span class="op">&amp;)</span>;</span>
<span id="cb2-9"><a href="#cb2-9"></a>  system_scheduler<span class="op">&amp;</span> <span class="kw">operator</span><span class="op">=(</span>system_scheduler<span class="op">&amp;&amp;)</span>;</span>
<span id="cb2-10"><a href="#cb2-10"></a></span>
<span id="cb2-11"><a href="#cb2-11"></a>  <span class="dt">bool</span> <span class="kw">operator</span><span class="op">==(</span><span class="kw">const</span> system_scheduler<span class="op">&amp;)</span> <span class="kw">const</span> <span class="kw">noexcept</span>;</span>
<span id="cb2-12"><a href="#cb2-12"></a></span>
<span id="cb2-13"><a href="#cb2-13"></a>  <span class="kw">friend</span> implementation<span class="op">-</span>defined<span class="op">-</span>system_sender tag_invoke<span class="op">(</span></span>
<span id="cb2-14"><a href="#cb2-14"></a>    std<span class="op">::</span>execution<span class="op">::</span>schedule_t, <span class="kw">const</span> system_scheduler<span class="op">&amp;)</span> <span class="kw">noexcept</span>;</span>
<span id="cb2-15"><a href="#cb2-15"></a>  <span class="kw">friend</span> std<span class="op">::</span>execution<span class="op">::</span>forward_progress_guarantee tag_invoke<span class="op">(</span></span>
<span id="cb2-16"><a href="#cb2-16"></a>    std<span class="op">::</span>execution<span class="op">::</span>get_forward_progress_guarantee_t,</span>
<span id="cb2-17"><a href="#cb2-17"></a>    <span class="kw">const</span> system_scheduler<span class="op">&amp;)</span> <span class="kw">noexcept</span>;</span>
<span id="cb2-18"><a href="#cb2-18"></a>  <span class="kw">friend</span> implementation<span class="op">-</span>defined<span class="op">-</span>bulk<span class="op">-</span>sender tag_invoke<span class="op">(</span></span>
<span id="cb2-19"><a href="#cb2-19"></a>    std<span class="op">::</span>execution<span class="op">::</span>bulk_t,</span>
<span id="cb2-20"><a href="#cb2-20"></a>    <span class="kw">const</span> system_scheduler<span class="op">&amp;</span>,</span>
<span id="cb2-21"><a href="#cb2-21"></a>    Sh<span class="op">&amp;&amp;</span> sh,</span>
<span id="cb2-22"><a href="#cb2-22"></a>    F<span class="op">&amp;&amp;</span> f<span class="op">)</span> <span class="kw">noexcept</span>;</span>
<span id="cb2-23"><a href="#cb2-23"></a><span class="op">}</span>;</span></code></pre></div>
<ul>
<li><code class="sourceCode default">system_scheduler</code> is not independely constructable, and must be obtained from a <code class="sourceCode default">system_context</code>. It is both move and copy constructable and assignable.</li>
<li>Two <code class="sourceCode default">system_scheduler</code>s compare equal if they share the same underlying <code class="sourceCode default">system_context</code>.</li>
<li>A <code class="sourceCode default">system_scheduler</code> has reference semantics with respect to its <code class="sourceCode default">system_context</code>. Calling any operation other than the destructor on a <code class="sourceCode default">system_scheduler</code> after the <code class="sourceCode default">system_context</code> it was created from is destroyed is undefined behavior, and that operation may access freed memory.</li>
<li>The <code class="sourceCode default">system_scheduler</code>:
<ul>
<li>satisfies the <code class="sourceCode default">scheduler</code> concept and implements the <code class="sourceCode default">schedule</code> customisation point to return an <code class="sourceCode default">implementation-defined</code> <code class="sourceCode default">sender</code> type.</li>
<li>implements the <code class="sourceCode default">get_forward_progress_guarantee</code> query to return <code class="sourceCode default">parallel</code>.</li>
<li>implements the <code class="sourceCode default">bulk</code> CPO to customise the <code class="sourceCode default">bulk</code> sender adapter such that:
<ul>
<li>When <code class="sourceCode default">execution::set_value(r, args...)</code> is called on the created <code class="sourceCode default">receiver</code>, an agent is created with parallel forward progress on the underlying <code class="sourceCode default">system_context</code> for each <code class="sourceCode default">i</code> of type <code class="sourceCode default">Shape</code> from <code class="sourceCode default">0</code> to <code class="sourceCode default">sh</code> that calls <code class="sourceCode default">f(i, args...)</code>.</li>
</ul></li>
</ul></li>
<li><code class="sourceCode default">schedule</code> calls on a <code class="sourceCode default">system_scheduler</code> are non-blocking operations.</li>
<li>If the underlying <code class="sourceCode default">system_context</code> is unable to make progress on work created through <code class="sourceCode default">system_scheduler</code> instances, and the sender retrieved from <code class="sourceCode default">scheduler</code> is connected to a <code class="sourceCode default">receiver</code> that supports the <code class="sourceCode default">get_delegatee_scheduler</code> query, work may scheduled on the <code class="sourceCode default">scheduler</code> returned by <code class="sourceCode default">get_delegatee_scheduler</code> at the time of the call to <code class="sourceCode default">start</code>, or at any later point before the work completes.</li>
</ul>
<h2 data-number="4.3" id="parallel-sender"><span class="header-section-number">4.3</span> Parallel sender<a href="#parallel-sender" class="self-link"></a></h2>
<div class="sourceCode" id="cb3"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb3-1"><a href="#cb3-1"></a><span class="kw">class</span> implementation<span class="op">-</span>defined<span class="op">-</span>system_sender <span class="op">{</span></span>
<span id="cb3-2"><a href="#cb3-2"></a><span class="kw">public</span><span class="op">:</span></span>
<span id="cb3-3"><a href="#cb3-3"></a>  <span class="kw">friend</span> pair<span class="op">&lt;</span>std<span class="op">::</span>execution<span class="op">::</span>system_scheduler, delegatee_scheduler<span class="op">&gt;</span> tag_invoke<span class="op">(</span></span>
<span id="cb3-4"><a href="#cb3-4"></a>    std<span class="op">::</span>execution<span class="op">::</span>get_completion_scheduler_t<span class="op">&lt;</span>set_value_t<span class="op">&gt;</span>,</span>
<span id="cb3-5"><a href="#cb3-5"></a>    <span class="kw">const</span> system_scheduler<span class="op">&amp;)</span> <span class="kw">noexcept</span>;</span>
<span id="cb3-6"><a href="#cb3-6"></a>  <span class="kw">friend</span> pair<span class="op">&lt;</span>std<span class="op">::</span>execution<span class="op">::</span>system_scheduler, delegatee_scheduler<span class="op">&gt;</span> tag_invoke<span class="op">(</span></span>
<span id="cb3-7"><a href="#cb3-7"></a>    std<span class="op">::</span>execution<span class="op">::</span>get_completion_scheduler_t<span class="op">&lt;</span>set_done_t<span class="op">&gt;</span>,</span>
<span id="cb3-8"><a href="#cb3-8"></a>    <span class="kw">const</span> system_scheduler<span class="op">&amp;)</span> <span class="kw">noexcept</span>;</span>
<span id="cb3-9"><a href="#cb3-9"></a></span>
<span id="cb3-10"><a href="#cb3-10"></a>  <span class="kw">template</span><span class="op">&lt;</span>receiver R<span class="op">&gt;</span></span>
<span id="cb3-11"><a href="#cb3-11"></a>        <span class="kw">requires</span> receiver_of<span class="op">&lt;</span>R<span class="op">&gt;</span></span>
<span id="cb3-12"><a href="#cb3-12"></a>  <span class="kw">friend</span> implementation<span class="op">-</span>defined<span class="op">-</span>operation_state</span>
<span id="cb3-13"><a href="#cb3-13"></a>    tag_invoke<span class="op">(</span>execution<span class="op">::</span>connect_t, implementation<span class="op">-</span>defined<span class="op">-</span>system_sender<span class="op">&amp;&amp;</span>, R<span class="op">&amp;&amp;)</span>;</span>
<span id="cb3-14"><a href="#cb3-14"></a></span>
<span id="cb3-15"><a href="#cb3-15"></a>  <span class="op">...</span></span>
<span id="cb3-16"><a href="#cb3-16"></a><span class="op">}</span>;</span></code></pre></div>
<p><code class="sourceCode default">schedule</code> on a <code class="sourceCode default">system_scheduler</code> returns some implementation-defined <code class="sourceCode default">sender</code> type.</p>
<p>This sender satisfies the following properties: - Implements the <code class="sourceCode default">get_completion_scheduler</code> query for the value and done channel where it returns a type that is a pair of an object that compares equal to itself, and a representation of delegatee scheduler that may be obtained from receivers connected with the sender. - If connected with a <code class="sourceCode default">receiver</code> that supports the <code class="sourceCode default">get_stop_token</code> query, if that <code class="sourceCode default">stop_token</code> is stopped, operations on which <code class="sourceCode default">start</code> has been called, but are not yet running (and are hence not yet guaranteed to make progress) <strong>must</strong> complete with <code class="sourceCode default">set_done</code> as soon as is practical. - <code class="sourceCode default">connect</code>ing the <code class="sourceCode default">sender</code> and calling <code class="sourceCode default">start()</code> on the resulting operation state are non-blocking operations.</p>
<h1 data-number="5" id="examples"><span class="header-section-number">5</span> Examples<a href="#examples" class="self-link"></a></h1>
<p>As a simple parallel scheduler we can use it locally, and <code class="sourceCode default">sync_wait</code> on the work to make sure that it is complete. With forward progress delegation this would also allow the scheduler to delegate work to the blocked thread. This example is derived from the Hello World example in <span class="citation" data-cites="P2300">[<a href="#ref-P2300" role="doc-biblioref">P2300R4</a>]</span>. Note that it only adds a well-defined context object, and queries that for the scheduler. Everything else is unchanged about the example.</p>
<div class="sourceCode" id="cb4"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb4-1"><a href="#cb4-1"></a><span class="kw">using</span> <span class="kw">namespace</span> std<span class="op">::</span>execution;</span>
<span id="cb4-2"><a href="#cb4-2"></a></span>
<span id="cb4-3"><a href="#cb4-3"></a>system_context ctx;</span>
<span id="cb4-4"><a href="#cb4-4"></a>scheduler <span class="kw">auto</span> sch <span class="op">=</span> ctx<span class="op">.</span>scheduler<span class="op">()</span>;</span>
<span id="cb4-5"><a href="#cb4-5"></a></span>
<span id="cb4-6"><a href="#cb4-6"></a>sender <span class="kw">auto</span> begin <span class="op">=</span> schedule<span class="op">(</span>sch<span class="op">)</span>;</span>
<span id="cb4-7"><a href="#cb4-7"></a>sender <span class="kw">auto</span> hi <span class="op">=</span> then<span class="op">(</span>begin, <span class="op">[]{</span></span>
<span id="cb4-8"><a href="#cb4-8"></a>    std<span class="op">::</span>cout <span class="op">&lt;&lt;</span> <span class="st">&quot;Hello world! Have an int.&quot;</span>;</span>
<span id="cb4-9"><a href="#cb4-9"></a>    <span class="cf">return</span> <span class="dv">13</span>;</span>
<span id="cb4-10"><a href="#cb4-10"></a><span class="op">})</span>;</span>
<span id="cb4-11"><a href="#cb4-11"></a>sender <span class="kw">auto</span> add_42 <span class="op">=</span> then<span class="op">(</span>hi, <span class="op">[](</span><span class="dt">int</span> arg<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> arg <span class="op">+</span> <span class="dv">42</span>; <span class="op">})</span>;</span>
<span id="cb4-12"><a href="#cb4-12"></a></span>
<span id="cb4-13"><a href="#cb4-13"></a><span class="kw">auto</span> <span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> this_thread<span class="op">::</span>sync_wait<span class="op">(</span>add_42<span class="op">).</span>value<span class="op">()</span>;</span></code></pre></div>
<p>We can structure the same thing using <code class="sourceCode default">execution::on</code>, which better matches structured concurrency:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb5-1"><a href="#cb5-1"></a><span class="kw">using</span> <span class="kw">namespace</span> std<span class="op">::</span>execution;</span>
<span id="cb5-2"><a href="#cb5-2"></a></span>
<span id="cb5-3"><a href="#cb5-3"></a>system_context ctx;</span>
<span id="cb5-4"><a href="#cb5-4"></a>scheduler <span class="kw">auto</span> sch <span class="op">=</span> ctx<span class="op">.</span>scheduler<span class="op">()</span>;</span>
<span id="cb5-5"><a href="#cb5-5"></a></span>
<span id="cb5-6"><a href="#cb5-6"></a>sender <span class="kw">auto</span> hi <span class="op">=</span> then<span class="op">(</span>just<span class="op">()</span>, <span class="op">[]{</span></span>
<span id="cb5-7"><a href="#cb5-7"></a>    std<span class="op">::</span>cout <span class="op">&lt;&lt;</span> <span class="st">&quot;Hello world! Have an int.&quot;</span>;</span>
<span id="cb5-8"><a href="#cb5-8"></a>    <span class="cf">return</span> <span class="dv">13</span>;</span>
<span id="cb5-9"><a href="#cb5-9"></a><span class="op">})</span>;</span>
<span id="cb5-10"><a href="#cb5-10"></a>sender <span class="kw">auto</span> add_42 <span class="op">=</span> then<span class="op">(</span>hi, <span class="op">[](</span><span class="dt">int</span> arg<span class="op">)</span> <span class="op">{</span> <span class="cf">return</span> arg <span class="op">+</span> <span class="dv">42</span>; <span class="op">})</span>;</span>
<span id="cb5-11"><a href="#cb5-11"></a></span>
<span id="cb5-12"><a href="#cb5-12"></a><span class="kw">auto</span> <span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> this_thread<span class="op">::</span>sync_wait<span class="op">(</span>on<span class="op">(</span>sch, add_42<span class="op">)).</span>value<span class="op">()</span>;</span></code></pre></div>
<p>The <code class="sourceCode default">system_scheduler</code> customises bulk, so we can use bulk dependent on the scheduler. Here we use it in structured form using the parameterless <code class="sourceCode default">get_scheduler</code> that retrieves the scheduler from the receiver, combined with <code class="sourceCode default">on</code>:</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb6-1"><a href="#cb6-1"></a><span class="kw">auto</span> bar<span class="op">()</span> <span class="op">{</span></span>
<span id="cb6-2"><a href="#cb6-2"></a>  <span class="cf">return</span></span>
<span id="cb6-3"><a href="#cb6-3"></a>    ex<span class="op">::</span>let_value<span class="op">(</span></span>
<span id="cb6-4"><a href="#cb6-4"></a>      ex<span class="op">::</span>get_scheduler<span class="op">()</span>,          <span class="co">// Fetch scheduler from receiver.</span></span>
<span id="cb6-5"><a href="#cb6-5"></a>      <span class="op">[](</span><span class="kw">auto</span> current_sched<span class="op">)</span> <span class="op">{</span></span>
<span id="cb6-6"><a href="#cb6-6"></a>        <span class="cf">return</span> bulk<span class="op">(</span></span>
<span id="cb6-7"><a href="#cb6-7"></a>          current_sched<span class="op">.</span>schedule<span class="op">()</span>,</span>
<span id="cb6-8"><a href="#cb6-8"></a>          <span class="dv">1</span>,                        <span class="co">// Only 1 bulk task as a lazy way of making cout safe</span></span>
<span id="cb6-9"><a href="#cb6-9"></a>          <span class="op">[](</span><span class="kw">auto</span> idx<span class="op">){</span></span>
<span id="cb6-10"><a href="#cb6-10"></a>            std<span class="op">::</span>cout <span class="op">&lt;&lt;</span> <span class="st">&quot;Index: &quot;</span> <span class="op">&lt;&lt;</span> idx <span class="op">&lt;&lt;</span> <span class="st">&quot;</span><span class="sc">\n</span><span class="st">&quot;</span>;</span>
<span id="cb6-11"><a href="#cb6-11"></a>          <span class="op">})</span></span>
<span id="cb6-12"><a href="#cb6-12"></a>      <span class="op">})</span>;</span>
<span id="cb6-13"><a href="#cb6-13"></a><span class="op">}</span></span>
<span id="cb6-14"><a href="#cb6-14"></a></span>
<span id="cb6-15"><a href="#cb6-15"></a><span class="dt">void</span> foo<span class="op">()</span></span>
<span id="cb6-16"><a href="#cb6-16"></a><span class="op">{</span></span>
<span id="cb6-17"><a href="#cb6-17"></a>  <span class="kw">using</span> <span class="kw">namespace</span> std<span class="op">::</span>execution;</span>
<span id="cb6-18"><a href="#cb6-18"></a></span>
<span id="cb6-19"><a href="#cb6-19"></a>  system_context ctx;</span>
<span id="cb6-20"><a href="#cb6-20"></a>  scheduler <span class="kw">auto</span> sch <span class="op">=</span> ;</span>
<span id="cb6-21"><a href="#cb6-21"></a></span>
<span id="cb6-22"><a href="#cb6-22"></a>  <span class="kw">auto</span> <span class="op">[</span>i<span class="op">]</span> <span class="op">=</span> this_thread<span class="op">::</span>sync_wait<span class="op">(</span></span>
<span id="cb6-23"><a href="#cb6-23"></a>    on<span class="op">(</span></span>
<span id="cb6-24"><a href="#cb6-24"></a>      ctx<span class="op">.</span>scheduler<span class="op">()</span>,                <span class="co">// Start bar on the system_scheduler</span></span>
<span id="cb6-25"><a href="#cb6-25"></a>      bar<span class="op">()))</span>                         <span class="co">// and propagate it through the receivers</span></span>
<span id="cb6-26"><a href="#cb6-26"></a>    <span class="op">.</span>value<span class="op">()</span>;</span>
<span id="cb6-27"><a href="#cb6-27"></a><span class="op">}</span></span></code></pre></div>
<p>Use <code class="sourceCode default">async_scope</code> and the delegation functionality of the context to build a loop to drive the context. This will be important if the context has no threads and we have setup the system for a single-threaded process:</p>
<div class="sourceCode" id="cb7"><pre class="sourceCode cpp"><code class="sourceCode cpp"><span id="cb7-1"><a href="#cb7-1"></a><span class="kw">using</span> <span class="kw">namespace</span> std<span class="op">::</span>execution;</span>
<span id="cb7-2"><a href="#cb7-2"></a></span>
<span id="cb7-3"><a href="#cb7-3"></a>system_context ctx;</span>
<span id="cb7-4"><a href="#cb7-4"></a></span>
<span id="cb7-5"><a href="#cb7-5"></a><span class="dt">int</span> result <span class="op">=</span> <span class="dv">0</span>;</span>
<span id="cb7-6"><a href="#cb7-6"></a></span>
<span id="cb7-7"><a href="#cb7-7"></a><span class="op">{</span></span>
<span id="cb7-8"><a href="#cb7-8"></a>  async_scope scope;</span>
<span id="cb7-9"><a href="#cb7-9"></a>  scheduler <span class="kw">auto</span> sch <span class="op">=</span> ctx<span class="op">.</span>scheduler<span class="op">()</span>;</span>
<span id="cb7-10"><a href="#cb7-10"></a></span>
<span id="cb7-11"><a href="#cb7-11"></a>  sender <span class="kw">auto</span> work <span class="op">=</span></span>
<span id="cb7-12"><a href="#cb7-12"></a>    then<span class="op">(</span>just<span class="op">()</span>, <span class="op">[&amp;](</span><span class="kw">auto</span> sched<span class="op">)</span> <span class="op">{</span></span>
<span id="cb7-13"><a href="#cb7-13"></a></span>
<span id="cb7-14"><a href="#cb7-14"></a>      <span class="dt">int</span> val <span class="op">=</span> <span class="dv">13</span>;</span>
<span id="cb7-15"><a href="#cb7-15"></a></span>
<span id="cb7-16"><a href="#cb7-16"></a>      <span class="kw">auto</span> print_sender <span class="op">=</span> then<span class="op">(</span>just<span class="op">()</span>, <span class="op">[</span>val<span class="op">]{</span></span>
<span id="cb7-17"><a href="#cb7-17"></a>        std<span class="op">::</span>cout <span class="op">&lt;&lt;</span> <span class="st">&quot;Hello world! Have an int with value: &quot;</span> <span class="op">&lt;&lt;</span> val <span class="op">&lt;&lt;</span> <span class="st">&quot;</span><span class="sc">\n</span><span class="st">&quot;</span>;</span>
<span id="cb7-18"><a href="#cb7-18"></a>      <span class="op">})</span>;</span>
<span id="cb7-19"><a href="#cb7-19"></a></span>
<span id="cb7-20"><a href="#cb7-20"></a>      <span class="co">// spawn the print sender on sched to make sure it</span></span>
<span id="cb7-21"><a href="#cb7-21"></a>      <span class="co">// completes before shutdown</span></span>
<span id="cb7-22"><a href="#cb7-22"></a>      scope<span class="op">.</span>spawn<span class="op">(</span>on<span class="op">(</span>sch, std<span class="op">::</span>move<span class="op">(</span>print_sender<span class="op">)))</span>;</span>
<span id="cb7-23"><a href="#cb7-23"></a></span>
<span id="cb7-24"><a href="#cb7-24"></a>      <span class="cf">return</span> val;</span>
<span id="cb7-25"><a href="#cb7-25"></a>    <span class="op">})</span>;</span>
<span id="cb7-26"><a href="#cb7-26"></a></span>
<span id="cb7-27"><a href="#cb7-27"></a>  scope<span class="op">.</span>spawn<span class="op">(</span>on<span class="op">(</span>sch, std<span class="op">::</span>move<span class="op">(</span>work<span class="op">)))</span>;</span>
<span id="cb7-28"><a href="#cb7-28"></a></span>
<span id="cb7-29"><a href="#cb7-29"></a>  <span class="co">// Loop to drain the context and subsequently check that the scope is empty</span></span>
<span id="cb7-30"><a href="#cb7-30"></a>  <span class="co">// We need a repeat algorithm to do this correctly, the following logic</span></span>
<span id="cb7-31"><a href="#cb7-31"></a>  <span class="co">// approximates what a repeat algorithm would achieve.</span></span>
<span id="cb7-32"><a href="#cb7-32"></a>  <span class="cf">while</span><span class="op">(</span>this_thread<span class="op">::</span>sync_wait<span class="op">(</span>ctx<span class="op">.</span>execute_chunk<span class="op">()).</span>value <span class="op">!=</span> <span class="dv">0</span><span class="op">)</span>;</span>
<span id="cb7-33"><a href="#cb7-33"></a>  this_thread<span class="op">::</span>sync_wait<span class="op">(</span>when_all<span class="op">(</span>scope<span class="op">.</span>empty<span class="op">()</span>, ctx<span class="op">.</span>execute_all<span class="op">()))</span>;</span>
<span id="cb7-34"><a href="#cb7-34"></a><span class="op">}</span>;</span>
<span id="cb7-35"><a href="#cb7-35"></a></span>
<span id="cb7-36"><a href="#cb7-36"></a><span class="co">// The scope ensured that all work is safely joined, so result contains 13</span></span>
<span id="cb7-37"><a href="#cb7-37"></a>std<span class="op">::</span>cout <span class="op">&lt;&lt;</span> <span class="st">&quot;Result: &quot;</span> <span class="op">&lt;&lt;</span> result <span class="op">&lt;&lt;</span> <span class="st">&quot;</span><span class="sc">\n</span><span class="st">&quot;</span>;</span>
<span id="cb7-38"><a href="#cb7-38"></a></span>
<span id="cb7-39"><a href="#cb7-39"></a><span class="co">// and destruction of the context is now safe</span></span></code></pre></div>
<h1 data-number="6" id="bibliography"><span class="header-section-number">6</span> References<a href="#bibliography" class="self-link"></a></h1>
<div id="refs" class="references hanging-indent" role="doc-bibliography">
<div id="ref-P2079R1">
<p>[P2079R1] Ruslan Arutyunyan, Michael Voss. 2020-08-15. Parallel Executor. <br />
<a href="https://wg21.link/p2079r1">https://wg21.link/p2079r1</a></p>
</div>
<div id="ref-P2300">
<p>[P2300R4] 2022. std::execution. <br />
<a href="https://wg21.link/p2300r4">https://wg21.link/p2300r4</a></p>
</div>
<div id="ref-P2519">
<p>[P2519R0] 2022. async_scope - Creating scopes for non-sequential concurrency. <br />
<a href="https://wg21.link/p2519">https://wg21.link/p2519</a></p>
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