forked from LeeHowes/CPP
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathp2079r2_system_execution_context.html
700 lines (693 loc) · 55.4 KB
/
p2079r2_system_execution_context.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang xml:lang>
<head>
<meta charset="utf-8" />
<meta name="generator" content="mpark/wg21" />
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes" />
<meta name="dcterms.date" content="2022-01-14" />
<title>System execution context</title>
<style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
span.underline{text-decoration: underline;}
div.column{display: inline-block; vertical-align: top; width: 50%;}
div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
ul.task-list{list-style: none;}
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { display: inline-block; line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
color: #aaaaaa;
}
pre.numberSource { margin-left: 3em; border-left: 1px solid #aaaaaa; padding-left: 4px; }
div.sourceCode
{ background-color: #f6f8fa; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span. { } /* Normal */
code span.al { color: #ff0000; } /* Alert */
code span.an { } /* Annotation */
code span.at { } /* Attribute */
code span.bn { color: #9f6807; } /* BaseN */
code span.bu { color: #9f6807; } /* BuiltIn */
code span.cf { color: #00607c; } /* ControlFlow */
code span.ch { color: #9f6807; } /* Char */
code span.cn { } /* Constant */
code span.co { color: #008000; font-style: italic; } /* Comment */
code span.cv { color: #008000; font-style: italic; } /* CommentVar */
code span.do { color: #008000; } /* Documentation */
code span.dt { color: #00607c; } /* DataType */
code span.dv { color: #9f6807; } /* DecVal */
code span.er { color: #ff0000; font-weight: bold; } /* Error */
code span.ex { } /* Extension */
code span.fl { color: #9f6807; } /* Float */
code span.fu { } /* Function */
code span.im { } /* Import */
code span.in { color: #008000; } /* Information */
code span.kw { color: #00607c; } /* Keyword */
code span.op { color: #af1915; } /* Operator */
code span.ot { } /* Other */
code span.pp { color: #6f4e37; } /* Preprocessor */
code span.re { } /* RegionMarker */
code span.sc { color: #9f6807; } /* SpecialChar */
code span.ss { color: #9f6807; } /* SpecialString */
code span.st { color: #9f6807; } /* String */
code span.va { } /* Variable */
code span.vs { color: #9f6807; } /* VerbatimString */
code span.wa { color: #008000; font-weight: bold; } /* Warning */
code.diff {color: #898887}
code.diff span.va {color: #006e28}
code.diff span.st {color: #bf0303}
</style>
<style type="text/css">
body {
margin: 5em;
font-family: serif;
hyphens: auto;
line-height: 1.35;
text-align: justify;
}
@media screen and (max-width: 30em) {
body {
margin: 1.5em;
}
}
div.wrapper {
max-width: 60em;
margin: auto;
}
ul {
list-style-type: none;
padding-left: 2em;
margin-top: -0.2em;
margin-bottom: -0.2em;
}
a {
text-decoration: none;
color: #4183C4;
}
a.hidden_link {
text-decoration: none;
color: inherit;
}
li {
margin-top: 0.6em;
margin-bottom: 0.6em;
}
h1, h2, h3, h4 {
position: relative;
line-height: 1;
}
a.self-link {
position: absolute;
top: 0;
left: calc(-1 * (3.5rem - 26px));
width: calc(3.5rem - 26px);
height: 2em;
text-align: center;
border: none;
transition: opacity .2s;
opacity: .5;
font-family: sans-serif;
font-weight: normal;
font-size: 83%;
}
a.self-link:hover { opacity: 1; }
a.self-link::before { content: "§"; }
ul > li:before {
content: "\2014";
position: absolute;
margin-left: -1.5em;
}
:target { background-color: #C9FBC9; }
:target .codeblock { background-color: #C9FBC9; }
:target ul { background-color: #C9FBC9; }
.abbr_ref { float: right; }
.folded_abbr_ref { float: right; }
:target .folded_abbr_ref { display: none; }
:target .unfolded_abbr_ref { float: right; display: inherit; }
.unfolded_abbr_ref { display: none; }
.secnum { display: inline-block; min-width: 35pt; }
.header-section-number { display: inline-block; min-width: 35pt; }
.annexnum { display: block; }
div.sourceLinkParent {
float: right;
}
a.sourceLink {
position: absolute;
opacity: 0;
margin-left: 10pt;
}
a.sourceLink:hover {
opacity: 1;
}
a.itemDeclLink {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
opacity: 0;
}
a.itemDeclLink:hover { opacity: 1; }
span.marginalizedparent {
position: relative;
left: -5em;
}
li span.marginalizedparent { left: -7em; }
li ul > li span.marginalizedparent { left: -9em; }
li ul > li ul > li span.marginalizedparent { left: -11em; }
li ul > li ul > li ul > li span.marginalizedparent { left: -13em; }
div.footnoteNumberParent {
position: relative;
left: -4.7em;
}
a.marginalized {
position: absolute;
font-size: 75%;
text-align: right;
width: 5em;
}
a.enumerated_item_num {
position: relative;
left: -3.5em;
display: inline-block;
margin-right: -3em;
text-align: right;
width: 3em;
}
div.para { margin-bottom: 0.6em; margin-top: 0.6em; text-align: justify; }
div.section { text-align: justify; }
div.sentence { display: inline; }
span.indexparent {
display: inline;
position: relative;
float: right;
right: -1em;
}
a.index {
position: absolute;
display: none;
}
a.index:before { content: "⟵"; }
a.index:target {
display: inline;
}
.indexitems {
margin-left: 2em;
text-indent: -2em;
}
div.itemdescr {
margin-left: 3em;
}
.bnf {
font-family: serif;
margin-left: 40pt;
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.ncbnf {
font-family: serif;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
}
.ncsimplebnf {
font-family: serif;
font-style: italic;
margin-top: 0.5em;
margin-bottom: 0.5em;
margin-left: 40pt;
background: inherit;
}
span.textnormal {
font-style: normal;
font-family: serif;
white-space: normal;
display: inline-block;
}
span.rlap {
display: inline-block;
width: 0px;
}
span.descr { font-style: normal; font-family: serif; }
span.grammarterm { font-style: italic; }
span.term { font-style: italic; }
span.terminal { font-family: monospace; font-style: normal; }
span.nonterminal { font-style: italic; }
span.tcode { font-family: monospace; font-style: normal; }
span.textbf { font-weight: bold; }
span.textsc { font-variant: small-caps; }
a.nontermdef { font-style: italic; font-family: serif; }
span.emph { font-style: italic; }
span.techterm { font-style: italic; }
span.mathit { font-style: italic; }
span.mathsf { font-family: sans-serif; }
span.mathrm { font-family: serif; font-style: normal; }
span.textrm { font-family: serif; }
span.textsl { font-style: italic; }
span.mathtt { font-family: monospace; font-style: normal; }
span.mbox { font-family: serif; font-style: normal; }
span.ungap { display: inline-block; width: 2pt; }
span.textit { font-style: italic; }
span.texttt { font-family: monospace; }
span.tcode_in_codeblock { font-family: monospace; font-style: normal; }
span.phantom { color: white; }
span.math { font-style: normal; }
span.mathblock {
display: block;
margin-left: auto;
margin-right: auto;
margin-top: 1.2em;
margin-bottom: 1.2em;
text-align: center;
}
span.mathalpha {
font-style: italic;
}
span.synopsis {
font-weight: bold;
margin-top: 0.5em;
display: block;
}
span.definition {
font-weight: bold;
display: block;
}
.codeblock {
margin-left: 1.2em;
line-height: 127%;
}
.outputblock {
margin-left: 1.2em;
line-height: 127%;
}
div.itemdecl {
margin-top: 2ex;
}
code.itemdeclcode {
white-space: pre;
display: block;
}
span.textsuperscript {
vertical-align: super;
font-size: smaller;
line-height: 0;
}
.footnotenum { vertical-align: super; font-size: smaller; line-height: 0; }
.footnote {
font-size: small;
margin-left: 2em;
margin-right: 2em;
margin-top: 0.6em;
margin-bottom: 0.6em;
}
div.minipage {
display: inline-block;
margin-right: 3em;
}
div.numberedTable {
text-align: center;
margin: 2em;
}
div.figure {
text-align: center;
margin: 2em;
}
table {
border: 1px solid black;
border-collapse: collapse;
margin-left: auto;
margin-right: auto;
margin-top: 0.8em;
text-align: left;
hyphens: none;
}
td, th {
padding-left: 1em;
padding-right: 1em;
vertical-align: top;
}
td.empty {
padding: 0px;
padding-left: 1px;
}
td.left {
text-align: left;
}
td.right {
text-align: right;
}
td.center {
text-align: center;
}
td.justify {
text-align: justify;
}
td.border {
border-left: 1px solid black;
}
tr.rowsep, td.cline {
border-top: 1px solid black;
}
tr.even, tr.odd {
border-bottom: 1px solid black;
}
tr.capsep {
border-top: 3px solid black;
border-top-style: double;
}
tr.header {
border-bottom: 3px solid black;
border-bottom-style: double;
}
th {
border-bottom: 1px solid black;
}
span.centry {
font-weight: bold;
}
div.table {
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
width: 90%;
}
span.indented {
display: block;
margin-left: 2em;
margin-bottom: 1em;
margin-top: 1em;
}
ol.enumeratea { list-style-type: none; background: inherit; }
ol.enumerate { list-style-type: none; background: inherit; }
code.sourceCode > span { display: inline; }
</style>
<link href="data:image/x-icon;base64,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" rel="icon" />
<!--[if lt IE 9]>
<script src="//cdnjs.cloudflare.com/ajax/libs/html5shiv/3.7.3/html5shiv-printshiv.min.js"></script>
<![endif]-->
</head>
<body>
<div class="wrapper">
<header id="title-block-header">
<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><<a href="mailto:lwh@fb.com" class="email">lwh@fb.com</a>><br>
Ruslan Arutyunyan<br><<a href="mailto:ruslan.arutyunyan@intel.com" class="email">ruslan.arutyunyan@intel.com</a>><br>
Michael Voss<br><<a href="mailto:michaelj.voss@intel.com" class="email">michaelj.voss@intel.com</a>><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">&)</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">&&)</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">&</span> <span class="kw">operator</span><span class="op">=(</span><span class="kw">const</span> system_context<span class="op">&)</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">&</span> <span class="kw">operator</span><span class="op">=(</span>system_context<span class="op">&&)</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<size_t>::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">&)</span>;</span>
<span id="cb2-7"><a href="#cb2-7"></a> system_scheduler<span class="op">(</span>system_scheduler<span class="op">&&)</span>;</span>
<span id="cb2-8"><a href="#cb2-8"></a> system_scheduler<span class="op">&</span> <span class="kw">operator</span><span class="op">=(</span><span class="kw">const</span> system_scheduler<span class="op">&)</span>;</span>
<span id="cb2-9"><a href="#cb2-9"></a> system_scheduler<span class="op">&</span> <span class="kw">operator</span><span class="op">=(</span>system_scheduler<span class="op">&&)</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">&)</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">&)</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">&)</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">&</span>,</span>
<span id="cb2-21"><a href="#cb2-21"></a> Sh<span class="op">&&</span> sh,</span>
<span id="cb2-22"><a href="#cb2-22"></a> F<span class="op">&&</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="system-sender"><span class="header-section-number">4.3</span> system sender<a href="#system-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"><</span>std<span class="op">::</span>execution<span class="op">::</span>system_scheduler, delegatee_scheduler<span class="op">></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"><</span>set_value_t<span class="op">></span>,</span>
<span id="cb3-5"><a href="#cb3-5"></a> <span class="kw">const</span> system_scheduler<span class="op">&)</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"><</span>std<span class="op">::</span>execution<span class="op">::</span>system_scheduler, delegatee_scheduler<span class="op">></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"><</span>set_done_t<span class="op">></span>,</span>
<span id="cb3-8"><a href="#cb3-8"></a> <span class="kw">const</span> system_scheduler<span class="op">&)</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"><</span>receiver R<span class="op">></span></span>
<span id="cb3-11"><a href="#cb3-11"></a> <span class="kw">requires</span> receiver_of<span class="op"><</span>R<span class="op">></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">&&</span>, R<span class="op">&&)</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"><<</span> <span class="st">"Hello world! Have an int."</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"><<</span> <span class="st">"Hello world! Have an int."</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"><<</span> <span class="st">"Index: "</span> <span class="op"><<</span> idx <span class="op"><<</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</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">[&](</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"><<</span> <span class="st">"Hello world! Have an int with value: "</span> <span class="op"><<</span> val <span class="op"><<</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</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"><<</span> <span class="st">"Result: "</span> <span class="op"><<</span> result <span class="op"><<</span> <span class="st">"</span><span class="sc">\n</span><span class="st">"</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>
</div>
</div>
</div>
</div>
</body>
</html>