include/boost/corosio/io_context.hpp

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1			//
2			// Copyright (c) 2025 Vinnie Falco ([email protected])
3			// Copyright (c) 2026 Steve Gerbino
4			// Copyright (c) 2026 Michael Vandeberg
5			//
6			// Distributed under the Boost Software License, Version 1.0. (See accompanying
7			// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
8			//
9			// Official repository: https://github.com/cppalliance/corosio
10			//
11
12			#ifndef BOOST_COROSIO_IO_CONTEXT_HPP
13			#define BOOST_COROSIO_IO_CONTEXT_HPP
14
15			#include <boost/corosio/detail/config.hpp>
16			#include <boost/corosio/detail/platform.hpp>
17			#include <boost/corosio/detail/scheduler.hpp>
18			#include <boost/capy/continuation.hpp>
19			#include <boost/capy/ex/execution_context.hpp>
20
21			#include <chrono>
22			#include <coroutine>
23			#include <cstddef>
24			#include <limits>
25			#include <thread>
26
27			namespace boost::corosio {
28
29			/** Locking-safety tier for an @ref io_context.
30
31			Selects which internal locks the scheduler and reactor elide, trading
32			thread-safety guarantees for reduced synchronization overhead. This is
33			the analog of Boost.Asio's `SAFE` / `UNSAFE_IO` / `UNSAFE` concurrency
34			hint constants. The tier is chosen explicitly, not derived from the
35			`concurrency_hint`. (The reverse does apply: a lockless tier reduces the
36			effective hint used for performance tuning to 1.)
37
38			@see io_context_options::locking
39			*/
40			enum class locking_mode
41			{
42			/** Full thread safety (default). All locks enabled; equivalent to
43			Boost.Asio's `SAFE`/`DEFAULT`. Any thread may use the context. */
44			safe,
45
46			/** Disable only the per-descriptor I/O locks; keep scheduler locking.
47			Equivalent to Boost.Asio's `UNSAFE_IO`. The context must be run
48			and driven by a single thread, but resolver and POSIX file
49			services remain available (they rely on scheduler locking, which
50			stays on). */
51			unsafe_io,
52
53			/** Disable all locking (fully lockless). Equivalent to Boost.Asio's
54			`UNSAFE`.
55
56			@par Restrictions
57			- Only one thread may call `run()` (or any run variant).
58			- Posting work from another thread is undefined behavior.
59			- DNS resolution returns `operation_not_supported`.
60			- POSIX file I/O returns `operation_not_supported`.
61			- Signal sets should not be shared across contexts. */
62			unsafe
63			};
64
65			/** Runtime tuning options for @ref io_context.
66
67			All fields have defaults that match the library's built-in
68			values, so constructing a default `io_context_options` produces
69			identical behavior to an unconfigured context.
70
71			Options that apply only to a specific backend family are
72			silently ignored when the active backend does not support them.
73
74			@par Example
75			@code
76			io_context_options opts;
77			opts.max_events_per_poll = 256; // larger batch per syscall
78			opts.inline_budget_max = 32; // more speculative completions
79			opts.thread_pool_size = 4; // more file-I/O workers
80
81			io_context ioc(opts);
82			@endcode
83
84			@see io_context, native_io_context
85			*/
86		50x	struct io_context_options
87			{
88			/** Maximum events fetched per reactor poll call.
89
90			Controls the buffer size passed to `epoll_wait()` or
91			`kevent()`. Larger values reduce syscall frequency under
92			high load; smaller values improve fairness between
93			connections. Ignored on IOCP and select backends.
94			*/
95		50x	unsigned max_events_per_poll = 128;
96
97			/** Starting inline completion budget per handler chain.
98
99			After a posted handler executes, the reactor grants this
100			many speculative inline completions before forcing a
101			re-queue. Applies to reactor backends only.
102
103			@note Constructing an `io_context` with `concurrency_hint > 1`
104			and all three budget fields at their defaults overrides
105			them to disable inline completion (post-everything mode),
106			since multi-thread workloads benefit from cross-thread
107			work-stealing. Setting any budget field to a non-default
108			value disables the override.
109			*/
110		50x	unsigned inline_budget_initial = 2;
111
112			/** Hard ceiling on adaptive inline budget ramp-up.
113
114			The budget doubles each cycle it is fully consumed, up to
115			this limit. Applies to reactor backends only.
116			*/
117		50x	unsigned inline_budget_max = 16;
118
119			/** Inline budget when no other thread assists the reactor.
120
121			When only one thread is running the event loop, this
122			value caps the inline budget to preserve fairness.
123			Applies to reactor backends only.
124			*/
125		50x	unsigned unassisted_budget = 4;
126
127			/** Thread pool size for blocking I/O (file I/O, DNS resolution).
128
129			Sets the number of worker threads in the shared thread pool
130			used by POSIX file services and DNS resolution. Must be at
131			least 1. Applies to POSIX backends only; ignored on IOCP
132			where file I/O uses native overlapped I/O.
133			*/
134		50x	unsigned thread_pool_size = 1;
135
136			/** Thread-safety tier. See @ref locking_mode for the tiers and their
137			restrictions.
138			*/
139		50x	locking_mode locking = locking_mode::safe;
140
141			/** Enable IORING_SETUP_SQPOLL on the io_uring backend.
142
143			With SQPOLL, the kernel forks a thread that busy-polls the
144			submission ring; submission becomes a userspace-only memory
145			store, eliminating the io_uring_enter syscall on the submit
146			path. Most useful for sustained traffic. Idle thread parks
147			after `sq_thread_idle_ms` of no activity.
148
149			Independent of `locking`. Default: off.
150
151			Ignored on non-io_uring backends.
152			*/
153		50x	bool enable_sqpoll = false;
154
155			/** SQ-poll idle timeout in milliseconds.
156
157			After this many ms of no submissions, the kernel polling
158			thread sleeps; next submit re-wakes it via SQ_WAKEUP. 0
159			means use the kernel default (1ms). Recommended for bursty
160			workloads: 100-1000ms (avoids park/unpark thrash).
161
162			Ignored unless `enable_sqpoll` is true. Ignored on
163			non-io_uring backends.
164			*/
165		50x	unsigned sq_thread_idle_ms = 0;
166
167			/** Pin the SQ-poll kernel thread to this CPU.
168
169			-1 means do not pin (kernel scheduler picks). Pinning off
170			the dispatch core is recommended on latency-sensitive
171			deployments to avoid cache contention.
172
173			Ignored unless `enable_sqpoll` is true. Ignored on
174			non-io_uring backends.
175			*/
176		50x	int sq_thread_cpu = -1;
177			};
178
179			namespace detail {
180			class timer_service;
181
182			/** Return the hint used for performance tuning: the lockless tiers are
183			single-threaded, so their effective hint is 1 whatever the caller passed.
184			*/
185			inline unsigned
186		33x	effective_concurrency_hint(
187			io_context_options const& opts, unsigned hint) noexcept
188			{
189	2/2 ✓ Branch 0 taken 15 times. ✓ Branch 1 taken 18 times.	33x	return opts.locking == locking_mode::safe ? hint : 1u;
190			}
191			} // namespace detail
192
193			/** An I/O context for running asynchronous operations.
194
195			The io_context provides an execution environment for async
196			operations. It maintains a queue of pending work items and
197			processes them when `run()` is called.
198
199			The default and unsigned constructors select the platform's
200			native backend:
201			- Windows: IOCP
202			- Linux: epoll
203			- BSD/macOS: kqueue
204			- Other POSIX: select
205
206			The template constructor accepts a backend tag value to
207			choose a specific backend at compile time:
208
209			@par Example
210			@code
211			io_context ioc; // platform default
212			io_context ioc2(corosio::epoll); // explicit backend
213			@endcode
214
215			@par Preconditions
216			The context must outlive every operation posted or dispatched
217			through its executor, and no thread may be executing a run
218			variant when the context is destroyed. Posting to the context
219			concurrently with, or after, its destruction is undefined
220			behavior. The safe teardown pattern is to stop submitting new
221			work, let every `run()` call return (each returns once no
222			outstanding work remains), and join the threads that ran the
223			loop before destroying the context. Work launched with
224			`capy::run` / `capy::run_async` is work-tracked, so a normal
225			`run()` completion already waits for it.
226
227			@par Thread Safety
228			Distinct objects: Safe.@n
229			Shared objects: Safe, unless the context was constructed with a
230			lockless @ref io_context_options::locking tier (`unsafe_io` or
231			`unsafe`), in which case a single thread must drive it.
232
233			@see epoll_t, select_t, kqueue_t, iocp_t
234			*/
235			class BOOST_COROSIO_DECL io_context : public capy::execution_context
236			{
237			/// Pre-create services that depend on options (before construct).
238			void apply_options_pre_(io_context_options const& opts);
239
240			/// Apply runtime tuning to the scheduler (after construct).
241			void apply_options_post_(
242			io_context_options const& opts,
243			unsigned concurrency_hint);
244
245			/** Apply only the decomposed threading configuration (locking tiers).
246			Used by the plain constructors, which — unlike the options
247			constructors — deliberately leave the reactor budget at its defaults
248			rather than engaging the multi-thread post-everything heuristic. */
249			void apply_threading_(io_context_options const& opts);
250
251			protected:
252			detail::scheduler* sched_;
253
254			public:
255			/** The executor type for this context. */
256			class executor_type;
257
258			/** Construct with default concurrency and platform backend.
259
260			Uses `std::thread::hardware_concurrency()` (floored to 1, in
261			case it reports 0) as the concurrency hint, and the default
262			@ref locking_mode::safe tier. Select a lockless tier via
263			@ref io_context_options::locking.
264			*/
265			io_context();
266
267			/** Construct with a concurrency hint and platform backend.
268
269			@param concurrency_hint Hint for the number of threads
270			that will call `run()`.
271			*/
272			explicit io_context(unsigned concurrency_hint);
273
274			/** Construct with runtime tuning options and platform backend.
275
276			@param opts Runtime options controlling scheduler and
277			service behavior.
278			@param concurrency_hint Hint for the number of threads
279			that will call `run()`.
280			*/
281			explicit io_context(
282			io_context_options const& opts,
283			unsigned concurrency_hint = std::thread::hardware_concurrency());
284
285			/** Construct with an explicit backend tag.
286
287			@param backend The backend tag value selecting the I/O
288			multiplexer (e.g. `corosio::epoll`).
289			@param concurrency_hint Hint for the number of threads
290			that will call `run()`.
291			*/
292			template<class Backend>
293			requires requires { Backend::construct; }
294		2288x	explicit io_context(
295			Backend backend,
296			unsigned concurrency_hint = std::thread::hardware_concurrency())
297		1154x	: capy::execution_context(this)
298		1154x	, sched_(nullptr)
299		1134x	{
300			(void)backend;
301	2/4 ✓ Branch 0 taken 577 times. ✗ Branch 1 not taken. ✓ Branch 2 taken 577 times. ✗ Branch 3 not taken.	1154x	sched_ = &Backend::construct(*this, concurrency_hint);
302			// Apply threading config only (locking tier). Unlike the options
303			// ctor, the plain path leaves the reactor budget at its defaults.
304	2/4 ✓ Branch 0 taken 577 times. ✗ Branch 1 not taken. ✓ Branch 2 taken 577 times. ✗ Branch 3 not taken.	1154x	apply_threading_(io_context_options{});
305		2288x	}
306
307			/** Construct with an explicit backend tag and runtime options.
308
309			@param backend The backend tag value selecting the I/O
310			multiplexer (e.g. `corosio::epoll`).
311			@param opts Runtime options controlling scheduler and
312			service behavior.
313			@param concurrency_hint Hint for the number of threads
314			that will call `run()`.
315			*/
316			template<class Backend>
317			requires requires { Backend::construct; }
318		34x	explicit io_context(
319			Backend backend,
320			io_context_options const& opts,
321			unsigned concurrency_hint = std::thread::hardware_concurrency())
322		18x	: capy::execution_context(this)
323		18x	, sched_(nullptr)
324		16x	{
325			(void)backend;
326	2/4 ✓ Branch 0 taken 9 times. ✗ Branch 1 not taken. ✓ Branch 2 taken 9 times. ✗ Branch 3 not taken.	18x	apply_options_pre_(opts);
327			// Effective hint (1 for lockless tiers); see effective_concurrency_hint.
328		18x	unsigned const eff =
329		18x	detail::effective_concurrency_hint(opts, concurrency_hint);
330	2/4 ✓ Branch 0 taken 9 times. ✗ Branch 1 not taken. ✓ Branch 2 taken 9 times. ✗ Branch 3 not taken.	18x	sched_ = &Backend::construct(*this, eff);
331	4/4 ✓ Branch 0 taken 8 times. ✓ Branch 1 taken 1 time. ✓ Branch 2 taken 8 times. ✓ Branch 3 taken 1 time.	18x	apply_options_post_(opts, eff);
332		34x	}
333
334			~io_context();
335
336			io_context(io_context const&) = delete;
337			io_context& operator=(io_context const&) = delete;
338
339			/** Return an executor for this context.
340
341			The returned executor can be used to dispatch coroutines
342			and post work items to this context.
343
344			@return An executor associated with this context.
345			*/
346			executor_type get_executor() const noexcept;
347
348			/** Signal the context to stop processing.
349
350			This causes `run()` to return as soon as possible. Any pending
351			work items remain queued.
352			*/
353		4x	void stop()
354			{
355		4x	sched_->stop();
356		4x	}
357
358			/** Return whether the context has been stopped.
359
360			@return `true` if `stop()` has been called and `restart()`
361			has not been called since.
362			*/
363		68x	bool stopped() const noexcept
364			{
365		68x	return sched_->stopped();
366			}
367
368			/** Restart the context after being stopped.
369
370			This function must be called before `run()` can be called
371			again after `stop()` has been called.
372			*/
373		2351x	void restart()
374			{
375		2351x	sched_->restart();
376		2351x	}
377
378			/** Process all pending work items.
379
380			This function blocks until all pending work items have been
381			executed or `stop()` is called. The context is stopped
382			when there is no more outstanding work.
383
384			@note The context must be restarted with `restart()` before
385			calling this function again after it returns.
386
387			@return The number of handlers executed.
388			*/
389		3160x	std::size_t run()
390			{
391		3160x	return sched_->run();
392			}
393
394			/** Process at most one pending work item.
395
396			This function blocks until one work item has been executed
397			or `stop()` is called. The context is stopped when there
398			is no more outstanding work.
399
400			@note The context must be restarted with `restart()` before
401			calling this function again after it returns.
402
403			@return The number of handlers executed (0 or 1).
404			*/
405		22x	std::size_t run_one()
406			{
407		22x	return sched_->run_one();
408			}
409
410			/** Process work items for the specified duration.
411
412			This function blocks until work items have been executed for
413			the specified duration, or `stop()` is called. The context
414			is stopped when there is no more outstanding work.
415
416			@note The context must be restarted with `restart()` before
417			calling this function again after it returns.
418
419			@param rel_time The duration for which to process work.
420
421			@return The number of handlers executed.
422			*/
423			template<class Rep, class Period>
424		10x	std::size_t run_for(std::chrono::duration<Rep, Period> const& rel_time)
425			{
426		10x	return run_until(std::chrono::steady_clock::now() + rel_time);
427			}
428
429			/** Process work items until the specified time.
430
431			This function blocks until the specified time is reached
432			or `stop()` is called. The context is stopped when there
433			is no more outstanding work.
434
435			@note The context must be restarted with `restart()` before
436			calling this function again after it returns.
437
438			@param abs_time The time point until which to process work.
439
440			@return The number of handlers executed.
441			*/
442			template<class Clock, class Duration>
443			std::size_t
444		10x	run_until(std::chrono::time_point<Clock, Duration> const& abs_time)
445			{
446		10x	std::size_t n = 0;
447	2/2 ✓ Branch 0 taken 18 times. ✓ Branch 1 taken 10 times.	28x	while (run_one_until(abs_time))
448	1/2 ✗ Branch 0 not taken. ✓ Branch 1 taken 18 times.	18x	if (n != (std::numeric_limits<std::size_t>::max)())
449		18x	++n;
450		10x	return n;
451			}
452
453			/** Process at most one work item for the specified duration.
454
455			This function blocks until one work item has been executed,
456			the specified duration has elapsed, or `stop()` is called.
457			The context is stopped when there is no more outstanding work.
458
459			@note The context must be restarted with `restart()` before
460			calling this function again after it returns.
461
462			@param rel_time The duration for which the call may block.
463
464			@return The number of handlers executed (0 or 1).
465			*/
466			template<class Rep, class Period>
467		6x	std::size_t run_one_for(std::chrono::duration<Rep, Period> const& rel_time)
468			{
469		6x	return run_one_until(std::chrono::steady_clock::now() + rel_time);
470			}
471
472			/** Process at most one work item until the specified time.
473
474			This function blocks until one work item has been executed,
475			the specified time is reached, or `stop()` is called.
476			The context is stopped when there is no more outstanding work.
477
478			@note The context must be restarted with `restart()` before
479			calling this function again after it returns.
480
481			@param abs_time The time point until which the call may block.
482
483			@return The number of handlers executed (0 or 1).
484			*/
485			template<class Clock, class Duration>
486			std::size_t
487		42x	run_one_until(std::chrono::time_point<Clock, Duration> const& abs_time)
488			{
489		42x	typename Clock::time_point now = Clock::now();
490		50x	for (;;)
491			{
492		50x	auto rel_time = abs_time - now;
493			using rel_type = decltype(rel_time);
494	2/2 ✓ Branch 0 taken 4 times. ✓ Branch 1 taken 46 times.	50x	if (rel_time < rel_type::zero())
495		4x	rel_time = rel_type::zero();
496	2/2 ✓ Branch 0 taken 24 times. ✓ Branch 1 taken 22 times.	46x	else if (rel_time > std::chrono::seconds(1))
497		22x	rel_time = std::chrono::seconds(1);
498
499		100x	std::size_t s = sched_->wait_one(
500			static_cast<long>(
501		50x	std::chrono::duration_cast<std::chrono::microseconds>(
502			rel_time)
503		50x	.count()));
504
505	4/4 ✓ Branch 0 taken 28 times. ✓ Branch 1 taken 22 times. ✓ Branch 2 taken 16 times. ✓ Branch 3 taken 12 times.	50x	if (s \|\| stopped())
506		38x	return s;
507
508		12x	now = Clock::now();
509	2/2 ✓ Branch 0 taken 4 times. ✓ Branch 1 taken 8 times.	12x	if (now >= abs_time)
510		4x	return 0;
511			}
512		42x	}
513
514			/** Process all ready work items without blocking.
515
516			This function executes all work items that are ready to run
517			without blocking for more work. The context is stopped
518			when there is no more outstanding work.
519
520			@note The context must be restarted with `restart()` before
521			calling this function again after it returns.
522
523			@return The number of handlers executed.
524			*/
525		16x	std::size_t poll()
526			{
527		16x	return sched_->poll();
528			}
529
530			/** Process at most one ready work item without blocking.
531
532			This function executes at most one work item that is ready
533			to run without blocking for more work. The context is
534			stopped when there is no more outstanding work.
535
536			@note The context must be restarted with `restart()` before
537			calling this function again after it returns.
538
539			@return The number of handlers executed (0 or 1).
540			*/
541		8x	std::size_t poll_one()
542			{
543		8x	return sched_->poll_one();
544			}
545			};
546
547			/** An executor for dispatching work to an I/O context.
548
549			The executor provides the interface for posting work items and
550			dispatching coroutines to the associated context. It satisfies
551			the `capy::Executor` concept.
552
553			Executors are lightweight handles that can be copied and compared
554			for equality. Two executors compare equal if they refer to the
555			same context.
556
557			@par Thread Safety
558			Distinct objects: Safe.@n
559			Shared objects: Safe.
560			*/
561			class io_context::executor_type
562			{
563		2039x	io_context* ctx_ = nullptr;
564
565			public:
566			/** Default constructor.
567
568			Constructs an executor not associated with any context.
569			*/
570		6117x	executor_type() = default;
571
572			/** Construct an executor from a context.
573
574			@param ctx The context to associate with this executor.
575			*/
576		9074x	explicit executor_type(io_context& ctx) noexcept : ctx_(&ctx) {}
577
578			/** Return a reference to the associated execution context.
579
580			@return Reference to the context.
581			*/
582		21639x	io_context& context() const noexcept
583			{
584		21639x	return *ctx_;
585			}
586
587			/** Check if the current thread is running this executor's context.
588
589			@return `true` if `run()` is being called on this thread.
590			*/
591		10701x	bool running_in_this_thread() const noexcept
592			{
593		10701x	return ctx_->sched_->running_in_this_thread();
594			}
595
596			/** Informs the executor that work is beginning.
597
598			Must be paired with `on_work_finished()`.
599			*/
600		10983x	void on_work_started() const noexcept
601			{
602		10983x	ctx_->sched_->work_started();
603		10983x	}
604
605			/** Informs the executor that work has completed.
606
607			@par Preconditions
608			A preceding call to `on_work_started()` on an equal executor.
609			*/
610		10931x	void on_work_finished() const noexcept
611			{
612		10931x	ctx_->sched_->work_finished();
613		10931x	}
614
615			/** Dispatch a continuation.
616
617			Returns a handle for symmetric transfer. If called from
618			within `run()`, returns `c.h`. Otherwise posts `c` for
619			later execution and returns `std::noop_coroutine()`.
620
621			@param c The continuation to dispatch.
622
623			@return A handle for symmetric transfer or `std::noop_coroutine()`.
624
625			@par Preconditions
626			The associated context must outlive this call. Dispatching
627			concurrently with, or after, the context's destruction is
628			undefined behavior.
629			*/
630		10697x	std::coroutine_handle<> dispatch(capy::continuation& c) const
631			{
632	2/2 ✓ Branch 0 taken 462 times. ✓ Branch 1 taken 10235 times.	10697x	if (running_in_this_thread())
633		462x	return c.h;
634		10235x	post(c);
635		10235x	return std::noop_coroutine();
636		10697x	}
637
638			/** Post a continuation for deferred execution.
639
640			Enqueues `c` directly on the scheduler's ready queue.
641			No heap allocation occurs.
642
643			@par Preconditions
644			The associated context must outlive this call. Posting
645			concurrently with, or after, the context's destruction is
646			undefined behavior.
647			*/
648		19523x	void post(capy::continuation& c) const
649			{
650		19523x	ctx_->sched_->post(c);
651		19523x	}
652
653			/** Post a bare coroutine handle for deferred execution.
654
655			Heap-allocates a scheduler_op to wrap the handle. A caller
656			that already owns a `scheduler_op` can post it directly via
657			the `post(scheduler_op*)` overload to avoid the allocation.
658
659			@param h The coroutine handle to post.
660
661			@par Preconditions
662			The associated context must outlive this call. Posting
663			concurrently with, or after, the context's destruction is
664			undefined behavior.
665			*/
666		3681x	void post(std::coroutine_handle<> h) const
667			{
668		3681x	ctx_->sched_->post(h);
669		3681x	}
670
671			/** Compare two executors for equality.
672
673			@return `true` if both executors refer to the same context.
674			*/
675		2x	bool operator==(executor_type const& other) const noexcept
676			{
677		2x	return ctx_ == other.ctx_;
678			}
679
680			/** Compare two executors for inequality.
681
682			@return `true` if the executors refer to different contexts.
683			*/
684			bool operator!=(executor_type const& other) const noexcept
685			{
686			return ctx_ != other.ctx_;
687			}
688			};
689
690			inline io_context::executor_type
691		4536x	io_context::get_executor() const noexcept
692			{
693		4536x	return executor_type(const_cast<io_context&>(*this));
694			}
695
696			} // namespace boost::corosio
697
698			#endif // BOOST_COROSIO_IO_CONTEXT_HPP
699