include/boost/corosio/native/detail/epoll/epoll_scheduler.hpp

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epoll_scheduler.hpp

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1		//
2		// Copyright (c) 2026 Steve Gerbino
3		// Copyright (c) 2026 Michael Vandeberg
4		//
5		// Distributed under the Boost Software License, Version 1.0. (See accompanying
6		// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7		//
8		// Official repository: https://github.com/cppalliance/corosio
9		//
10
11		#ifndef BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
12		#define BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
13
14		#include <boost/corosio/detail/platform.hpp>
15
16		#if BOOST_COROSIO_HAS_EPOLL
17
18		#include <boost/corosio/detail/config.hpp>
19		#include <boost/capy/ex/execution_context.hpp>
20
21		#include <boost/corosio/native/detail/reactor/reactor_scheduler.hpp>
22		#include <boost/corosio/native/detail/reactor/reactor_signal_pipe.hpp>
23
24		#include <boost/corosio/native/detail/epoll/epoll_traits.hpp>
25		#include <boost/corosio/detail/timer_service.hpp>
26		#include <boost/corosio/native/detail/make_err.hpp>
27		#include <boost/corosio/native/detail/posix/posix_resolver_service.hpp>
28		#include <boost/corosio/native/detail/posix/posix_signal_service.hpp>
29		#include <boost/corosio/native/detail/posix/posix_stream_file_service.hpp>
30		#include <boost/corosio/native/detail/posix/posix_random_access_file_service.hpp>
31
32		#include <boost/corosio/detail/except.hpp>
33
34		#include <atomic>
35		#include <chrono>
36		#include <cstdint>
37		#include <mutex>
38		#include <vector>
39
40		#include <errno.h>
41		#include <sys/epoll.h>
42		#include <sys/eventfd.h>
43		#include <sys/timerfd.h>
44		#include <unistd.h>
45
46		namespace boost::corosio::detail {
47
48		/** Linux scheduler using epoll for I/O multiplexing.
49
50		This scheduler implements the scheduler interface using Linux epoll
51		for efficient I/O event notification. It uses a single reactor model
52		where one thread runs epoll_wait while other threads
53		wait on a condition variable for handler work. This design provides:
54
55		- Handler parallelism: N posted handlers can execute on N threads
56		- No thundering herd: condition_variable wakes exactly one thread
57		- IOCP parity: Behavior matches Windows I/O completion port semantics
58
59		When threads call run(), they first try to execute queued handlers.
60		If the queue is empty and no reactor is running, one thread becomes
61		the reactor and runs epoll_wait. Other threads wait on a condition
62		variable until handlers are available.
63
64		@par Thread Safety
65		All public member functions are thread-safe.
66		*/
67		class BOOST_COROSIO_DECL epoll_scheduler final : public reactor_scheduler
68		{
69		public:
70		/** Construct the scheduler.
71
72		Creates an epoll instance, eventfd for reactor interruption,
73		and timerfd for kernel-managed timer expiry.
74
75		@param ctx Reference to the owning execution_context.
76		@param concurrency_hint Hint for expected thread count (unused).
77		*/
78		epoll_scheduler(capy::execution_context& ctx, int concurrency_hint = -1);
79
80		/// Destroy the scheduler.
81		~epoll_scheduler() override;
82
83		epoll_scheduler(epoll_scheduler const&) = delete;
84		epoll_scheduler& operator=(epoll_scheduler const&) = delete;
85
86		/// Shut down the scheduler, draining pending operations.
87		void shutdown() override;
88
89		/// Apply runtime configuration, resizing the event buffer.
90		void configure_reactor(
91		unsigned max_events,
92		unsigned budget_init,
93		unsigned budget_max,
94		unsigned unassisted) override;
95
96		/** Return the epoll file descriptor.
97
98		Used by socket services to register file descriptors
99		for I/O event notification.
100
101		@return The epoll file descriptor.
102		*/
103		int epoll_fd() const noexcept
104		{
105		return epoll_fd_;
106		}
107
108		/** Register a descriptor for persistent monitoring.
109
110		The fd is registered once and stays registered until explicitly
111		deregistered. Events are dispatched via reactor_descriptor_state which
112		tracks pending read/write/connect operations.
113
114		@param fd The file descriptor to register.
115		@param desc Pointer to descriptor data (stored in epoll_event.data.ptr).
116
117		@return The error if registration fails, otherwise a default
118		constructed error code.
119		*/
120		std::error_code
121		register_descriptor(int fd, reactor_descriptor_state* desc) const;
122
123		/** Deregister a persistently registered descriptor.
124
125		@param fd The file descriptor to deregister.
126		*/
127		void deregister_descriptor(int fd) const;
128
129		/// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp).
130	51x	void register_signal_reader(int read_fd) override
131		{
132	51x	if (auto ec = register_descriptor(read_fd, signal_pipe_reader_.arm()))
133	✗	detail::throw_system_error(ec, "epoll_ctl (register)");
134	51x	}
135
136		private:
137		void
138		run_task(lock_type& lock, context_type* ctx,
139		long timeout_us) override;
140		void interrupt_reactor() const override;
141		void update_timerfd() const;
142
143		int epoll_fd_;
144		int event_fd_;
145		int timer_fd_;
146
147		// Watches the global signal self-pipe's read end (armed lazily by
148		// register_signal_reader on the first signal registration).
149		reactor_signal_pipe_reader signal_pipe_reader_;
150
151		// Edge-triggered eventfd state
152		mutable std::atomic<bool> eventfd_armed_{false};
153
154		// Set when the earliest timer changes; flushed before epoll_wait
155		mutable std::atomic<bool> timerfd_stale_{false};
156
157		// Event buffer sized from max_events_per_poll_ (set at construction,
158		// resized by configure_reactor via io_context_options).
159		std::vector<epoll_event> event_buffer_;
160		};
161
162	867x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
163	867x	: epoll_fd_(-1)
164	867x	, event_fd_(-1)
165	867x	, timer_fd_(-1)
166	1734x	, event_buffer_(max_events_per_poll_)
167		{
168	867x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
169	867x	if (epoll_fd_ < 0)
170	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
171
172	867x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
173	867x	if (event_fd_ < 0)
174		{
175	✗	int errn = errno;
176	✗	::close(epoll_fd_);
177	✗	detail::throw_system_error(make_err(errn), "eventfd");
178		}
179
180	867x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
181	867x	if (timer_fd_ < 0)
182		{
183	✗	int errn = errno;
184	✗	::close(event_fd_);
185	✗	::close(epoll_fd_);
186	✗	detail::throw_system_error(make_err(errn), "timerfd_create");
187		}
188
189	867x	epoll_event ev{};
190	867x	ev.events = EPOLLIN \| EPOLLET;
191	867x	ev.data.ptr = nullptr;
192	867x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, event_fd_, &ev) < 0)
193		{
194	✗	int errn = errno;
195	✗	::close(timer_fd_);
196	✗	::close(event_fd_);
197	✗	::close(epoll_fd_);
198	✗	detail::throw_system_error(make_err(errn), "epoll_ctl");
199		}
200
201	867x	epoll_event timer_ev{};
202	867x	timer_ev.events = EPOLLIN \| EPOLLERR;
203	867x	timer_ev.data.ptr = &timer_fd_;
204	867x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, timer_fd_, &timer_ev) < 0)
205		{
206	✗	int errn = errno;
207	✗	::close(timer_fd_);
208	✗	::close(event_fd_);
209	✗	::close(epoll_fd_);
210	✗	detail::throw_system_error(make_err(errn), "epoll_ctl (timerfd)");
211		}
212
213	867x	timer_svc_ = &get_timer_service(ctx, *this);
214	867x	timer_svc_->set_on_earliest_changed(
215	3963x	timer_service::callback(this, [](void* p) {
216	3096x	auto* self = static_cast<epoll_scheduler*>(p);
217	3096x	self->timerfd_stale_.store(true, std::memory_order_release);
218	3096x	self->interrupt_reactor();
219	3096x	}));
220
221	867x	get_resolver_service(ctx, *this);
222	867x	get_signal_service(ctx, *this);
223	867x	get_stream_file_service(ctx, *this);
224	867x	get_random_access_file_service(ctx, *this);
225
226	867x	completed_ops_.push(&task_op_);
227	867x	}
228
229	1734x	inline epoll_scheduler::~epoll_scheduler()
230		{
231	867x	if (timer_fd_ >= 0)
232	867x	::close(timer_fd_);
233	867x	if (event_fd_ >= 0)
234	867x	::close(event_fd_);
235	867x	if (epoll_fd_ >= 0)
236	867x	::close(epoll_fd_);
237	1734x	}
238
239		inline void
240	867x	epoll_scheduler::shutdown()
241		{
242	867x	shutdown_drain();
243
244	867x	if (event_fd_ >= 0)
245	867x	interrupt_reactor();
246	867x	}
247
248		inline void
249	19x	epoll_scheduler::configure_reactor(
250		unsigned max_events,
251		unsigned budget_init,
252		unsigned budget_max,
253		unsigned unassisted)
254		{
255	19x	reactor_scheduler::configure_reactor(
256		max_events, budget_init, budget_max, unassisted);
257	18x	event_buffer_.resize(max_events_per_poll_);
258	18x	}
259
260		inline std::error_code
261	5108x	epoll_scheduler::register_descriptor(int fd, reactor_descriptor_state* desc) const
262		{
263	5108x	epoll_event ev{};
264	5108x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
265	5108x	ev.data.ptr = desc;
266
267	5108x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
268	1x	return make_err(errno);
269
270	5107x	desc->registered_events = ev.events;
271	5107x	desc->fd = fd;
272	5107x	desc->scheduler_ = this;
273	5107x	desc->mutex.set_enabled(reactor_io_locking_);
274	5107x	desc->ready_events_.store(0, std::memory_order_relaxed);
275
276	5107x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
277	5107x	desc->impl_ref_.reset();
278	5107x	desc->read_ready = false;
279	5107x	desc->write_ready = false;
280	5107x	return {};
281	5107x	}
282
283		inline void
284	5056x	epoll_scheduler::deregister_descriptor(int fd) const
285		{
286	5056x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
287	5056x	}
288
289		inline void
290	4757x	epoll_scheduler::interrupt_reactor() const
291		{
292	4757x	bool expected = false;
293	4757x	if (eventfd_armed_.compare_exchange_strong(
294		expected, true, std::memory_order_release,
295		std::memory_order_relaxed))
296		{
297	3530x	std::uint64_t val = 1;
298	3530x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
299		}
300	4757x	}
301
302		inline void
303	4828x	epoll_scheduler::update_timerfd() const
304		{
305	4828x	auto nearest = timer_svc_->nearest_expiry();
306
307	4828x	itimerspec ts{};
308	4828x	int flags = 0;
309
310	4828x	if (nearest == timer_service::time_point::max())
311		{
312		// No timers — disarm by setting to 0 (relative)
313		}
314		else
315		{
316	4649x	auto now = std::chrono::steady_clock::now();
317	4649x	if (nearest <= now)
318		{
319		// Use 1ns instead of 0 — zero disarms the timerfd
320	438x	ts.it_value.tv_nsec = 1;
321		}
322		else
323		{
324	4211x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
325	4211x	nearest - now)
326	4211x	.count();
327	4211x	ts.it_value.tv_sec = nsec / 1000000000;
328	4211x	ts.it_value.tv_nsec = nsec % 1000000000;
329	4211x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
330	✗	ts.it_value.tv_nsec = 1;
331		}
332		}
333
334	4828x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
335	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
336	4828x	}
337
338		inline void
339	29696x	epoll_scheduler::run_task(
340		lock_type& lock, context_type* ctx, long timeout_us)
341		{
342		int timeout_ms;
343	29696x	if (task_interrupted_)
344	23347x	timeout_ms = 0;
345	6349x	else if (timeout_us < 0)
346	6345x	timeout_ms = -1;
347		else
348	4x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
349
350	29696x	if (lock.owns_lock())
351	6351x	lock.unlock();
352
353	29696x	task_cleanup on_exit{this, &lock, ctx};
354
355		// Flush deferred timerfd programming before blocking
356	29696x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
357	2422x	update_timerfd();
358
359	29696x	int nfds = ::epoll_wait(
360		epoll_fd_, event_buffer_.data(),
361	29696x	static_cast<int>(event_buffer_.size()), timeout_ms);
362
363	29696x	if (nfds < 0 && errno != EINTR)
364	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
365
366	29696x	bool check_timers = false;
367	29696x	ready_queue local_ops;
368
369	69126x	for (int i = 0; i < nfds; ++i)
370		{
371	39430x	if (event_buffer_[i].data.ptr == nullptr)
372		{
373		std::uint64_t val;
374		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
375	2663x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
376	2663x	eventfd_armed_.store(false, std::memory_order_relaxed);
377	2663x	continue;
378	2663x	}
379
380	36767x	if (event_buffer_[i].data.ptr == &timer_fd_)
381		{
382		std::uint64_t expirations;
383		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
384		[[maybe_unused]] auto r =
385	2406x	::read(timer_fd_, &expirations, sizeof(expirations));
386	2406x	check_timers = true;
387	2406x	continue;
388	2406x	}
389
390		auto* desc =
391	34361x	static_cast<reactor_descriptor_state*>(event_buffer_[i].data.ptr);
392	34361x	desc->add_ready_events(event_buffer_[i].events);
393
394	34361x	bool expected = false;
395	34361x	if (desc->is_enqueued_.compare_exchange_strong(
396		expected, true, std::memory_order_release,
397		std::memory_order_relaxed))
398		{
399	34361x	local_ops.push(desc);
400		}
401		}
402
403	29696x	if (check_timers)
404		{
405	2406x	timer_svc_->process_expired();
406	2406x	update_timerfd();
407		}
408
409	29696x	lock.lock();
410
411	29696x	completed_ops_.splice(local_ops);
412	29696x	}
413
414		} // namespace boost::corosio::detail
415
416		#endif // BOOST_COROSIO_HAS_EPOLL
417
418		#endif // BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
419