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		void register_descriptor(int fd, reactor_descriptor_state* desc) const;
118
119		/** Deregister a persistently registered descriptor.
120
121		@param fd The file descriptor to deregister.
122		*/
123		void deregister_descriptor(int fd) const;
124
125		/// Watch the read end of the POSIX signal self-pipe (see scheduler.hpp).
126	51x	void register_signal_reader(int read_fd) override
127		{
128	51x	register_descriptor(read_fd, signal_pipe_reader_.arm());
129	51x	}
130
131		private:
132		void
133		run_task(lock_type& lock, context_type* ctx,
134		long timeout_us) override;
135		void interrupt_reactor() const override;
136		void update_timerfd() const;
137
138		int epoll_fd_;
139		int event_fd_;
140		int timer_fd_;
141
142		// Watches the global signal self-pipe's read end (armed lazily by
143		// register_signal_reader on the first signal registration).
144		reactor_signal_pipe_reader signal_pipe_reader_;
145
146		// Edge-triggered eventfd state
147		mutable std::atomic<bool> eventfd_armed_{false};
148
149		// Set when the earliest timer changes; flushed before epoll_wait
150		mutable std::atomic<bool> timerfd_stale_{false};
151
152		// Event buffer sized from max_events_per_poll_ (set at construction,
153		// resized by configure_reactor via io_context_options).
154		std::vector<epoll_event> event_buffer_;
155		};
156
157	825x	inline epoll_scheduler::epoll_scheduler(capy::execution_context& ctx, int)
158	825x	: epoll_fd_(-1)
159	825x	, event_fd_(-1)
160	825x	, timer_fd_(-1)
161	1650x	, event_buffer_(max_events_per_poll_)
162		{
163	825x	epoll_fd_ = ::epoll_create1(EPOLL_CLOEXEC);
164	825x	if (epoll_fd_ < 0)
165	✗	detail::throw_system_error(make_err(errno), "epoll_create1");
166
167	825x	event_fd_ = ::eventfd(0, EFD_NONBLOCK \| EFD_CLOEXEC);
168	825x	if (event_fd_ < 0)
169		{
170	✗	int errn = errno;
171	✗	::close(epoll_fd_);
172	✗	detail::throw_system_error(make_err(errn), "eventfd");
173		}
174
175	825x	timer_fd_ = ::timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK \| TFD_CLOEXEC);
176	825x	if (timer_fd_ < 0)
177		{
178	✗	int errn = errno;
179	✗	::close(event_fd_);
180	✗	::close(epoll_fd_);
181	✗	detail::throw_system_error(make_err(errn), "timerfd_create");
182		}
183
184	825x	epoll_event ev{};
185	825x	ev.events = EPOLLIN \| EPOLLET;
186	825x	ev.data.ptr = nullptr;
187	825x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, event_fd_, &ev) < 0)
188		{
189	✗	int errn = errno;
190	✗	::close(timer_fd_);
191	✗	::close(event_fd_);
192	✗	::close(epoll_fd_);
193	✗	detail::throw_system_error(make_err(errn), "epoll_ctl");
194		}
195
196	825x	epoll_event timer_ev{};
197	825x	timer_ev.events = EPOLLIN \| EPOLLERR;
198	825x	timer_ev.data.ptr = &timer_fd_;
199	825x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, timer_fd_, &timer_ev) < 0)
200		{
201	✗	int errn = errno;
202	✗	::close(timer_fd_);
203	✗	::close(event_fd_);
204	✗	::close(epoll_fd_);
205	✗	detail::throw_system_error(make_err(errn), "epoll_ctl (timerfd)");
206		}
207
208	825x	timer_svc_ = &get_timer_service(ctx, *this);
209	825x	timer_svc_->set_on_earliest_changed(
210	6366x	timer_service::callback(this, [](void* p) {
211	5541x	auto* self = static_cast<epoll_scheduler*>(p);
212	5541x	self->timerfd_stale_.store(true, std::memory_order_release);
213	5541x	self->interrupt_reactor();
214	5541x	}));
215
216	825x	get_resolver_service(ctx, *this);
217	825x	get_signal_service(ctx, *this);
218	825x	get_stream_file_service(ctx, *this);
219	825x	get_random_access_file_service(ctx, *this);
220
221	825x	completed_ops_.push(&task_op_);
222	825x	}
223
224	1650x	inline epoll_scheduler::~epoll_scheduler()
225		{
226	825x	if (timer_fd_ >= 0)
227	825x	::close(timer_fd_);
228	825x	if (event_fd_ >= 0)
229	825x	::close(event_fd_);
230	825x	if (epoll_fd_ >= 0)
231	825x	::close(epoll_fd_);
232	1650x	}
233
234		inline void
235	825x	epoll_scheduler::shutdown()
236		{
237	825x	shutdown_drain();
238
239	825x	if (event_fd_ >= 0)
240	825x	interrupt_reactor();
241	825x	}
242
243		inline void
244	19x	epoll_scheduler::configure_reactor(
245		unsigned max_events,
246		unsigned budget_init,
247		unsigned budget_max,
248		unsigned unassisted)
249		{
250	19x	reactor_scheduler::configure_reactor(
251		max_events, budget_init, budget_max, unassisted);
252	18x	event_buffer_.resize(max_events_per_poll_);
253	18x	}
254
255		inline void
256	10003x	epoll_scheduler::register_descriptor(int fd, reactor_descriptor_state* desc) const
257		{
258	10003x	epoll_event ev{};
259	10003x	ev.events = EPOLLIN \| EPOLLOUT \| EPOLLET \| EPOLLERR \| EPOLLHUP;
260	10003x	ev.data.ptr = desc;
261
262	10003x	if (::epoll_ctl(epoll_fd_, EPOLL_CTL_ADD, fd, &ev) < 0)
263	✗	detail::throw_system_error(make_err(errno), "epoll_ctl (register)");
264
265	10003x	desc->registered_events = ev.events;
266	10003x	desc->fd = fd;
267	10003x	desc->scheduler_ = this;
268	10003x	desc->mutex.set_enabled(reactor_io_locking_);
269	10003x	desc->ready_events_.store(0, std::memory_order_relaxed);
270
271	10003x	conditionally_enabled_mutex::scoped_lock lock(desc->mutex);
272	10003x	desc->impl_ref_.reset();
273	10003x	desc->read_ready = false;
274	10003x	desc->write_ready = false;
275	10003x	}
276
277		inline void
278	9952x	epoll_scheduler::deregister_descriptor(int fd) const
279		{
280	9952x	::epoll_ctl(epoll_fd_, EPOLL_CTL_DEL, fd, nullptr);
281	9952x	}
282
283		inline void
284	7112x	epoll_scheduler::interrupt_reactor() const
285		{
286	7112x	bool expected = false;
287	7112x	if (eventfd_armed_.compare_exchange_strong(
288		expected, true, std::memory_order_release,
289		std::memory_order_relaxed))
290		{
291	5968x	std::uint64_t val = 1;
292	5968x	[[maybe_unused]] auto r = ::write(event_fd_, &val, sizeof(val));
293		}
294	7112x	}
295
296		inline void
297	9811x	epoll_scheduler::update_timerfd() const
298		{
299	9811x	auto nearest = timer_svc_->nearest_expiry();
300
301	9811x	itimerspec ts{};
302	9811x	int flags = 0;
303
304	9811x	if (nearest == timer_service::time_point::max())
305		{
306		// No timers — disarm by setting to 0 (relative)
307		}
308		else
309		{
310	9639x	auto now = std::chrono::steady_clock::now();
311	9639x	if (nearest <= now)
312		{
313		// Use 1ns instead of 0 — zero disarms the timerfd
314	860x	ts.it_value.tv_nsec = 1;
315		}
316		else
317		{
318	8779x	auto nsec = std::chrono::duration_cast<std::chrono::nanoseconds>(
319	8779x	nearest - now)
320	8779x	.count();
321	8779x	ts.it_value.tv_sec = nsec / 1000000000;
322	8779x	ts.it_value.tv_nsec = nsec % 1000000000;
323	8779x	if (ts.it_value.tv_sec == 0 && ts.it_value.tv_nsec == 0)
324	✗	ts.it_value.tv_nsec = 1;
325		}
326		}
327
328	9811x	if (::timerfd_settime(timer_fd_, flags, &ts, nullptr) < 0)
329	✗	detail::throw_system_error(make_err(errno), "timerfd_settime");
330	9811x	}
331
332		inline void
333	55110x	epoll_scheduler::run_task(
334		lock_type& lock, context_type* ctx, long timeout_us)
335		{
336		int timeout_ms;
337	55110x	if (task_interrupted_)
338	41568x	timeout_ms = 0;
339	13542x	else if (timeout_us < 0)
340	13538x	timeout_ms = -1;
341		else
342	4x	timeout_ms = static_cast<int>((timeout_us + 999) / 1000);
343
344	55110x	if (lock.owns_lock())
345	13544x	lock.unlock();
346
347	55110x	task_cleanup on_exit{this, &lock, ctx};
348
349		// Flush deferred timerfd programming before blocking
350	55110x	if (timerfd_stale_.exchange(false, std::memory_order_acquire))
351	4913x	update_timerfd();
352
353	55110x	int nfds = ::epoll_wait(
354		epoll_fd_, event_buffer_.data(),
355	55110x	static_cast<int>(event_buffer_.size()), timeout_ms);
356
357	55110x	if (nfds < 0 && errno != EINTR)
358	✗	detail::throw_system_error(make_err(errno), "epoll_wait");
359
360	55110x	bool check_timers = false;
361	55110x	ready_queue local_ops;
362
363	125755x	for (int i = 0; i < nfds; ++i)
364		{
365	70645x	if (event_buffer_[i].data.ptr == nullptr)
366		{
367		std::uint64_t val;
368		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
369	5143x	[[maybe_unused]] auto r = ::read(event_fd_, &val, sizeof(val));
370	5143x	eventfd_armed_.store(false, std::memory_order_relaxed);
371	5143x	continue;
372	5143x	}
373
374	65502x	if (event_buffer_[i].data.ptr == &timer_fd_)
375		{
376		std::uint64_t expirations;
377		// NOLINTNEXTLINE(clang-analyzer-unix.BlockInCriticalSection)
378		[[maybe_unused]] auto r =
379	4898x	::read(timer_fd_, &expirations, sizeof(expirations));
380	4898x	check_timers = true;
381	4898x	continue;
382	4898x	}
383
384		auto* desc =
385	60604x	static_cast<reactor_descriptor_state*>(event_buffer_[i].data.ptr);
386	60604x	desc->add_ready_events(event_buffer_[i].events);
387
388	60604x	bool expected = false;
389	60604x	if (desc->is_enqueued_.compare_exchange_strong(
390		expected, true, std::memory_order_release,
391		std::memory_order_relaxed))
392		{
393	60604x	local_ops.push(desc);
394		}
395		}
396
397	55110x	if (check_timers)
398		{
399	4898x	timer_svc_->process_expired();
400	4898x	update_timerfd();
401		}
402
403	55110x	lock.lock();
404
405	55110x	completed_ops_.splice(local_ops);
406	55110x	}
407
408		} // namespace boost::corosio::detail
409
410		#endif // BOOST_COROSIO_HAS_EPOLL
411
412		#endif // BOOST_COROSIO_NATIVE_DETAIL_EPOLL_EPOLL_SCHEDULER_HPP
413