src/ex/thread_pool.cpp

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thread_pool.cpp

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1		//
2		// Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
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/boostorg/capy
9		//
10
11		#include <boost/capy/ex/thread_pool.hpp>
12		#include <boost/capy/continuation.hpp>
13		#include <boost/capy/detail/thread_local_ptr.hpp>
14		#include <boost/capy/ex/frame_allocator.hpp>
15		#include <boost/capy/test/thread_name.hpp>
16		#include <algorithm>
17		#include <atomic>
18		#include <condition_variable>
19		#include <cstdio>
20		#include <mutex>
21		#include <thread>
22		#include <vector>
23
24		/*
25		Thread pool implementation using a shared work queue.
26
27		Work items are continuations linked via their intrusive next pointer,
28		stored in a single queue protected by a mutex. No per-post heap
29		allocation: the continuation is owned by the caller and linked
30		directly. Worker threads wait on a condition_variable until work
31		is available or stop is requested.
32
33		Threads are started lazily on first post() via std::call_once to avoid
34		spawning threads for pools that are constructed but never used. Each
35		thread is named with a configurable prefix plus index for debugger
36		visibility.
37
38		Work tracking: on_work_started/on_work_finished maintain the atomic
39		outstanding_work_ counter. on_work_started is lock-free; the worker
40		that drives the count to zero takes mutex_ and re-reads the count
41		before deciding to stop, so the count and the stop decision stay
42		consistent even if work is started in between. join() blocks until
43		this counter reaches zero, then signals workers to stop and joins
44		threads.
45
46		Two shutdown paths:
47		- join(): waits for outstanding work to drain, then stops workers.
48		- stop(): immediately signals workers to exit; queued work is abandoned.
49		- Destructor: stop() then join() (abandon + wait for threads).
50		*/
51
52		namespace boost {
53		namespace capy {
54
55		//------------------------------------------------------------------------------
56
57		class thread_pool::impl
58		{
59		// Identifies the pool owning the current worker thread, or
60		// nullptr if the calling thread is not a pool worker. Checked
61		// by dispatch() to decide between symmetric transfer (inline
62		// resume) and post.
63		static inline detail::thread_local_ptr<impl const> current_;
64
65		// Intrusive queue of continuations: the next link is stored in
66		// continuation::reserved (typed continuation* round-tripped through
67		// void*). No per-post allocation: the continuation is owned by the caller.
68		continuation* head_ = nullptr;
69		continuation* tail_ = nullptr;
70
71	20845x	void push(continuation* c) noexcept
72		{
73	20845x	c->reserved = nullptr;
74	20845x	if(tail_)
75	1623x	tail_->reserved = c;
76		else
77	19222x	head_ = c;
78	20845x	tail_ = c;
79	20845x	}
80
81	21115x	continuation* pop() noexcept
82		{
83	21115x	if(!head_)
84	270x	return nullptr;
85	20845x	continuation* c = head_;
86	20845x	head_ = static_cast<continuation*>(head_->reserved);
87	20845x	if(!head_)
88	19222x	tail_ = nullptr;
89	20845x	return c;
90		}
91
92	41126x	bool empty() const noexcept
93		{
94	41126x	return head_ == nullptr;
95		}
96
97		std::mutex mutex_;
98		std::condition_variable work_cv_;
99		std::condition_variable done_cv_;
100		std::vector<std::thread> threads_;
101		std::atomic<std::size_t> outstanding_work_{0};
102		bool stop_{false};
103		bool joined_{false};
104		std::size_t num_threads_;
105		char thread_name_prefix_[13]{}; // 12 chars max + null terminator
106		std::once_flag start_flag_;
107
108		public:
109	270x	~impl() = default;
110
111		bool
112	453x	running_in_this_thread() const noexcept
113		{
114	453x	return current_.get() == this;
115		}
116
117		// Destroy abandoned coroutine frames. Must be called
118		// before execution_context::shutdown()/destroy() so
119		// that suspended-frame destructors touching services
120		// (e.g. cancelling registrations) run while those
121		// services are still valid.
122		void
123	270x	drain_abandoned() noexcept
124		{
125	483x	while(auto* c = pop())
126		{
127	213x	auto h = c->h;
128	213x	if(h && h != std::noop_coroutine())
129	162x	h.destroy();
130	213x	}
131	270x	}
132
133	270x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	270x	: num_threads_(num_threads)
135		{
136	270x	if(num_threads_ == 0)
137	4x	num_threads_ = std::max(
138	2x	std::thread::hardware_concurrency(), 1u);
139
140		// Truncate prefix to 12 chars, leaving room for up to 3-digit index.
141	270x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	270x	thread_name_prefix_[n] = '\0';
143	270x	}
144
145		void
146	20845x	post(continuation& c)
147		{
148	20845x	ensure_started();
149		{
150	20845x	std::lock_guard<std::mutex> lock(mutex_);
151	20845x	push(&c);
152	20845x	}
153	20845x	work_cv_.notify_one();
154	20845x	}
155
156		void
157	453x	on_work_started() noexcept
158		{
159	453x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
160	453x	}
161
162		void
163	453x	on_work_finished() noexcept
164		{
165	453x	if(outstanding_work_.fetch_sub(
166	453x	1, std::memory_order_acq_rel) == 1)
167		{
168		// fetch_sub's result can be stale: a concurrent
169		// on_work_started() may raise the count before we take the
170		// lock, so re-read it here rather than trust the decrement.
171	201x	std::lock_guard<std::mutex> lock(mutex_);
172	201x	if(outstanding_work_.load(
173	201x	std::memory_order_acquire) == 0 && joined_ && !stop_)
174		{
175	69x	stop_ = true;
176	69x	done_cv_.notify_all();
177	69x	work_cv_.notify_all();
178		}
179	201x	}
180	453x	}
181
182		void
183	410x	join() noexcept
184		{
185		{
186	410x	std::unique_lock<std::mutex> lock(mutex_);
187	410x	if(joined_)
188	140x	return;
189	270x	joined_ = true;
190
191	270x	if(outstanding_work_.load(
192	270x	std::memory_order_acquire) == 0)
193		{
194	148x	stop_ = true;
195	148x	work_cv_.notify_all();
196		}
197		else
198		{
199	122x	done_cv_.wait(lock, [this]{
200	192x	return stop_;
201		});
202		}
203	410x	}
204
205	574x	for(auto& t : threads_)
206	304x	if(t.joinable())
207	304x	t.join();
208		}
209
210		void
211	272x	stop() noexcept
212		{
213		{
214	272x	std::lock_guard<std::mutex> lock(mutex_);
215	272x	stop_ = true;
216	272x	}
217	272x	work_cv_.notify_all();
218	272x	done_cv_.notify_all();
219	272x	}
220
221		private:
222		void
223	20845x	ensure_started()
224		{
225	20845x	std::call_once(start_flag_, [this]{
226	223x	threads_.reserve(num_threads_);
227	527x	for(std::size_t i = 0; i < num_threads_; ++i)
228	608x	threads_.emplace_back([this, i]{ run(i); });
229	223x	});
230	20845x	}
231
232		void
233	304x	run(std::size_t index)
234		{
235		// Build name; set_current_thread_name truncates to platform limits.
236		char name[16];
237	304x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
238	304x	set_current_thread_name(name);
239
240		// Mark this thread as a worker of this pool so dispatch()
241		// can symmetric-transfer when called from within pool work.
242		struct scoped_pool
243		{
244	304x	scoped_pool(impl const* p) noexcept { current_.set(p); }
245	304x	~scoped_pool() noexcept { current_.set(nullptr); }
246	304x	} guard(this);
247
248		for(;;)
249		{
250	20936x	continuation* c = nullptr;
251		{
252	20936x	std::unique_lock<std::mutex> lock(mutex_);
253	20936x	work_cv_.wait(lock, [this]{
254	61521x	return !empty() \|\|
255	61521x	stop_;
256		});
257	20936x	if(stop_)
258	608x	return;
259	20632x	c = pop();
260	20936x	}
261	20632x	if(c)
262	20632x	safe_resume(c->h);
263	20632x	}
264	304x	}
265		};
266
267		//------------------------------------------------------------------------------
268
269	270x	thread_pool::
270		~thread_pool()
271		{
272	270x	impl_->stop();
273	270x	impl_->join();
274	270x	impl_->drain_abandoned();
275	270x	shutdown();
276	270x	destroy();
277	270x	delete impl_;
278	270x	}
279
280	270x	thread_pool::
281	270x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
282	270x	: impl_(new impl(num_threads, thread_name_prefix))
283		{
284	270x	this->set_frame_allocator(std::allocator<void>{});
285	270x	}
286
287		void
288	140x	thread_pool::
289		join() noexcept
290		{
291	140x	impl_->join();
292	140x	}
293
294		void
295	2x	thread_pool::
296		stop() noexcept
297		{
298	2x	impl_->stop();
299	2x	}
300
301		//------------------------------------------------------------------------------
302
303		thread_pool::executor_type
304	11676x	thread_pool::
305		get_executor() const noexcept
306		{
307	11676x	return executor_type(
308	11676x	const_cast<thread_pool&>(*this));
309		}
310
311		void
312	453x	thread_pool::executor_type::
313		on_work_started() const noexcept
314		{
315	453x	pool_->impl_->on_work_started();
316	453x	}
317
318		void
319	453x	thread_pool::executor_type::
320		on_work_finished() const noexcept
321		{
322	453x	pool_->impl_->on_work_finished();
323	453x	}
324
325		void
326	20397x	thread_pool::executor_type::
327		post(continuation& c) const
328		{
329	20397x	pool_->impl_->post(c);
330	20397x	}
331
332		std::coroutine_handle<>
333	453x	thread_pool::executor_type::
334		dispatch(continuation& c) const
335		{
336	453x	if(pool_->impl_->running_in_this_thread())
337	5x	return c.h;
338	448x	pool_->impl_->post(c);
339	448x	return std::noop_coroutine();
340		}
341
342		} // capy
343		} // boost
344