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	20629x	void push(continuation* c) noexcept
72		{
73	20629x	c->reserved = nullptr;
74	20629x	if(tail_)
75	2242x	tail_->reserved = c;
76		else
77	18387x	head_ = c;
78	20629x	tail_ = c;
79	20629x	}
80
81	20900x	continuation* pop() noexcept
82		{
83	20900x	if(!head_)
84	271x	return nullptr;
85	20629x	continuation* c = head_;
86	20629x	head_ = static_cast<continuation*>(head_->reserved);
87	20629x	if(!head_)
88	18387x	tail_ = nullptr;
89	20629x	return c;
90		}
91
92	40423x	bool empty() const noexcept
93		{
94	40423x	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	271x	~impl() = default;
110
111		bool
112	454x	running_in_this_thread() const noexcept
113		{
114	454x	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	271x	drain_abandoned() noexcept
124		{
125	477x	while(auto* c = pop())
126		{
127	206x	auto h = c->h;
128	206x	if(h && h != std::noop_coroutine())
129	155x	h.destroy();
130	206x	}
131	271x	}
132
133	271x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	271x	: num_threads_(num_threads)
135		{
136	271x	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	271x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	271x	thread_name_prefix_[n] = '\0';
143	271x	}
144
145		void
146	20629x	post(continuation& c)
147		{
148	20629x	ensure_started();
149		{
150	20629x	std::lock_guard<std::mutex> lock(mutex_);
151	20629x	push(&c);
152	20629x	}
153	20629x	work_cv_.notify_one();
154	20629x	}
155
156		void
157	454x	on_work_started() noexcept
158		{
159	454x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
160	454x	}
161
162		void
163	454x	on_work_finished() noexcept
164		{
165	454x	if(outstanding_work_.fetch_sub(
166	454x	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	202x	std::lock_guard<std::mutex> lock(mutex_);
172	202x	if(outstanding_work_.load(
173	202x	std::memory_order_acquire) == 0 && joined_ && !stop_)
174		{
175	74x	stop_ = true;
176	74x	done_cv_.notify_all();
177	74x	work_cv_.notify_all();
178		}
179	202x	}
180	454x	}
181
182		void
183	412x	join() noexcept
184		{
185		{
186	412x	std::unique_lock<std::mutex> lock(mutex_);
187	412x	if(joined_)
188	141x	return;
189	271x	joined_ = true;
190
191	271x	if(outstanding_work_.load(
192	271x	std::memory_order_acquire) == 0)
193		{
194	143x	stop_ = true;
195	143x	work_cv_.notify_all();
196		}
197		else
198		{
199	128x	done_cv_.wait(lock, [this]{
200	203x	return stop_;
201		});
202		}
203	412x	}
204
205	576x	for(auto& t : threads_)
206	305x	if(t.joinable())
207	305x	t.join();
208		}
209
210		void
211	273x	stop() noexcept
212		{
213		{
214	273x	std::lock_guard<std::mutex> lock(mutex_);
215	273x	stop_ = true;
216	273x	}
217	273x	work_cv_.notify_all();
218	273x	done_cv_.notify_all();
219	273x	}
220
221		private:
222		void
223	20629x	ensure_started()
224		{
225	20629x	std::call_once(start_flag_, [this]{
226	224x	threads_.reserve(num_threads_);
227	529x	for(std::size_t i = 0; i < num_threads_; ++i)
228	610x	threads_.emplace_back([this, i]{ run(i); });
229	224x	});
230	20629x	}
231
232		void
233	305x	run(std::size_t index)
234		{
235		// Build name; set_current_thread_name truncates to platform limits.
236		char name[16];
237	305x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
238	305x	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	305x	scoped_pool(impl const* p) noexcept { current_.set(p); }
245	305x	~scoped_pool() noexcept { current_.set(nullptr); }
246	305x	} guard(this);
247
248		for(;;)
249		{
250	20728x	continuation* c = nullptr;
251		{
252	20728x	std::unique_lock<std::mutex> lock(mutex_);
253	20728x	work_cv_.wait(lock, [this]{
254	60328x	return !empty() \|\|
255	60328x	stop_;
256		});
257	20728x	if(stop_)
258	610x	return;
259	20423x	c = pop();
260	20728x	}
261	20423x	if(c)
262	20423x	safe_resume(c->h);
263	20423x	}
264	305x	}
265		};
266
267		//------------------------------------------------------------------------------
268
269	271x	thread_pool::
270		~thread_pool()
271		{
272	271x	impl_->stop();
273	271x	impl_->join();
274	271x	impl_->drain_abandoned();
275	271x	shutdown();
276	271x	destroy();
277	271x	delete impl_;
278	271x	}
279
280	271x	thread_pool::
281	271x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
282	271x	: impl_(new impl(num_threads, thread_name_prefix))
283		{
284	271x	this->set_frame_allocator(std::allocator<void>{});
285	271x	}
286
287		void
288	141x	thread_pool::
289		join() noexcept
290		{
291	141x	impl_->join();
292	141x	}
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	11677x	thread_pool::
305		get_executor() const noexcept
306		{
307	11677x	return executor_type(
308	11677x	const_cast<thread_pool&>(*this));
309		}
310
311		void
312	454x	thread_pool::executor_type::
313		on_work_started() const noexcept
314		{
315	454x	pool_->impl_->on_work_started();
316	454x	}
317
318		void
319	454x	thread_pool::executor_type::
320		on_work_finished() const noexcept
321		{
322	454x	pool_->impl_->on_work_finished();
323	454x	}
324
325		void
326	20180x	thread_pool::executor_type::
327		post(continuation& c) const
328		{
329	20180x	pool_->impl_->post(c);
330	20180x	}
331
332		std::coroutine_handle<>
333	454x	thread_pool::executor_type::
334		dispatch(continuation& c) const
335		{
336	454x	if(pool_->impl_->running_in_this_thread())
337	5x	return c.h;
338	449x	pool_->impl_->post(c);
339	449x	return std::noop_coroutine();
340		}
341
342		} // capy
343		} // boost
344