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	21048x	void push(continuation* c) noexcept
72		{
73	21048x	c->reserved = nullptr;
74	21048x	if(tail_)
75	2711x	tail_->reserved = c;
76		else
77	18337x	head_ = c;
78	21048x	tail_ = c;
79	21048x	}
80
81	21321x	continuation* pop() noexcept
82		{
83	21321x	if(!head_)
84	273x	return nullptr;
85	21048x	continuation* c = head_;
86	21048x	head_ = static_cast<continuation*>(head_->reserved);
87	21048x	if(!head_)
88	18337x	tail_ = nullptr;
89	21048x	return c;
90		}
91
92	41318x	bool empty() const noexcept
93		{
94	41318x	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	273x	~impl() = default;
110
111		bool
112	456x	running_in_this_thread() const noexcept
113		{
114	456x	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	273x	drain_abandoned() noexcept
124		{
125	476x	while(auto* c = pop())
126		{
127	203x	auto h = c->h;
128	203x	if(h && h != std::noop_coroutine())
129	152x	h.destroy();
130	203x	}
131	273x	}
132
133	273x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	273x	: num_threads_(num_threads)
135		{
136	273x	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	273x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	273x	thread_name_prefix_[n] = '\0';
143	273x	}
144
145		void
146	21048x	post(continuation& c)
147		{
148	21048x	ensure_started();
149		{
150	21048x	std::lock_guard<std::mutex> lock(mutex_);
151	21048x	push(&c);
152	21048x	}
153	21048x	work_cv_.notify_one();
154	21048x	}
155
156		void
157	456x	on_work_started() noexcept
158		{
159	456x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
160	456x	}
161
162		void
163	456x	on_work_finished() noexcept
164		{
165	456x	if(outstanding_work_.fetch_sub(
166	456x	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	204x	std::lock_guard<std::mutex> lock(mutex_);
172	204x	if(outstanding_work_.load(
173	204x	std::memory_order_acquire) == 0 && joined_ && !stop_)
174		{
175	73x	stop_ = true;
176	73x	done_cv_.notify_all();
177	73x	work_cv_.notify_all();
178		}
179	204x	}
180	456x	}
181
182		void
183	416x	join() noexcept
184		{
185		{
186	416x	std::unique_lock<std::mutex> lock(mutex_);
187	416x	if(joined_)
188	143x	return;
189	273x	joined_ = true;
190
191	273x	if(outstanding_work_.load(
192	273x	std::memory_order_acquire) == 0)
193		{
194	145x	stop_ = true;
195	145x	work_cv_.notify_all();
196		}
197		else
198		{
199	128x	done_cv_.wait(lock, [this]{
200	202x	return stop_;
201		});
202		}
203	416x	}
204
205	580x	for(auto& t : threads_)
206	307x	if(t.joinable())
207	307x	t.join();
208		}
209
210		void
211	275x	stop() noexcept
212		{
213		{
214	275x	std::lock_guard<std::mutex> lock(mutex_);
215	275x	stop_ = true;
216	275x	}
217	275x	work_cv_.notify_all();
218	275x	done_cv_.notify_all();
219	275x	}
220
221		private:
222		void
223	21048x	ensure_started()
224		{
225	21048x	std::call_once(start_flag_, [this]{
226	226x	threads_.reserve(num_threads_);
227	533x	for(std::size_t i = 0; i < num_threads_; ++i)
228	614x	threads_.emplace_back([this, i]{ run(i); });
229	226x	});
230	21048x	}
231
232		void
233	307x	run(std::size_t index)
234		{
235		// Build name; set_current_thread_name truncates to platform limits.
236		char name[16];
237	307x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
238	307x	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	307x	scoped_pool(impl const* p) noexcept { current_.set(p); }
245	307x	~scoped_pool() noexcept { current_.set(nullptr); }
246	307x	} guard(this);
247
248		for(;;)
249		{
250	21152x	continuation* c = nullptr;
251		{
252	21152x	std::unique_lock<std::mutex> lock(mutex_);
253	21152x	work_cv_.wait(lock, [this]{
254	61692x	return !empty() \|\|
255	61692x	stop_;
256		});
257	21152x	if(stop_)
258	614x	return;
259	20845x	c = pop();
260	21152x	}
261	20845x	if(c)
262	20845x	safe_resume(c->h);
263	20845x	}
264	307x	}
265		};
266
267		//------------------------------------------------------------------------------
268
269	273x	thread_pool::
270		~thread_pool()
271		{
272	273x	impl_->stop();
273	273x	impl_->join();
274	273x	impl_->drain_abandoned();
275	273x	shutdown();
276	273x	destroy();
277	273x	delete impl_;
278	273x	}
279
280	273x	thread_pool::
281	273x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
282	273x	: impl_(new impl(num_threads, thread_name_prefix))
283		{
284	273x	this->set_frame_allocator(std::allocator<void>{});
285	273x	}
286
287		void
288	143x	thread_pool::
289		join() noexcept
290		{
291	143x	impl_->join();
292	143x	}
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	11679x	thread_pool::
305		get_executor() const noexcept
306		{
307	11679x	return executor_type(
308	11679x	const_cast<thread_pool&>(*this));
309		}
310
311		void
312	456x	thread_pool::executor_type::
313		on_work_started() const noexcept
314		{
315	456x	pool_->impl_->on_work_started();
316	456x	}
317
318		void
319	456x	thread_pool::executor_type::
320		on_work_finished() const noexcept
321		{
322	456x	pool_->impl_->on_work_finished();
323	456x	}
324
325		void
326	20597x	thread_pool::executor_type::
327		post(continuation& c) const
328		{
329	20597x	pool_->impl_->post(c);
330	20597x	}
331
332		std::coroutine_handle<>
333	456x	thread_pool::executor_type::
334		dispatch(continuation& c) const
335		{
336	456x	if(pool_->impl_->running_in_this_thread())
337	5x	return c.h;
338	451x	pool_->impl_->post(c);
339	451x	return std::noop_coroutine();
340		}
341
342		} // capy
343		} // boost
344