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	20511x	void push(continuation* c) noexcept
72		{
73	20511x	c->reserved = nullptr;
74	20511x	if(tail_)
75	6144x	tail_->reserved = c;
76		else
77	14367x	head_ = c;
78	20511x	tail_ = c;
79	20511x	}
80
81	20900x	continuation* pop() noexcept
82		{
83	20900x	if(!head_)
84	389x	return nullptr;
85	20511x	continuation* c = head_;
86	20511x	head_ = static_cast<continuation*>(head_->reserved);
87	20511x	if(!head_)
88	14367x	tail_ = nullptr;
89	20511x	return c;
90		}
91
92	35237x	bool empty() const noexcept
93		{
94	35237x	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	389x	~impl() = default;
110
111		bool
112	677x	running_in_this_thread() const noexcept
113		{
114	677x	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	389x	drain_abandoned() noexcept
124		{
125	570x	while(auto* c = pop())
126		{
127	181x	auto h = c->h;
128	181x	if(h && h != std::noop_coroutine())
129	130x	h.destroy();
130	181x	}
131	389x	}
132
133	389x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	389x	: num_threads_(num_threads)
135		{
136	389x	if(num_threads_ == 0)
137	8x	num_threads_ = std::max(
138	4x	std::thread::hardware_concurrency(), 1u);
139
140		// Truncate prefix to 12 chars, leaving room for up to 3-digit index.
141	389x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	389x	thread_name_prefix_[n] = '\0';
143	389x	}
144
145		void
146	20511x	post(continuation& c)
147		{
148	20511x	ensure_started();
149		{
150	20511x	std::lock_guard<std::mutex> lock(mutex_);
151	20511x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	20511x	work_cv_.notify_one();
155	20511x	}
156	20511x	}
157
158		void
159	676x	on_work_started() noexcept
160		{
161	676x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	676x	}
163
164		void
165	676x	on_work_finished() noexcept
166		{
167	676x	if(outstanding_work_.fetch_sub(
168	676x	1, std::memory_order_acq_rel) == 1)
169		{
170		// fetch_sub's result can be stale: a concurrent
171		// on_work_started() may raise the count before we take the
172		// lock, so re-read it here rather than trust the decrement.
173	313x	std::lock_guard<std::mutex> lock(mutex_);
174	313x	if(outstanding_work_.load(
175	313x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	189x	stop_ = true;
178	189x	done_cv_.notify_all();
179	189x	work_cv_.notify_all();
180		}
181	313x	}
182	676x	}
183
184		void
185	644x	join() noexcept
186		{
187		{
188	644x	std::unique_lock<std::mutex> lock(mutex_);
189	644x	if(joined_)
190	255x	return;
191	389x	joined_ = true;
192
193	389x	if(outstanding_work_.load(
194	389x	std::memory_order_acquire) == 0)
195		{
196	146x	stop_ = true;
197	146x	work_cv_.notify_all();
198		}
199		else
200		{
201	243x	done_cv_.wait(lock, [this]{
202	433x	return stop_;
203		});
204		}
205	644x	}
206
207	832x	for(auto& t : threads_)
208	443x	if(t.joinable())
209	443x	t.join();
210		}
211
212		void
213	391x	stop() noexcept
214		{
215		{
216	391x	std::lock_guard<std::mutex> lock(mutex_);
217	391x	stop_ = true;
218	391x	}
219	391x	work_cv_.notify_all();
220	391x	done_cv_.notify_all();
221	391x	}
222
223		private:
224		void
225	20511x	ensure_started()
226		{
227	20511x	std::call_once(start_flag_, [this]{
228	339x	threads_.reserve(num_threads_);
229	782x	for(std::size_t i = 0; i < num_threads_; ++i)
230	886x	threads_.emplace_back([this, i]{ run(i); });
231	339x	});
232	20511x	}
233
234		void
235	443x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	443x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	443x	set_current_thread_name(name);
241
242		// Mark this thread as a worker of this pool so dispatch()
243		// can symmetric-transfer when called from within pool work.
244		struct scoped_pool
245		{
246	443x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	443x	~scoped_pool() noexcept { current_.set(nullptr); }
248	443x	} guard(this);
249
250		for(;;)
251		{
252	20773x	continuation* c = nullptr;
253		{
254	20773x	std::unique_lock<std::mutex> lock(mutex_);
255	20773x	work_cv_.wait(lock, [this]{
256	50051x	return !empty() \|\|
257	50051x	stop_;
258		});
259	20773x	if(stop_)
260	886x	return;
261	20330x	c = pop();
262	20773x	}
263	20330x	if(c)
264	20330x	safe_resume(c->h);
265	20330x	}
266	443x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	389x	thread_pool::
272		~thread_pool()
273		{
274	389x	impl_->stop();
275	389x	impl_->join();
276	389x	impl_->drain_abandoned();
277	389x	shutdown();
278	389x	destroy();
279	389x	delete impl_;
280	389x	}
281
282	389x	thread_pool::
283	389x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	389x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	389x	this->set_frame_allocator(std::allocator<void>{});
287	389x	}
288
289		void
290	255x	thread_pool::
291		join() noexcept
292		{
293	255x	impl_->join();
294	255x	}
295
296		void
297	2x	thread_pool::
298		stop() noexcept
299		{
300	2x	impl_->stop();
301	2x	}
302
303		//------------------------------------------------------------------------------
304
305		thread_pool::executor_type
306	11801x	thread_pool::
307		get_executor() const noexcept
308		{
309	11801x	return executor_type(
310	11801x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	676x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	676x	pool_->impl_->on_work_started();
318	676x	}
319
320		void
321	676x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	676x	pool_->impl_->on_work_finished();
325	676x	}
326
327		void
328	19841x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	19841x	pool_->impl_->post(c);
332	19841x	}
333
334		std::coroutine_handle<>
335	677x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	677x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	670x	pool_->impl_->post(c);
341	670x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346