src/ex/thread_pool.cpp

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thread_pool.cpp

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1		//
2		// Copyright (c) 2025 Vinnie Falco ([email protected])
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	14572x	void push(continuation* c) noexcept
72		{
73	14572x	c->reserved = nullptr;
74	14572x	if(tail_)
75	8721x	tail_->reserved = c;
76		else
77	5851x	head_ = c;
78	14572x	tail_ = c;
79	14572x	}
80
81	14957x	continuation* pop() noexcept
82		{
83	14957x	if(!head_)
84	385x	return nullptr;
85	14572x	continuation* c = head_;
86	14572x	head_ = static_cast<continuation*>(head_->reserved);
87	14572x	if(!head_)
88	5851x	tail_ = nullptr;
89	14572x	return c;
90		}
91
92	21283x	bool empty() const noexcept
93		{
94	21283x	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	385x	~impl() = default;
110
111		bool
112	673x	running_in_this_thread() const noexcept
113		{
114	673x	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	385x	drain_abandoned() noexcept
124		{
125	679x	while(auto* c = pop())
126		{
127	294x	auto h = c->h;
128	294x	if(h && h != std::noop_coroutine())
129	243x	h.destroy();
130	294x	}
131	385x	}
132
133	385x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	385x	: num_threads_(num_threads)
135		{
136	385x	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	385x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	385x	thread_name_prefix_[n] = '\0';
143	385x	}
144
145		void
146	14572x	post(continuation& c)
147		{
148	14572x	ensure_started();
149		{
150	14572x	std::lock_guard<std::mutex> lock(mutex_);
151	14572x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	14572x	work_cv_.notify_one();
155	14572x	}
156	14572x	}
157
158		void
159	672x	on_work_started() noexcept
160		{
161	672x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	672x	}
163
164		void
165	672x	on_work_finished() noexcept
166		{
167	672x	if(outstanding_work_.fetch_sub(
168	672x	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	309x	std::lock_guard<std::mutex> lock(mutex_);
174	309x	if(outstanding_work_.load(
175	309x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	181x	stop_ = true;
178	181x	done_cv_.notify_all();
179	181x	work_cv_.notify_all();
180		}
181	309x	}
182	672x	}
183
184		void
185	636x	join() noexcept
186		{
187		{
188	636x	std::unique_lock<std::mutex> lock(mutex_);
189	636x	if(joined_)
190	251x	return;
191	385x	joined_ = true;
192
193	385x	if(outstanding_work_.load(
194	385x	std::memory_order_acquire) == 0)
195		{
196	151x	stop_ = true;
197	151x	work_cv_.notify_all();
198		}
199		else
200		{
201	234x	done_cv_.wait(lock, [this]{
202	416x	return stop_;
203		});
204		}
205	636x	}
206
207	824x	for(auto& t : threads_)
208	439x	if(t.joinable())
209	439x	t.join();
210		}
211
212		void
213	387x	stop() noexcept
214		{
215		{
216	387x	std::lock_guard<std::mutex> lock(mutex_);
217	387x	stop_ = true;
218	387x	}
219	387x	work_cv_.notify_all();
220	387x	done_cv_.notify_all();
221	387x	}
222
223		private:
224		void
225	14572x	ensure_started()
226		{
227	14572x	std::call_once(start_flag_, [this]{
228	335x	threads_.reserve(num_threads_);
229	774x	for(std::size_t i = 0; i < num_threads_; ++i)
230	878x	threads_.emplace_back([this, i]{ run(i); });
231	335x	});
232	14572x	}
233
234		void
235	439x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	439x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	439x	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	439x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	439x	~scoped_pool() noexcept { current_.set(nullptr); }
248	439x	} guard(this);
249
250		for(;;)
251		{
252	14717x	continuation* c = nullptr;
253		{
254	14717x	std::unique_lock<std::mutex> lock(mutex_);
255	14717x	work_cv_.wait(lock, [this]{
256	28184x	return !empty() \|\|
257	28184x	stop_;
258		});
259	14717x	if(stop_)
260	878x	return;
261	14278x	c = pop();
262	14717x	}
263	14278x	if(c)
264	14278x	safe_resume(c->h);
265	14278x	}
266	439x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	385x	thread_pool::
272		~thread_pool()
273		{
274	385x	impl_->stop();
275	385x	impl_->join();
276	385x	impl_->drain_abandoned();
277	385x	shutdown();
278	385x	destroy();
279	385x	delete impl_;
280	385x	}
281
282	385x	thread_pool::
283	385x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	385x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	385x	this->set_frame_allocator(std::allocator<void>{});
287	385x	}
288
289		void
290	251x	thread_pool::
291		join() noexcept
292		{
293	251x	impl_->join();
294	251x	}
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	11797x	thread_pool::
307		get_executor() const noexcept
308		{
309	11797x	return executor_type(
310	11797x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	672x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	672x	pool_->impl_->on_work_started();
318	672x	}
319
320		void
321	672x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	672x	pool_->impl_->on_work_finished();
325	672x	}
326
327		void
328	13906x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	13906x	pool_->impl_->post(c);
332	13906x	}
333
334		std::coroutine_handle<>
335	673x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	673x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	666x	pool_->impl_->post(c);
341	666x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346