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/abandon.hpp>
15		#include <boost/capy/ex/frame_allocator.hpp>
16		#include <boost/capy/test/thread_name.hpp>
17		#include <algorithm>
18		#include <atomic>
19		#include <condition_variable>
20		#include <cstdio>
21		#include <mutex>
22		#include <thread>
23		#include <vector>
24
25		/*
26		Thread pool implementation using a shared work queue.
27
28		Work items are continuations linked via their intrusive next pointer,
29		stored in a single queue protected by a mutex. No per-post heap
30		allocation: the continuation is owned by the caller and linked
31		directly. Worker threads wait on a condition_variable until work
32		is available or stop is requested.
33
34		Threads are started lazily on first post(), under mutex_, to avoid
35		spawning threads for pools that are constructed but never used. A
36		stopped pool starts none. Each thread is named
37		with a configurable prefix plus index for debugger visibility.
38
39		Work tracking: on_work_started/on_work_finished maintain the atomic
40		outstanding_work_ counter. on_work_started is lock-free, and so is
41		on_work_finished while the count stays above one. The release that
42		may take it to zero is made under mutex_, so join() can't see zero,
43		return and let the pool be destroyed while that call still uses it.
44		join() blocks until this counter reaches zero, then signals workers
45		to stop and joins threads.
46
47		Two shutdown paths:
48		- join(): waits for outstanding work to drain, then stops workers.
49		- stop(): immediately signals workers to exit; queued work stays queued.
50		- Destructor: stop() then join(), then queued work is abandoned.
51		*/
52
53		namespace boost {
54		namespace capy {
55
56		//------------------------------------------------------------------------------
57
58		class thread_pool::impl
59		{
60		// Identifies the pool owning the current worker thread, or
61		// nullptr if the calling thread is not a pool worker. Checked
62		// by dispatch() to decide between symmetric transfer (inline
63		// resume) and post.
64		static inline detail::thread_local_ptr<impl const> current_;
65
66		// Intrusive queue of continuations: the next link is stored in
67		// continuation::reserved (typed continuation* round-tripped through
68		// void*). No per-post allocation: the continuation is owned by the caller.
69		continuation* head_ = nullptr;
70		continuation* tail_ = nullptr;
71
72	21233x	void push(continuation* c) noexcept
73		{
74	21233x	c->reserved = nullptr;
75	21233x	if(tail_)
76	6796x	tail_->reserved = c;
77		else
78	14437x	head_ = c;
79	21233x	tail_ = c;
80	21233x	}
81
82	24041x	continuation* pop() noexcept
83		{
84	24041x	if(!head_)
85	2808x	return nullptr;
86	21233x	continuation* c = head_;
87	21233x	head_ = static_cast<continuation*>(head_->reserved);
88	21233x	if(!head_)
89	14437x	tail_ = nullptr;
90	21233x	return c;
91		}
92
93	35012x	bool empty() const noexcept
94		{
95	35012x	return head_ == nullptr;
96		}
97
98		std::mutex mutex_;
99		std::condition_variable work_cv_;
100		std::condition_variable done_cv_;
101		std::vector<std::thread> threads_;
102		std::atomic<std::size_t> outstanding_work_{0};
103		bool stop_{false};
104		bool joined_{false};
105		std::size_t num_threads_;
106		char thread_name_prefix_[13]{}; // 12 chars max + null terminator
107		std::atomic<bool> started_{false};
108
109		public:
110	1404x	~impl() = default;
111
112		bool
113	3185x	running_in_this_thread() const noexcept
114		{
115	3185x	return current_.get() == this;
116		}
117
118		// Abandon queued coroutines. Each is taken under the lock, as other
119		// threads may still post, and abandoned outside it, as abandoning
120		// can post.
121		void
122	2808x	drain_abandoned() noexcept
123		{
124		for(;;)
125		{
126		continuation* c;
127		{
128	3462x	std::lock_guard<std::mutex> lock(mutex_);
129	3462x	c = pop();
130	3462x	}
131	3462x	if(!c)
132	2808x	return;
133	654x	if(c->h && c->h != std::noop_coroutine())
134	403x	capy::abandon(*c);
135	654x	}
136		}
137
138	1404x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
139	1404x	: num_threads_(num_threads)
140		{
141	1404x	if(num_threads_ == 0)
142	8x	num_threads_ = std::max(
143	4x	std::thread::hardware_concurrency(), 1u);
144
145		// Truncate prefix to 12 chars, leaving room for up to 3-digit index.
146	1404x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
147	1404x	thread_name_prefix_[n] = '\0';
148	1404x	}
149
150		void
151	21233x	post(continuation& c)
152		{
153	21233x	if(!started_.load(std::memory_order_acquire))
154		{
155		{
156	574x	std::lock_guard<std::mutex> lock(mutex_);
157	574x	if(stop_)
158		{
159		// Stopped before any thread started: nothing will
160		// run it, and the destructor's drain abandons it.
161	219x	push(&c);
162	219x	return;
163		}
164	574x	}
165	355x	ensure_started();
166		}
167	21014x	std::lock_guard<std::mutex> lock(mutex_);
168	21014x	push(&c);
169		// Under the lock so the pool cannot drain, join, and
170		// destroy the condition variable mid-signal.
171	21014x	work_cv_.notify_one();
172	21014x	}
173
174		void
175	4062x	on_work_started() noexcept
176		{
177	4062x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
178	4062x	}
179
180		void
181	3762x	on_work_finished() noexcept
182		{
183	3762x	auto n = outstanding_work_.load(std::memory_order_relaxed);
184	3763x	while(n > 1)
185		{
186	1262x	if(outstanding_work_.compare_exchange_weak(
187		n, n - 1, std::memory_order_acq_rel))
188	630x	return;
189		}
190	3132x	std::lock_guard<std::mutex> lock(mutex_);
191	3132x	outstanding_work_.fetch_sub(1, std::memory_order_acq_rel);
192		// Re-read: a concurrent on_work_started() may have raised it.
193	3132x	if(outstanding_work_.load(
194	3132x	std::memory_order_acquire) == 0 && joined_ && !stop_)
195		{
196	732x	stop_ = true;
197	732x	done_cv_.notify_all();
198	732x	work_cv_.notify_all();
199		}
200	3132x	}
201
202		void
203	2665x	join() noexcept
204		{
205		{
206	2665x	std::unique_lock<std::mutex> lock(mutex_);
207	2665x	if(joined_)
208	1261x	return;
209	1404x	joined_ = true;
210
211	1404x	if(outstanding_work_.load(
212	1404x	std::memory_order_acquire) == 0)
213		{
214	317x	stop_ = true;
215	317x	work_cv_.notify_all();
216		}
217		else
218		{
219	1087x	done_cv_.wait(lock, [this]{
220	2120x	return stop_;
221		});
222		}
223	2665x	}
224
225	1880x	for(auto& t : threads_)
226	476x	if(t.joinable())
227	476x	t.join();
228		}
229
230		void
231	1707x	stop() noexcept
232		{
233		// Under the lock: the join() this releases may return and let
234		// the pool be destroyed as soon as the lock is free.
235	1707x	std::lock_guard<std::mutex> lock(mutex_);
236	1707x	stop_ = true;
237	1707x	work_cv_.notify_all();
238	1707x	done_cv_.notify_all();
239	1707x	}
240
241		private:
242		void
243	355x	ensure_started()
244		{
245		// Under the lock, so join() either sees every thread started
246		// here or makes this start none. A joined pool still starts them:
247		// join() may be waiting on work that needs them.
248	355x	std::lock_guard<std::mutex> lock(mutex_);
249	355x	if(started_.load(std::memory_order_relaxed) \|\| stop_)
250	7x	return;
251	348x	threads_.reserve(num_threads_);
252	824x	for(std::size_t i = 0; i < num_threads_; ++i)
253	952x	threads_.emplace_back([this, i]{ run(i); });
254	348x	started_.store(true, std::memory_order_release);
255	355x	}
256
257		void
258	476x	run(std::size_t index)
259		{
260		// Build name; set_current_thread_name truncates to platform limits.
261		char name[16];
262	476x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
263	476x	set_current_thread_name(name);
264
265		// Mark this thread as a worker of this pool so dispatch()
266		// can symmetric-transfer when called from within pool work.
267		struct scoped_pool
268		{
269	476x	scoped_pool(impl const* p) noexcept { current_.set(p); }
270	476x	~scoped_pool() noexcept { current_.set(nullptr); }
271	476x	} guard(this);
272
273		for(;;)
274		{
275	21055x	continuation* c = nullptr;
276		{
277	21055x	std::unique_lock<std::mutex> lock(mutex_);
278	21055x	work_cv_.wait(lock, [this]{
279	49347x	return !empty() \|\|
280	49347x	stop_;
281		});
282	21055x	if(stop_)
283	952x	return;
284	20579x	c = pop();
285	21055x	}
286	20579x	if(c)
287	20579x	safe_resume(c->h);
288	20579x	}
289	476x	}
290		};
291
292		//------------------------------------------------------------------------------
293
294	1404x	thread_pool::
295		~thread_pool()
296		{
297	1404x	impl_->stop();
298	1404x	impl_->join();
299	1404x	shutdown();
300		// Takes what the unwinds post too, and the work of any service they
301		// create, which is shut down as it is created.
302	1404x	impl_->drain_abandoned();
303	1404x	destroy();
304		// Services give up their work in shutdown(), but anything a
305		// destructor posts anyway is still destroyed.
306	1404x	impl_->drain_abandoned();
307	1404x	delete impl_;
308	1404x	}
309
310	1404x	thread_pool::
311	1404x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
312	1404x	: impl_(new impl(num_threads, thread_name_prefix))
313		{
314	1404x	this->set_frame_allocator(std::allocator<void>{});
315	1404x	}
316
317		void
318	1261x	thread_pool::
319		join() noexcept
320		{
321	1261x	impl_->join();
322	1261x	}
323
324		void
325	303x	thread_pool::
326		stop() noexcept
327		{
328	303x	impl_->stop();
329	303x	}
330
331		//------------------------------------------------------------------------------
332
333		thread_pool::executor_type
334	15321x	thread_pool::
335		get_executor() const noexcept
336		{
337	15321x	return executor_type(
338	15321x	const_cast<thread_pool&>(*this));
339		}
340
341		void
342	4062x	thread_pool::executor_type::
343		on_work_started() const noexcept
344		{
345	4062x	pool_->impl_->on_work_started();
346	4062x	}
347
348		void
349	3762x	thread_pool::executor_type::
350		on_work_finished() const noexcept
351		{
352	3762x	pool_->impl_->on_work_finished();
353	3762x	}
354
355		void
356	18055x	thread_pool::executor_type::
357		post(continuation& c) const
358		{
359	18055x	pool_->impl_->post(c);
360	18055x	}
361
362		std::coroutine_handle<>
363	3185x	thread_pool::executor_type::
364		dispatch(continuation& c) const
365		{
366	3185x	if(pool_->impl_->running_in_this_thread())
367	7x	return c.h;
368	3178x	pool_->impl_->post(c);
369	3178x	return std::noop_coroutine();
370		}
371
372		} // capy
373		} // boost
374