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	23945x	void push(continuation* c) noexcept
73		{
74	23945x	c->reserved = nullptr;
75	23945x	if(tail_)
76	3075x	tail_->reserved = c;
77		else
78	20870x	head_ = c;
79	23945x	tail_ = c;
80	23945x	}
81
82	26747x	continuation* pop() noexcept
83		{
84	26747x	if(!head_)
85	2802x	return nullptr;
86	23945x	continuation* c = head_;
87	23945x	head_ = static_cast<continuation*>(head_->reserved);
88	23945x	if(!head_)
89	20870x	tail_ = nullptr;
90	23945x	return c;
91		}
92
93	44837x	bool empty() const noexcept
94		{
95	44837x	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	1401x	~impl() = default;
111
112		bool
113	3184x	running_in_this_thread() const noexcept
114		{
115	3184x	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	2802x	drain_abandoned() noexcept
123		{
124		for(;;)
125		{
126		continuation* c;
127		{
128	3409x	std::lock_guard<std::mutex> lock(mutex_);
129	3409x	c = pop();
130	3409x	}
131	3409x	if(!c)
132	2802x	return;
133	607x	if(c->h && c->h != std::noop_coroutine())
134	356x	capy::abandon(*c);
135	607x	}
136		}
137
138	1401x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
139	1401x	: num_threads_(num_threads)
140		{
141	1401x	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	1401x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
147	1401x	thread_name_prefix_[n] = '\0';
148	1401x	}
149
150		void
151	23945x	post(continuation& c)
152		{
153	23945x	if(!started_.load(std::memory_order_acquire))
154		{
155		{
156	567x	std::lock_guard<std::mutex> lock(mutex_);
157	567x	if(stop_)
158		{
159		// Stopped before any thread started: nothing will
160		// run it, and the destructor's drain abandons it.
161	220x	push(&c);
162	220x	return;
163		}
164	567x	}
165	347x	ensure_started();
166		}
167	23725x	std::lock_guard<std::mutex> lock(mutex_);
168	23725x	push(&c);
169		// Under the lock so the pool cannot drain, join, and
170		// destroy the condition variable mid-signal.
171	23725x	work_cv_.notify_one();
172	23725x	}
173
174		void
175	4061x	on_work_started() noexcept
176		{
177	4061x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
178	4061x	}
179
180		void
181	3761x	on_work_finished() noexcept
182		{
183	3761x	auto n = outstanding_work_.load(std::memory_order_relaxed);
184	3766x	while(n > 1)
185		{
186	902x	if(outstanding_work_.compare_exchange_weak(
187		n, n - 1, std::memory_order_acq_rel))
188	446x	return;
189		}
190	3315x	std::lock_guard<std::mutex> lock(mutex_);
191	3315x	outstanding_work_.fetch_sub(1, std::memory_order_acq_rel);
192		// Re-read: a concurrent on_work_started() may have raised it.
193	3315x	if(outstanding_work_.load(
194	3315x	std::memory_order_acquire) == 0 && joined_ && !stop_)
195		{
196	646x	stop_ = true;
197	646x	done_cv_.notify_all();
198	646x	work_cv_.notify_all();
199		}
200	3315x	}
201
202		void
203	2659x	join() noexcept
204		{
205		{
206	2659x	std::unique_lock<std::mutex> lock(mutex_);
207	2659x	if(joined_)
208	1258x	return;
209	1401x	joined_ = true;
210
211	1401x	if(outstanding_work_.load(
212	1401x	std::memory_order_acquire) == 0)
213		{
214	396x	stop_ = true;
215	396x	work_cv_.notify_all();
216		}
217		else
218		{
219	1005x	done_cv_.wait(lock, [this]{
220	1952x	return stop_;
221		});
222		}
223	2659x	}
224
225	1870x	for(auto& t : threads_)
226	469x	if(t.joinable())
227	469x	t.join();
228		}
229
230		void
231	1704x	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	1704x	std::lock_guard<std::mutex> lock(mutex_);
236	1704x	stop_ = true;
237	1704x	work_cv_.notify_all();
238	1704x	done_cv_.notify_all();
239	1704x	}
240
241		private:
242		void
243	347x	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	347x	std::lock_guard<std::mutex> lock(mutex_);
249	347x	if(started_.load(std::memory_order_relaxed) \|\| stop_)
250	3x	return;
251	344x	threads_.reserve(num_threads_);
252	813x	for(std::size_t i = 0; i < num_threads_; ++i)
253	938x	threads_.emplace_back([this, i]{ run(i); });
254	344x	started_.store(true, std::memory_order_release);
255	347x	}
256
257		void
258	469x	run(std::size_t index)
259		{
260		// Build name; set_current_thread_name truncates to platform limits.
261		char name[16];
262	469x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
263	469x	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	469x	scoped_pool(impl const* p) noexcept { current_.set(p); }
270	469x	~scoped_pool() noexcept { current_.set(nullptr); }
271	469x	} guard(this);
272
273		for(;;)
274		{
275	23807x	continuation* c = nullptr;
276		{
277	23807x	std::unique_lock<std::mutex> lock(mutex_);
278	23807x	work_cv_.wait(lock, [this]{
279	66235x	return !empty() \|\|
280	66235x	stop_;
281		});
282	23807x	if(stop_)
283	938x	return;
284	23338x	c = pop();
285	23807x	}
286	23338x	if(c)
287	23338x	safe_resume(c->h);
288	23338x	}
289	469x	}
290		};
291
292		//------------------------------------------------------------------------------
293
294	1401x	thread_pool::
295		~thread_pool()
296		{
297	1401x	impl_->stop();
298	1401x	impl_->join();
299	1401x	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	1401x	impl_->drain_abandoned();
303	1401x	destroy();
304		// Services give up their work in shutdown(), but anything a
305		// destructor posts anyway is still destroyed.
306	1401x	impl_->drain_abandoned();
307	1401x	delete impl_;
308	1401x	}
309
310	1401x	thread_pool::
311	1401x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
312	1401x	: impl_(new impl(num_threads, thread_name_prefix))
313		{
314	1401x	this->set_frame_allocator(std::allocator<void>{});
315	1401x	}
316
317		void
318	1258x	thread_pool::
319		join() noexcept
320		{
321	1258x	impl_->join();
322	1258x	}
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	15318x	thread_pool::
335		get_executor() const noexcept
336		{
337	15318x	return executor_type(
338	15318x	const_cast<thread_pool&>(*this));
339		}
340
341		void
342	4061x	thread_pool::executor_type::
343		on_work_started() const noexcept
344		{
345	4061x	pool_->impl_->on_work_started();
346	4061x	}
347
348		void
349	3761x	thread_pool::executor_type::
350		on_work_finished() const noexcept
351		{
352	3761x	pool_->impl_->on_work_finished();
353	3761x	}
354
355		void
356	20768x	thread_pool::executor_type::
357		post(continuation& c) const
358		{
359	20768x	pool_->impl_->post(c);
360	20768x	}
361
362		std::coroutine_handle<>
363	3184x	thread_pool::executor_type::
364		dispatch(continuation& c) const
365		{
366	3184x	if(pool_->impl_->running_in_this_thread())
367	7x	return c.h;
368	3177x	pool_->impl_->post(c);
369	3177x	return std::noop_coroutine();
370		}
371
372		} // capy
373		} // boost
374