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	24089x	void push(continuation* c) noexcept
73		{
74	24089x	c->reserved = nullptr;
75	24089x	if(tail_)
76	3344x	tail_->reserved = c;
77		else
78	20745x	head_ = c;
79	24089x	tail_ = c;
80	24089x	}
81
82	26955x	continuation* pop() noexcept
83		{
84	26955x	if(!head_)
85	2866x	return nullptr;
86	24089x	continuation* c = head_;
87	24089x	head_ = static_cast<continuation*>(head_->reserved);
88	24089x	if(!head_)
89	20745x	tail_ = nullptr;
90	24089x	return c;
91		}
92
93	44736x	bool empty() const noexcept
94		{
95	44736x	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	1433x	~impl() = default;
111
112		bool
113	3211x	running_in_this_thread() const noexcept
114		{
115	3211x	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	2866x	drain_abandoned() noexcept
123		{
124		for(;;)
125		{
126		continuation* c;
127		{
128	3415x	std::lock_guard<std::mutex> lock(mutex_);
129	3415x	c = pop();
130	3415x	}
131	3415x	if(!c)
132	2866x	return;
133	549x	if(c->h && c->h != std::noop_coroutine())
134	296x	capy::abandon(*c);
135	549x	}
136		}
137
138	1433x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
139	1433x	: num_threads_(num_threads)
140		{
141	1433x	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	1433x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
147	1433x	thread_name_prefix_[n] = '\0';
148	1433x	}
149
150		void
151	24091x	post(continuation& c)
152		{
153	24091x	if(!started_.load(std::memory_order_acquire))
154		{
155		{
156	600x	std::lock_guard<std::mutex> lock(mutex_);
157	600x	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	600x	}
165	380x	ensure_started();
166		}
167	23869x	std::lock_guard<std::mutex> lock(mutex_);
168	23869x	push(&c);
169		// Under the lock so the pool cannot drain, join, and
170		// destroy the condition variable mid-signal.
171	23869x	work_cv_.notify_one();
172	23869x	}
173
174		void
175	4091x	on_work_started() noexcept
176		{
177	4091x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
178	4091x	}
179
180		void
181	3791x	on_work_finished() noexcept
182		{
183	3791x	auto n = outstanding_work_.load(std::memory_order_relaxed);
184	3797x	while(n > 1)
185		{
186	904x	if(outstanding_work_.compare_exchange_weak(
187		n, n - 1, std::memory_order_acq_rel))
188	446x	return;
189		}
190	3345x	std::lock_guard<std::mutex> lock(mutex_);
191	3345x	outstanding_work_.fetch_sub(1, std::memory_order_acq_rel);
192		// Re-read: a concurrent on_work_started() may have raised it.
193	3345x	if(outstanding_work_.load(
194	3345x	std::memory_order_acquire) == 0 && joined_ && !stop_)
195		{
196	700x	stop_ = true;
197	700x	done_cv_.notify_all();
198	700x	work_cv_.notify_all();
199		}
200	3345x	}
201
202		void
203	2727x	join() noexcept
204		{
205		{
206	2727x	std::unique_lock<std::mutex> lock(mutex_);
207	2727x	if(joined_)
208	1294x	return;
209	1433x	joined_ = true;
210
211	1433x	if(outstanding_work_.load(
212	1433x	std::memory_order_acquire) == 0)
213		{
214	379x	stop_ = true;
215	379x	work_cv_.notify_all();
216		}
217		else
218		{
219	1054x	done_cv_.wait(lock, [this]{
220	2055x	return stop_;
221		});
222		}
223	2727x	}
224
225	1933x	for(auto& t : threads_)
226	500x	if(t.joinable())
227	500x	t.join();
228		}
229
230		void
231	1736x	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	1736x	std::lock_guard<std::mutex> lock(mutex_);
236	1736x	stop_ = true;
237	1736x	work_cv_.notify_all();
238	1736x	done_cv_.notify_all();
239	1736x	}
240
241		private:
242		void
243	380x	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	380x	std::lock_guard<std::mutex> lock(mutex_);
249	380x	if(started_.load(std::memory_order_relaxed) \|\| stop_)
250	2x	return;
251		// Start at the threads already running: if an earlier call
252		// threw partway, this one adds only the missing ones.
253	378x	threads_.reserve(num_threads_);
254	878x	for(std::size_t i = threads_.size(); i < num_threads_; ++i)
255	1002x	threads_.emplace_back([this, i]{ run(i); });
256	376x	started_.store(true, std::memory_order_release);
257	380x	}
258
259		void
260	500x	run(std::size_t index)
261		{
262		// Build name; set_current_thread_name truncates to platform limits.
263		char name[16];
264	500x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
265	500x	set_current_thread_name(name);
266
267		// Mark this thread as a worker of this pool so dispatch()
268		// can symmetric-transfer when called from within pool work.
269		struct scoped_pool
270		{
271	500x	scoped_pool(impl const* p) noexcept { current_.set(p); }
272	500x	~scoped_pool() noexcept { current_.set(nullptr); }
273	500x	} guard(this);
274
275		for(;;)
276		{
277	24040x	continuation* c = nullptr;
278		{
279	24040x	std::unique_lock<std::mutex> lock(mutex_);
280	24040x	work_cv_.wait(lock, [this]{
281	65839x	return !empty() \|\|
282	65839x	stop_;
283		});
284	24040x	if(stop_)
285	1000x	return;
286	23540x	c = pop();
287	24040x	}
288	23540x	if(c)
289	23540x	safe_resume(c->h);
290	23540x	}
291	500x	}
292		};
293
294		//------------------------------------------------------------------------------
295
296	1433x	thread_pool::
297		~thread_pool()
298		{
299	1433x	impl_->stop();
300	1433x	impl_->join();
301	1433x	shutdown();
302		// Takes what the unwinds post too, and the work of any service they
303		// create, which is shut down as it is created.
304	1433x	impl_->drain_abandoned();
305	1433x	destroy();
306		// Services give up their work in shutdown(), but anything a
307		// destructor posts anyway is still destroyed.
308	1433x	impl_->drain_abandoned();
309	1433x	delete impl_;
310	1433x	}
311
312	1433x	thread_pool::
313	1433x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
314	1433x	: impl_(new impl(num_threads, thread_name_prefix))
315		{
316	1433x	this->set_frame_allocator(std::allocator<void>{});
317	1433x	}
318
319		void
320	1294x	thread_pool::
321		join() noexcept
322		{
323	1294x	impl_->join();
324	1294x	}
325
326		void
327	303x	thread_pool::
328		stop() noexcept
329		{
330	303x	impl_->stop();
331	303x	}
332
333		//------------------------------------------------------------------------------
334
335		thread_pool::executor_type
336	15352x	thread_pool::
337		get_executor() const noexcept
338		{
339	15352x	return executor_type(
340	15352x	const_cast<thread_pool&>(*this));
341		}
342
343		void
344	4091x	thread_pool::executor_type::
345		on_work_started() const noexcept
346		{
347	4091x	pool_->impl_->on_work_started();
348	4091x	}
349
350		void
351	3791x	thread_pool::executor_type::
352		on_work_finished() const noexcept
353		{
354	3791x	pool_->impl_->on_work_finished();
355	3791x	}
356
357		void
358	20884x	thread_pool::executor_type::
359		post(continuation& c) const
360		{
361	20884x	pool_->impl_->post(c);
362	20882x	}
363
364		std::coroutine_handle<>
365	3211x	thread_pool::executor_type::
366		dispatch(continuation& c) const
367		{
368	3211x	if(pool_->impl_->running_in_this_thread())
369	4x	return c.h;
370	3207x	pool_->impl_->post(c);
371	3207x	return std::noop_coroutine();
372		}
373
374		} // capy
375		} // boost
376