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	19442x	void push(continuation* c) noexcept
72		{
73	19442x	c->reserved = nullptr;
74	19442x	if(tail_)
75	5396x	tail_->reserved = c;
76		else
77	14046x	head_ = c;
78	19442x	tail_ = c;
79	19442x	}
80
81	19837x	continuation* pop() noexcept
82		{
83	19837x	if(!head_)
84	395x	return nullptr;
85	19442x	continuation* c = head_;
86	19442x	head_ = static_cast<continuation*>(head_->reserved);
87	19442x	if(!head_)
88	14046x	tail_ = nullptr;
89	19442x	return c;
90		}
91
92	33804x	bool empty() const noexcept
93		{
94	33804x	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	395x	~impl() = default;
110
111		bool
112	679x	running_in_this_thread() const noexcept
113		{
114	679x	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	395x	drain_abandoned() noexcept
124		{
125	582x	while(auto* c = pop())
126		{
127	187x	auto h = c->h;
128	187x	if(h && h != std::noop_coroutine())
129	136x	h.destroy();
130	187x	}
131	395x	}
132
133	395x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	395x	: num_threads_(num_threads)
135		{
136	395x	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	395x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	395x	thread_name_prefix_[n] = '\0';
143	395x	}
144
145		void
146	19442x	post(continuation& c)
147		{
148	19442x	ensure_started();
149		{
150	19442x	std::lock_guard<std::mutex> lock(mutex_);
151	19442x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	19442x	work_cv_.notify_one();
155	19442x	}
156	19442x	}
157
158		void
159	678x	on_work_started() noexcept
160		{
161	678x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	678x	}
163
164		void
165	678x	on_work_finished() noexcept
166		{
167	678x	if(outstanding_work_.fetch_sub(
168	678x	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	316x	std::lock_guard<std::mutex> lock(mutex_);
174	316x	if(outstanding_work_.load(
175	316x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	185x	stop_ = true;
178	185x	done_cv_.notify_all();
179	185x	work_cv_.notify_all();
180		}
181	316x	}
182	678x	}
183
184		void
185	652x	join() noexcept
186		{
187		{
188	652x	std::unique_lock<std::mutex> lock(mutex_);
189	652x	if(joined_)
190	257x	return;
191	395x	joined_ = true;
192
193	395x	if(outstanding_work_.load(
194	395x	std::memory_order_acquire) == 0)
195		{
196	157x	stop_ = true;
197	157x	work_cv_.notify_all();
198		}
199		else
200		{
201	238x	done_cv_.wait(lock, [this]{
202	424x	return stop_;
203		});
204		}
205	652x	}
206
207	840x	for(auto& t : threads_)
208	445x	if(t.joinable())
209	445x	t.join();
210		}
211
212		void
213	397x	stop() noexcept
214		{
215		{
216	397x	std::lock_guard<std::mutex> lock(mutex_);
217	397x	stop_ = true;
218	397x	}
219	397x	work_cv_.notify_all();
220	397x	done_cv_.notify_all();
221	397x	}
222
223		private:
224		void
225	19442x	ensure_started()
226		{
227	19442x	std::call_once(start_flag_, [this]{
228	341x	threads_.reserve(num_threads_);
229	786x	for(std::size_t i = 0; i < num_threads_; ++i)
230	890x	threads_.emplace_back([this, i]{ run(i); });
231	341x	});
232	19442x	}
233
234		void
235	445x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	445x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	445x	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	445x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	445x	~scoped_pool() noexcept { current_.set(nullptr); }
248	445x	} guard(this);
249
250		for(;;)
251		{
252	19700x	continuation* c = nullptr;
253		{
254	19700x	std::unique_lock<std::mutex> lock(mutex_);
255	19700x	work_cv_.wait(lock, [this]{
256	48266x	return !empty() \|\|
257	48266x	stop_;
258		});
259	19700x	if(stop_)
260	890x	return;
261	19255x	c = pop();
262	19700x	}
263	19255x	if(c)
264	19255x	safe_resume(c->h);
265	19255x	}
266	445x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	395x	thread_pool::
272		~thread_pool()
273		{
274	395x	impl_->stop();
275	395x	impl_->join();
276	395x	impl_->drain_abandoned();
277	395x	shutdown();
278	395x	destroy();
279	395x	delete impl_;
280	395x	}
281
282	395x	thread_pool::
283	395x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	395x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	395x	this->set_frame_allocator(std::allocator<void>{});
287	395x	}
288
289		void
290	257x	thread_pool::
291		join() noexcept
292		{
293	257x	impl_->join();
294	257x	}
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	11809x	thread_pool::
307		get_executor() const noexcept
308		{
309	11809x	return executor_type(
310	11809x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	678x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	678x	pool_->impl_->on_work_started();
318	678x	}
319
320		void
321	678x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	678x	pool_->impl_->on_work_finished();
325	678x	}
326
327		void
328	18770x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	18770x	pool_->impl_->post(c);
332	18770x	}
333
334		std::coroutine_handle<>
335	679x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	679x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	672x	pool_->impl_->post(c);
341	672x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346