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	17764x	void push(continuation* c) noexcept
72		{
73	17764x	c->reserved = nullptr;
74	17764x	if(tail_)
75	6733x	tail_->reserved = c;
76		else
77	11031x	head_ = c;
78	17764x	tail_ = c;
79	17764x	}
80
81	18144x	continuation* pop() noexcept
82		{
83	18144x	if(!head_)
84	380x	return nullptr;
85	17764x	continuation* c = head_;
86	17764x	head_ = static_cast<continuation*>(head_->reserved);
87	17764x	if(!head_)
88	11031x	tail_ = nullptr;
89	17764x	return c;
90		}
91
92	29038x	bool empty() const noexcept
93		{
94	29038x	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	380x	~impl() = default;
110
111		bool
112	668x	running_in_this_thread() const noexcept
113		{
114	668x	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	380x	drain_abandoned() noexcept
124		{
125	668x	while(auto* c = pop())
126		{
127	288x	auto h = c->h;
128	288x	if(h && h != std::noop_coroutine())
129	157x	h.destroy();
130	288x	}
131	380x	}
132
133	380x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	380x	: num_threads_(num_threads)
135		{
136	380x	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	380x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	380x	thread_name_prefix_[n] = '\0';
143	380x	}
144
145		void
146	17764x	post(continuation& c)
147		{
148	17764x	ensure_started();
149		{
150	17764x	std::lock_guard<std::mutex> lock(mutex_);
151	17764x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	17764x	work_cv_.notify_one();
155	17764x	}
156	17764x	}
157
158		void
159	667x	on_work_started() noexcept
160		{
161	667x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	667x	}
163
164		void
165	667x	on_work_finished() noexcept
166		{
167	667x	if(outstanding_work_.fetch_sub(
168	667x	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	304x	std::lock_guard<std::mutex> lock(mutex_);
174	304x	if(outstanding_work_.load(
175	304x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	154x	stop_ = true;
178	154x	done_cv_.notify_all();
179	154x	work_cv_.notify_all();
180		}
181	304x	}
182	667x	}
183
184		void
185	626x	join() noexcept
186		{
187		{
188	626x	std::unique_lock<std::mutex> lock(mutex_);
189	626x	if(joined_)
190	246x	return;
191	380x	joined_ = true;
192
193	380x	if(outstanding_work_.load(
194	380x	std::memory_order_acquire) == 0)
195		{
196	174x	stop_ = true;
197	174x	work_cv_.notify_all();
198		}
199		else
200		{
201	206x	done_cv_.wait(lock, [this]{
202	361x	return stop_;
203		});
204		}
205	626x	}
206
207	814x	for(auto& t : threads_)
208	434x	if(t.joinable())
209	434x	t.join();
210		}
211
212		void
213	382x	stop() noexcept
214		{
215		{
216	382x	std::lock_guard<std::mutex> lock(mutex_);
217	382x	stop_ = true;
218	382x	}
219	382x	work_cv_.notify_all();
220	382x	done_cv_.notify_all();
221	382x	}
222
223		private:
224		void
225	17764x	ensure_started()
226		{
227	17764x	std::call_once(start_flag_, [this]{
228	330x	threads_.reserve(num_threads_);
229	764x	for(std::size_t i = 0; i < num_threads_; ++i)
230	868x	threads_.emplace_back([this, i]{ run(i); });
231	330x	});
232	17764x	}
233
234		void
235	434x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	434x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	434x	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	434x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	434x	~scoped_pool() noexcept { current_.set(nullptr); }
248	434x	} guard(this);
249
250		for(;;)
251		{
252	17910x	continuation* c = nullptr;
253		{
254	17910x	std::unique_lock<std::mutex> lock(mutex_);
255	17910x	work_cv_.wait(lock, [this]{
256	40503x	return !empty() \|\|
257	40503x	stop_;
258		});
259	17910x	if(stop_)
260	868x	return;
261	17476x	c = pop();
262	17910x	}
263	17476x	if(c)
264	17476x	safe_resume(c->h);
265	17476x	}
266	434x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	380x	thread_pool::
272		~thread_pool()
273		{
274	380x	impl_->stop();
275	380x	impl_->join();
276	380x	impl_->drain_abandoned();
277	380x	shutdown();
278	380x	destroy();
279	380x	delete impl_;
280	380x	}
281
282	380x	thread_pool::
283	380x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	380x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	380x	this->set_frame_allocator(std::allocator<void>{});
287	380x	}
288
289		void
290	246x	thread_pool::
291		join() noexcept
292		{
293	246x	impl_->join();
294	246x	}
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	11792x	thread_pool::
307		get_executor() const noexcept
308		{
309	11792x	return executor_type(
310	11792x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	667x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	667x	pool_->impl_->on_work_started();
318	667x	}
319
320		void
321	667x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	667x	pool_->impl_->on_work_finished();
325	667x	}
326
327		void
328	17103x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	17103x	pool_->impl_->post(c);
332	17103x	}
333
334		std::coroutine_handle<>
335	668x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	668x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	661x	pool_->impl_->post(c);
341	661x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346