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	18250x	void push(continuation* c) noexcept
72		{
73	18250x	c->reserved = nullptr;
74	18250x	if(tail_)
75	3480x	tail_->reserved = c;
76		else
77	14770x	head_ = c;
78	18250x	tail_ = c;
79	18250x	}
80
81	18647x	continuation* pop() noexcept
82		{
83	18647x	if(!head_)
84	397x	return nullptr;
85	18250x	continuation* c = head_;
86	18250x	head_ = static_cast<continuation*>(head_->reserved);
87	18250x	if(!head_)
88	14770x	tail_ = nullptr;
89	18250x	return c;
90		}
91
92	33461x	bool empty() const noexcept
93		{
94	33461x	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	397x	~impl() = default;
110
111		bool
112	685x	running_in_this_thread() const noexcept
113		{
114	685x	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	397x	drain_abandoned() noexcept
124		{
125	575x	while(auto* c = pop())
126		{
127	178x	auto h = c->h;
128	178x	if(h && h != std::noop_coroutine())
129	128x	h.destroy();
130	178x	}
131	397x	}
132
133	397x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	397x	: num_threads_(num_threads)
135		{
136	397x	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	397x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	397x	thread_name_prefix_[n] = '\0';
143	397x	}
144
145		void
146	18250x	post(continuation& c)
147		{
148	18250x	ensure_started();
149		{
150	18250x	std::lock_guard<std::mutex> lock(mutex_);
151	18250x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	18250x	work_cv_.notify_one();
155	18250x	}
156	18250x	}
157
158		void
159	684x	on_work_started() noexcept
160		{
161	684x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	684x	}
163
164		void
165	684x	on_work_finished() noexcept
166		{
167	684x	if(outstanding_work_.fetch_sub(
168	684x	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	321x	std::lock_guard<std::mutex> lock(mutex_);
174	321x	if(outstanding_work_.load(
175	321x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	191x	stop_ = true;
178	191x	done_cv_.notify_all();
179	191x	work_cv_.notify_all();
180		}
181	321x	}
182	684x	}
183
184		void
185	660x	join() noexcept
186		{
187		{
188	660x	std::unique_lock<std::mutex> lock(mutex_);
189	660x	if(joined_)
190	263x	return;
191	397x	joined_ = true;
192
193	397x	if(outstanding_work_.load(
194	397x	std::memory_order_acquire) == 0)
195		{
196	152x	stop_ = true;
197	152x	work_cv_.notify_all();
198		}
199		else
200		{
201	245x	done_cv_.wait(lock, [this]{
202	437x	return stop_;
203		});
204		}
205	660x	}
206
207	848x	for(auto& t : threads_)
208	451x	if(t.joinable())
209	451x	t.join();
210		}
211
212		void
213	399x	stop() noexcept
214		{
215		{
216	399x	std::lock_guard<std::mutex> lock(mutex_);
217	399x	stop_ = true;
218	399x	}
219	399x	work_cv_.notify_all();
220	399x	done_cv_.notify_all();
221	399x	}
222
223		private:
224		void
225	18250x	ensure_started()
226		{
227	18250x	std::call_once(start_flag_, [this]{
228	347x	threads_.reserve(num_threads_);
229	798x	for(std::size_t i = 0; i < num_threads_; ++i)
230	902x	threads_.emplace_back([this, i]{ run(i); });
231	347x	});
232	18250x	}
233
234		void
235	451x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	451x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	451x	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	451x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	451x	~scoped_pool() noexcept { current_.set(nullptr); }
248	451x	} guard(this);
249
250		for(;;)
251		{
252	18523x	continuation* c = nullptr;
253		{
254	18523x	std::unique_lock<std::mutex> lock(mutex_);
255	18523x	work_cv_.wait(lock, [this]{
256	48758x	return !empty() \|\|
257	48758x	stop_;
258		});
259	18523x	if(stop_)
260	902x	return;
261	18072x	c = pop();
262	18523x	}
263	18072x	if(c)
264	18072x	safe_resume(c->h);
265	18072x	}
266	451x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	397x	thread_pool::
272		~thread_pool()
273		{
274	397x	impl_->stop();
275	397x	impl_->join();
276	397x	impl_->drain_abandoned();
277	397x	shutdown();
278	397x	destroy();
279	397x	delete impl_;
280	397x	}
281
282	397x	thread_pool::
283	397x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	397x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	397x	this->set_frame_allocator(std::allocator<void>{});
287	397x	}
288
289		void
290	263x	thread_pool::
291		join() noexcept
292		{
293	263x	impl_->join();
294	263x	}
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	684x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	684x	pool_->impl_->on_work_started();
318	684x	}
319
320		void
321	684x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	684x	pool_->impl_->on_work_finished();
325	684x	}
326
327		void
328	17572x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	17572x	pool_->impl_->post(c);
332	17572x	}
333
334		std::coroutine_handle<>
335	685x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	685x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	678x	pool_->impl_->post(c);
341	678x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346