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	20272x	void push(continuation* c) noexcept
72		{
73	20272x	c->reserved = nullptr;
74	20272x	if(tail_)
75	6108x	tail_->reserved = c;
76		else
77	14164x	head_ = c;
78	20272x	tail_ = c;
79	20272x	}
80
81	20631x	continuation* pop() noexcept
82		{
83	20631x	if(!head_)
84	359x	return nullptr;
85	20272x	continuation* c = head_;
86	20272x	head_ = static_cast<continuation*>(head_->reserved);
87	20272x	if(!head_)
88	14164x	tail_ = nullptr;
89	20272x	return c;
90		}
91
92	34728x	bool empty() const noexcept
93		{
94	34728x	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	359x	~impl() = default;
110
111		bool
112	647x	running_in_this_thread() const noexcept
113		{
114	647x	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	359x	drain_abandoned() noexcept
124		{
125	702x	while(auto* c = pop())
126		{
127	343x	auto h = c->h;
128	343x	if(h && h != std::noop_coroutine())
129	212x	h.destroy();
130	343x	}
131	359x	}
132
133	359x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	359x	: num_threads_(num_threads)
135		{
136	359x	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	359x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	359x	thread_name_prefix_[n] = '\0';
143	359x	}
144
145		void
146	20272x	post(continuation& c)
147		{
148	20272x	ensure_started();
149		{
150	20272x	std::lock_guard<std::mutex> lock(mutex_);
151	20272x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	20272x	work_cv_.notify_one();
155	20272x	}
156	20272x	}
157
158		void
159	646x	on_work_started() noexcept
160		{
161	646x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	646x	}
163
164		void
165	646x	on_work_finished() noexcept
166		{
167	646x	if(outstanding_work_.fetch_sub(
168	646x	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	283x	std::lock_guard<std::mutex> lock(mutex_);
174	283x	if(outstanding_work_.load(
175	283x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	111x	stop_ = true;
178	111x	done_cv_.notify_all();
179	111x	work_cv_.notify_all();
180		}
181	283x	}
182	646x	}
183
184		void
185	584x	join() noexcept
186		{
187		{
188	584x	std::unique_lock<std::mutex> lock(mutex_);
189	584x	if(joined_)
190	225x	return;
191	359x	joined_ = true;
192
193	359x	if(outstanding_work_.load(
194	359x	std::memory_order_acquire) == 0)
195		{
196	196x	stop_ = true;
197	196x	work_cv_.notify_all();
198		}
199		else
200		{
201	163x	done_cv_.wait(lock, [this]{
202	275x	return stop_;
203		});
204		}
205	584x	}
206
207	772x	for(auto& t : threads_)
208	413x	if(t.joinable())
209	413x	t.join();
210		}
211
212		void
213	361x	stop() noexcept
214		{
215		{
216	361x	std::lock_guard<std::mutex> lock(mutex_);
217	361x	stop_ = true;
218	361x	}
219	361x	work_cv_.notify_all();
220	361x	done_cv_.notify_all();
221	361x	}
222
223		private:
224		void
225	20272x	ensure_started()
226		{
227	20272x	std::call_once(start_flag_, [this]{
228	309x	threads_.reserve(num_threads_);
229	722x	for(std::size_t i = 0; i < num_threads_; ++i)
230	826x	threads_.emplace_back([this, i]{ run(i); });
231	309x	});
232	20272x	}
233
234		void
235	413x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	413x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	413x	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	413x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	413x	~scoped_pool() noexcept { current_.set(nullptr); }
248	413x	} guard(this);
249
250		for(;;)
251		{
252	20342x	continuation* c = nullptr;
253		{
254	20342x	std::unique_lock<std::mutex> lock(mutex_);
255	20342x	work_cv_.wait(lock, [this]{
256	49425x	return !empty() \|\|
257	49425x	stop_;
258		});
259	20342x	if(stop_)
260	826x	return;
261	19929x	c = pop();
262	20342x	}
263	19929x	if(c)
264	19929x	safe_resume(c->h);
265	19929x	}
266	413x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	359x	thread_pool::
272		~thread_pool()
273		{
274	359x	impl_->stop();
275	359x	impl_->join();
276	359x	impl_->drain_abandoned();
277	359x	shutdown();
278	359x	destroy();
279	359x	delete impl_;
280	359x	}
281
282	359x	thread_pool::
283	359x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	359x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	359x	this->set_frame_allocator(std::allocator<void>{});
287	359x	}
288
289		void
290	225x	thread_pool::
291		join() noexcept
292		{
293	225x	impl_->join();
294	225x	}
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	11771x	thread_pool::
307		get_executor() const noexcept
308		{
309	11771x	return executor_type(
310	11771x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	646x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	646x	pool_->impl_->on_work_started();
318	646x	}
319
320		void
321	646x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	646x	pool_->impl_->on_work_finished();
325	646x	}
326
327		void
328	19632x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	19632x	pool_->impl_->post(c);
332	19632x	}
333
334		std::coroutine_handle<>
335	647x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	647x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	640x	pool_->impl_->post(c);
341	640x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346