src/ex/thread_pool.cpp

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thread_pool.cpp

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1		//
2		// Copyright (c) 2025 Vinnie Falco ([email protected])
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	18154x	void push(continuation* c) noexcept
72		{
73	18154x	c->reserved = nullptr;
74	18154x	if(tail_)
75	3904x	tail_->reserved = c;
76		else
77	14250x	head_ = c;
78	18154x	tail_ = c;
79	18154x	}
80
81	18541x	continuation* pop() noexcept
82		{
83	18541x	if(!head_)
84	387x	return nullptr;
85	18154x	continuation* c = head_;
86	18154x	head_ = static_cast<continuation*>(head_->reserved);
87	18154x	if(!head_)
88	14250x	tail_ = nullptr;
89	18154x	return c;
90		}
91
92	32545x	bool empty() const noexcept
93		{
94	32545x	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	387x	~impl() = default;
110
111		bool
112	675x	running_in_this_thread() const noexcept
113		{
114	675x	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	387x	drain_abandoned() noexcept
124		{
125	649x	while(auto* c = pop())
126		{
127	262x	auto h = c->h;
128	262x	if(h && h != std::noop_coroutine())
129	209x	h.destroy();
130	262x	}
131	387x	}
132
133	387x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	387x	: num_threads_(num_threads)
135		{
136	387x	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	387x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	387x	thread_name_prefix_[n] = '\0';
143	387x	}
144
145		void
146	18154x	post(continuation& c)
147		{
148	18154x	ensure_started();
149		{
150	18154x	std::lock_guard<std::mutex> lock(mutex_);
151	18154x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	18154x	work_cv_.notify_one();
155	18154x	}
156	18154x	}
157
158		void
159	674x	on_work_started() noexcept
160		{
161	674x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	674x	}
163
164		void
165	674x	on_work_finished() noexcept
166		{
167	674x	if(outstanding_work_.fetch_sub(
168	674x	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	311x	std::lock_guard<std::mutex> lock(mutex_);
174	311x	if(outstanding_work_.load(
175	311x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	196x	stop_ = true;
178	196x	done_cv_.notify_all();
179	196x	work_cv_.notify_all();
180		}
181	311x	}
182	674x	}
183
184		void
185	640x	join() noexcept
186		{
187		{
188	640x	std::unique_lock<std::mutex> lock(mutex_);
189	640x	if(joined_)
190	253x	return;
191	387x	joined_ = true;
192
193	387x	if(outstanding_work_.load(
194	387x	std::memory_order_acquire) == 0)
195		{
196	138x	stop_ = true;
197	138x	work_cv_.notify_all();
198		}
199		else
200		{
201	249x	done_cv_.wait(lock, [this]{
202	446x	return stop_;
203		});
204		}
205	640x	}
206
207	828x	for(auto& t : threads_)
208	441x	if(t.joinable())
209	441x	t.join();
210		}
211
212		void
213	389x	stop() noexcept
214		{
215		{
216	389x	std::lock_guard<std::mutex> lock(mutex_);
217	389x	stop_ = true;
218	389x	}
219	389x	work_cv_.notify_all();
220	389x	done_cv_.notify_all();
221	389x	}
222
223		private:
224		void
225	18154x	ensure_started()
226		{
227	18154x	std::call_once(start_flag_, [this]{
228	337x	threads_.reserve(num_threads_);
229	778x	for(std::size_t i = 0; i < num_threads_; ++i)
230	882x	threads_.emplace_back([this, i]{ run(i); });
231	337x	});
232	18154x	}
233
234		void
235	441x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	441x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	441x	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	441x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	441x	~scoped_pool() noexcept { current_.set(nullptr); }
248	441x	} guard(this);
249
250		for(;;)
251		{
252	18333x	continuation* c = nullptr;
253		{
254	18333x	std::unique_lock<std::mutex> lock(mutex_);
255	18333x	work_cv_.wait(lock, [this]{
256	47090x	return !empty() \|\|
257	47090x	stop_;
258		});
259	18333x	if(stop_)
260	882x	return;
261	17892x	c = pop();
262	18333x	}
263	17892x	if(c)
264	17892x	safe_resume(c->h);
265	17892x	}
266	441x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	387x	thread_pool::
272		~thread_pool()
273		{
274	387x	impl_->stop();
275	387x	impl_->join();
276	387x	impl_->drain_abandoned();
277	387x	shutdown();
278	387x	destroy();
279	387x	delete impl_;
280	387x	}
281
282	387x	thread_pool::
283	387x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	387x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	387x	this->set_frame_allocator(std::allocator<void>{});
287	387x	}
288
289		void
290	253x	thread_pool::
291		join() noexcept
292		{
293	253x	impl_->join();
294	253x	}
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	11799x	thread_pool::
307		get_executor() const noexcept
308		{
309	11799x	return executor_type(
310	11799x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	674x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	674x	pool_->impl_->on_work_started();
318	674x	}
319
320		void
321	674x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	674x	pool_->impl_->on_work_finished();
325	674x	}
326
327		void
328	17486x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	17486x	pool_->impl_->post(c);
332	17486x	}
333
334		std::coroutine_handle<>
335	675x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	675x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	668x	pool_->impl_->post(c);
341	668x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346