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	18574x	void push(continuation* c) noexcept
72		{
73	18574x	c->reserved = nullptr;
74	18574x	if(tail_)
75	3962x	tail_->reserved = c;
76		else
77	14612x	head_ = c;
78	18574x	tail_ = c;
79	18574x	}
80
81	18972x	continuation* pop() noexcept
82		{
83	18972x	if(!head_)
84	398x	return nullptr;
85	18574x	continuation* c = head_;
86	18574x	head_ = static_cast<continuation*>(head_->reserved);
87	18574x	if(!head_)
88	14612x	tail_ = nullptr;
89	18574x	return c;
90		}
91
92	33576x	bool empty() const noexcept
93		{
94	33576x	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	398x	~impl() = default;
110
111		bool
112	686x	running_in_this_thread() const noexcept
113		{
114	686x	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	398x	drain_abandoned() noexcept
124		{
125	594x	while(auto* c = pop())
126		{
127	196x	auto h = c->h;
128	196x	if(h && h != std::noop_coroutine())
129	145x	h.destroy();
130	196x	}
131	398x	}
132
133	398x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	398x	: num_threads_(num_threads)
135		{
136	398x	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	398x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	398x	thread_name_prefix_[n] = '\0';
143	398x	}
144
145		void
146	18574x	post(continuation& c)
147		{
148	18574x	ensure_started();
149		{
150	18574x	std::lock_guard<std::mutex> lock(mutex_);
151	18574x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	18574x	work_cv_.notify_one();
155	18574x	}
156	18574x	}
157
158		void
159	685x	on_work_started() noexcept
160		{
161	685x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	685x	}
163
164		void
165	685x	on_work_finished() noexcept
166		{
167	685x	if(outstanding_work_.fetch_sub(
168	685x	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	322x	std::lock_guard<std::mutex> lock(mutex_);
174	322x	if(outstanding_work_.load(
175	322x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	178x	stop_ = true;
178	178x	done_cv_.notify_all();
179	178x	work_cv_.notify_all();
180		}
181	322x	}
182	685x	}
183
184		void
185	662x	join() noexcept
186		{
187		{
188	662x	std::unique_lock<std::mutex> lock(mutex_);
189	662x	if(joined_)
190	264x	return;
191	398x	joined_ = true;
192
193	398x	if(outstanding_work_.load(
194	398x	std::memory_order_acquire) == 0)
195		{
196	170x	stop_ = true;
197	170x	work_cv_.notify_all();
198		}
199		else
200		{
201	228x	done_cv_.wait(lock, [this]{
202	407x	return stop_;
203		});
204		}
205	662x	}
206
207	850x	for(auto& t : threads_)
208	452x	if(t.joinable())
209	452x	t.join();
210		}
211
212		void
213	400x	stop() noexcept
214		{
215		{
216	400x	std::lock_guard<std::mutex> lock(mutex_);
217	400x	stop_ = true;
218	400x	}
219	400x	work_cv_.notify_all();
220	400x	done_cv_.notify_all();
221	400x	}
222
223		private:
224		void
225	18574x	ensure_started()
226		{
227	18574x	std::call_once(start_flag_, [this]{
228	348x	threads_.reserve(num_threads_);
229	800x	for(std::size_t i = 0; i < num_threads_; ++i)
230	904x	threads_.emplace_back([this, i]{ run(i); });
231	348x	});
232	18574x	}
233
234		void
235	452x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	452x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	452x	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	452x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	452x	~scoped_pool() noexcept { current_.set(nullptr); }
248	452x	} guard(this);
249
250		for(;;)
251		{
252	18830x	continuation* c = nullptr;
253		{
254	18830x	std::unique_lock<std::mutex> lock(mutex_);
255	18830x	work_cv_.wait(lock, [this]{
256	48687x	return !empty() \|\|
257	48687x	stop_;
258		});
259	18830x	if(stop_)
260	904x	return;
261	18378x	c = pop();
262	18830x	}
263	18378x	if(c)
264	18378x	safe_resume(c->h);
265	18378x	}
266	452x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	398x	thread_pool::
272		~thread_pool()
273		{
274	398x	impl_->stop();
275	398x	impl_->join();
276	398x	impl_->drain_abandoned();
277	398x	shutdown();
278	398x	destroy();
279	398x	delete impl_;
280	398x	}
281
282	398x	thread_pool::
283	398x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	398x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	398x	this->set_frame_allocator(std::allocator<void>{});
287	398x	}
288
289		void
290	264x	thread_pool::
291		join() noexcept
292		{
293	264x	impl_->join();
294	264x	}
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	11810x	thread_pool::
307		get_executor() const noexcept
308		{
309	11810x	return executor_type(
310	11810x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	685x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	685x	pool_->impl_->on_work_started();
318	685x	}
319
320		void
321	685x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	685x	pool_->impl_->on_work_finished();
325	685x	}
326
327		void
328	17895x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	17895x	pool_->impl_->post(c);
332	17895x	}
333
334		std::coroutine_handle<>
335	686x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	686x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	679x	pool_->impl_->post(c);
341	679x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346