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	18554x	void push(continuation* c) noexcept
72		{
73	18554x	c->reserved = nullptr;
74	18554x	if(tail_)
75	4640x	tail_->reserved = c;
76		else
77	13914x	head_ = c;
78	18554x	tail_ = c;
79	18554x	}
80
81	18953x	continuation* pop() noexcept
82		{
83	18953x	if(!head_)
84	399x	return nullptr;
85	18554x	continuation* c = head_;
86	18554x	head_ = static_cast<continuation*>(head_->reserved);
87	18554x	if(!head_)
88	13914x	tail_ = nullptr;
89	18554x	return c;
90		}
91
92	33045x	bool empty() const noexcept
93		{
94	33045x	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	399x	~impl() = default;
110
111		bool
112	687x	running_in_this_thread() const noexcept
113		{
114	687x	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	399x	drain_abandoned() noexcept
124		{
125	593x	while(auto* c = pop())
126		{
127	194x	auto h = c->h;
128	194x	if(h && h != std::noop_coroutine())
129	143x	h.destroy();
130	194x	}
131	399x	}
132
133	399x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	399x	: num_threads_(num_threads)
135		{
136	399x	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	399x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	399x	thread_name_prefix_[n] = '\0';
143	399x	}
144
145		void
146	18554x	post(continuation& c)
147		{
148	18554x	ensure_started();
149		{
150	18554x	std::lock_guard<std::mutex> lock(mutex_);
151	18554x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	18554x	work_cv_.notify_one();
155	18554x	}
156	18554x	}
157
158		void
159	686x	on_work_started() noexcept
160		{
161	686x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	686x	}
163
164		void
165	686x	on_work_finished() noexcept
166		{
167	686x	if(outstanding_work_.fetch_sub(
168	686x	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	323x	std::lock_guard<std::mutex> lock(mutex_);
174	323x	if(outstanding_work_.load(
175	323x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	177x	stop_ = true;
178	177x	done_cv_.notify_all();
179	177x	work_cv_.notify_all();
180		}
181	323x	}
182	686x	}
183
184		void
185	664x	join() noexcept
186		{
187		{
188	664x	std::unique_lock<std::mutex> lock(mutex_);
189	664x	if(joined_)
190	265x	return;
191	399x	joined_ = true;
192
193	399x	if(outstanding_work_.load(
194	399x	std::memory_order_acquire) == 0)
195		{
196	170x	stop_ = true;
197	170x	work_cv_.notify_all();
198		}
199		else
200		{
201	229x	done_cv_.wait(lock, [this]{
202	407x	return stop_;
203		});
204		}
205	664x	}
206
207	852x	for(auto& t : threads_)
208	453x	if(t.joinable())
209	453x	t.join();
210		}
211
212		void
213	401x	stop() noexcept
214		{
215		{
216	401x	std::lock_guard<std::mutex> lock(mutex_);
217	401x	stop_ = true;
218	401x	}
219	401x	work_cv_.notify_all();
220	401x	done_cv_.notify_all();
221	401x	}
222
223		private:
224		void
225	18554x	ensure_started()
226		{
227	18554x	std::call_once(start_flag_, [this]{
228	349x	threads_.reserve(num_threads_);
229	802x	for(std::size_t i = 0; i < num_threads_; ++i)
230	906x	threads_.emplace_back([this, i]{ run(i); });
231	349x	});
232	18554x	}
233
234		void
235	453x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	453x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	453x	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	453x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	453x	~scoped_pool() noexcept { current_.set(nullptr); }
248	453x	} guard(this);
249
250		for(;;)
251		{
252	18813x	continuation* c = nullptr;
253		{
254	18813x	std::unique_lock<std::mutex> lock(mutex_);
255	18813x	work_cv_.wait(lock, [this]{
256	47639x	return !empty() \|\|
257	47639x	stop_;
258		});
259	18813x	if(stop_)
260	906x	return;
261	18360x	c = pop();
262	18813x	}
263	18360x	if(c)
264	18360x	safe_resume(c->h);
265	18360x	}
266	453x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	399x	thread_pool::
272		~thread_pool()
273		{
274	399x	impl_->stop();
275	399x	impl_->join();
276	399x	impl_->drain_abandoned();
277	399x	shutdown();
278	399x	destroy();
279	399x	delete impl_;
280	399x	}
281
282	399x	thread_pool::
283	399x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	399x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	399x	this->set_frame_allocator(std::allocator<void>{});
287	399x	}
288
289		void
290	265x	thread_pool::
291		join() noexcept
292		{
293	265x	impl_->join();
294	265x	}
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	11811x	thread_pool::
307		get_executor() const noexcept
308		{
309	11811x	return executor_type(
310	11811x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	686x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	686x	pool_->impl_->on_work_started();
318	686x	}
319
320		void
321	686x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	686x	pool_->impl_->on_work_finished();
325	686x	}
326
327		void
328	17874x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	17874x	pool_->impl_->post(c);
332	17874x	}
333
334		std::coroutine_handle<>
335	687x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	687x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	680x	pool_->impl_->post(c);
341	680x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346