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	18077x	void push(continuation* c) noexcept
72		{
73	18077x	c->reserved = nullptr;
74	18077x	if(tail_)
75	5204x	tail_->reserved = c;
76		else
77	12873x	head_ = c;
78	18077x	tail_ = c;
79	18077x	}
80
81	18412x	continuation* pop() noexcept
82		{
83	18412x	if(!head_)
84	335x	return nullptr;
85	18077x	continuation* c = head_;
86	18077x	head_ = static_cast<continuation*>(head_->reserved);
87	18077x	if(!head_)
88	12873x	tail_ = nullptr;
89	18077x	return c;
90		}
91
92	31165x	bool empty() const noexcept
93		{
94	31165x	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	335x	~impl() = default;
110
111		bool
112	517x	running_in_this_thread() const noexcept
113		{
114	517x	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	335x	drain_abandoned() noexcept
124		{
125	500x	while(auto* c = pop())
126		{
127	165x	auto h = c->h;
128	165x	if(h && h != std::noop_coroutine())
129	114x	h.destroy();
130	165x	}
131	335x	}
132
133	335x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	335x	: num_threads_(num_threads)
135		{
136	335x	if(num_threads_ == 0)
137	4x	num_threads_ = std::max(
138	2x	std::thread::hardware_concurrency(), 1u);
139
140		// Truncate prefix to 12 chars, leaving room for up to 3-digit index.
141	335x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	335x	thread_name_prefix_[n] = '\0';
143	335x	}
144
145		void
146	18077x	post(continuation& c)
147		{
148	18077x	ensure_started();
149		{
150	18077x	std::lock_guard<std::mutex> lock(mutex_);
151	18077x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	18077x	work_cv_.notify_one();
155	18077x	}
156	18077x	}
157
158		void
159	517x	on_work_started() noexcept
160		{
161	517x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	517x	}
163
164		void
165	517x	on_work_finished() noexcept
166		{
167	517x	if(outstanding_work_.fetch_sub(
168	517x	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	265x	std::lock_guard<std::mutex> lock(mutex_);
174	265x	if(outstanding_work_.load(
175	265x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	137x	stop_ = true;
178	137x	done_cv_.notify_all();
179	137x	work_cv_.notify_all();
180		}
181	265x	}
182	517x	}
183
184		void
185	540x	join() noexcept
186		{
187		{
188	540x	std::unique_lock<std::mutex> lock(mutex_);
189	540x	if(joined_)
190	205x	return;
191	335x	joined_ = true;
192
193	335x	if(outstanding_work_.load(
194	335x	std::memory_order_acquire) == 0)
195		{
196	147x	stop_ = true;
197	147x	work_cv_.notify_all();
198		}
199		else
200		{
201	188x	done_cv_.wait(lock, [this]{
202	326x	return stop_;
203		});
204		}
205	540x	}
206
207	704x	for(auto& t : threads_)
208	369x	if(t.joinable())
209	369x	t.join();
210		}
211
212		void
213	337x	stop() noexcept
214		{
215		{
216	337x	std::lock_guard<std::mutex> lock(mutex_);
217	337x	stop_ = true;
218	337x	}
219	337x	work_cv_.notify_all();
220	337x	done_cv_.notify_all();
221	337x	}
222
223		private:
224		void
225	18077x	ensure_started()
226		{
227	18077x	std::call_once(start_flag_, [this]{
228	288x	threads_.reserve(num_threads_);
229	657x	for(std::size_t i = 0; i < num_threads_; ++i)
230	738x	threads_.emplace_back([this, i]{ run(i); });
231	288x	});
232	18077x	}
233
234		void
235	369x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	369x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	369x	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	369x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	369x	~scoped_pool() noexcept { current_.set(nullptr); }
248	369x	} guard(this);
249
250		for(;;)
251		{
252	18281x	continuation* c = nullptr;
253		{
254	18281x	std::unique_lock<std::mutex> lock(mutex_);
255	18281x	work_cv_.wait(lock, [this]{
256	44333x	return !empty() \|\|
257	44333x	stop_;
258		});
259	18281x	if(stop_)
260	738x	return;
261	17912x	c = pop();
262	18281x	}
263	17912x	if(c)
264	17912x	safe_resume(c->h);
265	17912x	}
266	369x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	335x	thread_pool::
272		~thread_pool()
273		{
274	335x	impl_->stop();
275	335x	impl_->join();
276	335x	impl_->drain_abandoned();
277	335x	shutdown();
278	335x	destroy();
279	335x	delete impl_;
280	335x	}
281
282	335x	thread_pool::
283	335x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	335x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	335x	this->set_frame_allocator(std::allocator<void>{});
287	335x	}
288
289		void
290	205x	thread_pool::
291		join() noexcept
292		{
293	205x	impl_->join();
294	205x	}
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	11741x	thread_pool::
307		get_executor() const noexcept
308		{
309	11741x	return executor_type(
310	11741x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	517x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	517x	pool_->impl_->on_work_started();
318	517x	}
319
320		void
321	517x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	517x	pool_->impl_->on_work_finished();
325	517x	}
326
327		void
328	17565x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	17565x	pool_->impl_->post(c);
332	17565x	}
333
334		std::coroutine_handle<>
335	517x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	517x	if(pool_->impl_->running_in_this_thread())
339	5x	return c.h;
340	512x	pool_->impl_->post(c);
341	512x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346