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	16637x	void push(continuation* c) noexcept
72		{
73	16637x	c->reserved = nullptr;
74	16637x	if(tail_)
75	7747x	tail_->reserved = c;
76		else
77	8890x	head_ = c;
78	16637x	tail_ = c;
79	16637x	}
80
81	17005x	continuation* pop() noexcept
82		{
83	17005x	if(!head_)
84	368x	return nullptr;
85	16637x	continuation* c = head_;
86	16637x	head_ = static_cast<continuation*>(head_->reserved);
87	16637x	if(!head_)
88	8890x	tail_ = nullptr;
89	16637x	return c;
90		}
91
92	25742x	bool empty() const noexcept
93		{
94	25742x	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	368x	~impl() = default;
110
111		bool
112	656x	running_in_this_thread() const noexcept
113		{
114	656x	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	368x	drain_abandoned() noexcept
124		{
125	667x	while(auto* c = pop())
126		{
127	299x	auto h = c->h;
128	299x	if(h && h != std::noop_coroutine())
129	236x	h.destroy();
130	299x	}
131	368x	}
132
133	368x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	368x	: num_threads_(num_threads)
135		{
136	368x	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	368x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	368x	thread_name_prefix_[n] = '\0';
143	368x	}
144
145		void
146	16637x	post(continuation& c)
147		{
148	16637x	ensure_started();
149		{
150	16637x	std::lock_guard<std::mutex> lock(mutex_);
151	16637x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	16637x	work_cv_.notify_one();
155	16637x	}
156	16637x	}
157
158		void
159	655x	on_work_started() noexcept
160		{
161	655x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	655x	}
163
164		void
165	655x	on_work_finished() noexcept
166		{
167	655x	if(outstanding_work_.fetch_sub(
168	655x	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	306x	std::lock_guard<std::mutex> lock(mutex_);
174	306x	if(outstanding_work_.load(
175	306x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	121x	stop_ = true;
178	121x	done_cv_.notify_all();
179	121x	work_cv_.notify_all();
180		}
181	306x	}
182	655x	}
183
184		void
185	602x	join() noexcept
186		{
187		{
188	602x	std::unique_lock<std::mutex> lock(mutex_);
189	602x	if(joined_)
190	234x	return;
191	368x	joined_ = true;
192
193	368x	if(outstanding_work_.load(
194	368x	std::memory_order_acquire) == 0)
195		{
196	192x	stop_ = true;
197	192x	work_cv_.notify_all();
198		}
199		else
200		{
201	176x	done_cv_.wait(lock, [this]{
202	298x	return stop_;
203		});
204		}
205	602x	}
206
207	790x	for(auto& t : threads_)
208	422x	if(t.joinable())
209	422x	t.join();
210		}
211
212		void
213	370x	stop() noexcept
214		{
215		{
216	370x	std::lock_guard<std::mutex> lock(mutex_);
217	370x	stop_ = true;
218	370x	}
219	370x	work_cv_.notify_all();
220	370x	done_cv_.notify_all();
221	370x	}
222
223		private:
224		void
225	16637x	ensure_started()
226		{
227	16637x	std::call_once(start_flag_, [this]{
228	318x	threads_.reserve(num_threads_);
229	740x	for(std::size_t i = 0; i < num_threads_; ++i)
230	844x	threads_.emplace_back([this, i]{ run(i); });
231	318x	});
232	16637x	}
233
234		void
235	422x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	422x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	422x	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	422x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	422x	~scoped_pool() noexcept { current_.set(nullptr); }
248	422x	} guard(this);
249
250		for(;;)
251		{
252	16760x	continuation* c = nullptr;
253		{
254	16760x	std::unique_lock<std::mutex> lock(mutex_);
255	16760x	work_cv_.wait(lock, [this]{
256	35040x	return !empty() \|\|
257	35040x	stop_;
258		});
259	16760x	if(stop_)
260	844x	return;
261	16338x	c = pop();
262	16760x	}
263	16338x	if(c)
264	16338x	safe_resume(c->h);
265	16338x	}
266	422x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	368x	thread_pool::
272		~thread_pool()
273		{
274	368x	impl_->stop();
275	368x	impl_->join();
276	368x	impl_->drain_abandoned();
277	368x	shutdown();
278	368x	destroy();
279	368x	delete impl_;
280	368x	}
281
282	368x	thread_pool::
283	368x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	368x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	368x	this->set_frame_allocator(std::allocator<void>{});
287	368x	}
288
289		void
290	234x	thread_pool::
291		join() noexcept
292		{
293	234x	impl_->join();
294	234x	}
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	11780x	thread_pool::
307		get_executor() const noexcept
308		{
309	11780x	return executor_type(
310	11780x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	655x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	655x	pool_->impl_->on_work_started();
318	655x	}
319
320		void
321	655x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	655x	pool_->impl_->on_work_finished();
325	655x	}
326
327		void
328	15988x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	15988x	pool_->impl_->post(c);
332	15988x	}
333
334		std::coroutine_handle<>
335	656x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	656x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	649x	pool_->impl_->post(c);
341	649x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346