src/ex/thread_pool.cpp

100.0% Lines (140/140) 100.0% List of functions (29/29)

[ ] prev/next file

thread_pool.cpp

f(x) Functions (29)

Function Calls Lines Blocks

Line	Hits	Source Code
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	16806x	void push(continuation* c) noexcept
72		{
73	16806x	c->reserved = nullptr;
74	16806x	if(tail_)
75	5918x	tail_->reserved = c;
76		else
77	10888x	head_ = c;
78	16806x	tail_ = c;
79	16806x	}
80
81	17071x	continuation* pop() noexcept
82		{
83	17071x	if(!head_)
84	265x	return nullptr;
85	16806x	continuation* c = head_;
86	16806x	head_ = static_cast<continuation*>(head_->reserved);
87	16806x	if(!head_)
88	10888x	tail_ = nullptr;
89	16806x	return c;
90		}
91
92	28926x	bool empty() const noexcept
93		{
94	28926x	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	265x	~impl() = default;
110
111		bool
112	448x	running_in_this_thread() const noexcept
113		{
114	448x	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	265x	drain_abandoned() noexcept
124		{
125	496x	while(auto* c = pop())
126		{
127	231x	auto h = c->h;
128	231x	if(h && h != std::noop_coroutine())
129	180x	h.destroy();
130	231x	}
131	265x	}
132
133	265x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	265x	: num_threads_(num_threads)
135		{
136	265x	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	265x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	265x	thread_name_prefix_[n] = '\0';
143	265x	}
144
145		void
146	16806x	post(continuation& c)
147		{
148	16806x	ensure_started();
149		{
150	16806x	std::lock_guard<std::mutex> lock(mutex_);
151	16806x	push(&c);
152	16806x	}
153	16806x	work_cv_.notify_one();
154	16806x	}
155
156		void
157	448x	on_work_started() noexcept
158		{
159	448x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
160	448x	}
161
162		void
163	448x	on_work_finished() noexcept
164		{
165	448x	if(outstanding_work_.fetch_sub(
166	448x	1, std::memory_order_acq_rel) == 1)
167		{
168		// fetch_sub's result can be stale: a concurrent
169		// on_work_started() may raise the count before we take the
170		// lock, so re-read it here rather than trust the decrement.
171	196x	std::lock_guard<std::mutex> lock(mutex_);
172	196x	if(outstanding_work_.load(
173	196x	std::memory_order_acquire) == 0 && joined_ && !stop_)
174		{
175	90x	stop_ = true;
176	90x	done_cv_.notify_all();
177	90x	work_cv_.notify_all();
178		}
179	196x	}
180	448x	}
181
182		void
183	400x	join() noexcept
184		{
185		{
186	400x	std::unique_lock<std::mutex> lock(mutex_);
187	400x	if(joined_)
188	135x	return;
189	265x	joined_ = true;
190
191	265x	if(outstanding_work_.load(
192	265x	std::memory_order_acquire) == 0)
193		{
194	121x	stop_ = true;
195	121x	work_cv_.notify_all();
196		}
197		else
198		{
199	144x	done_cv_.wait(lock, [this]{
200	235x	return stop_;
201		});
202		}
203	400x	}
204
205	564x	for(auto& t : threads_)
206	299x	if(t.joinable())
207	299x	t.join();
208		}
209
210		void
211	267x	stop() noexcept
212		{
213		{
214	267x	std::lock_guard<std::mutex> lock(mutex_);
215	267x	stop_ = true;
216	267x	}
217	267x	work_cv_.notify_all();
218	267x	done_cv_.notify_all();
219	267x	}
220
221		private:
222		void
223	16806x	ensure_started()
224		{
225	16806x	std::call_once(start_flag_, [this]{
226	218x	threads_.reserve(num_threads_);
227	517x	for(std::size_t i = 0; i < num_threads_; ++i)
228	598x	threads_.emplace_back([this, i]{ run(i); });
229	218x	});
230	16806x	}
231
232		void
233	299x	run(std::size_t index)
234		{
235		// Build name; set_current_thread_name truncates to platform limits.
236		char name[16];
237	299x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
238	299x	set_current_thread_name(name);
239
240		// Mark this thread as a worker of this pool so dispatch()
241		// can symmetric-transfer when called from within pool work.
242		struct scoped_pool
243		{
244	299x	scoped_pool(impl const* p) noexcept { current_.set(p); }
245	299x	~scoped_pool() noexcept { current_.set(nullptr); }
246	299x	} guard(this);
247
248		for(;;)
249		{
250	16874x	continuation* c = nullptr;
251		{
252	16874x	std::unique_lock<std::mutex> lock(mutex_);
253	16874x	work_cv_.wait(lock, [this]{
254	41174x	return !empty() \|\|
255	41174x	stop_;
256		});
257	16874x	if(stop_)
258	598x	return;
259	16575x	c = pop();
260	16874x	}
261	16575x	if(c)
262	16575x	safe_resume(c->h);
263	16575x	}
264	299x	}
265		};
266
267		//------------------------------------------------------------------------------
268
269	265x	thread_pool::
270		~thread_pool()
271		{
272	265x	impl_->stop();
273	265x	impl_->join();
274	265x	impl_->drain_abandoned();
275	265x	shutdown();
276	265x	destroy();
277	265x	delete impl_;
278	265x	}
279
280	265x	thread_pool::
281	265x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
282	265x	: impl_(new impl(num_threads, thread_name_prefix))
283		{
284	265x	this->set_frame_allocator(std::allocator<void>{});
285	265x	}
286
287		void
288	135x	thread_pool::
289		join() noexcept
290		{
291	135x	impl_->join();
292	135x	}
293
294		void
295	2x	thread_pool::
296		stop() noexcept
297		{
298	2x	impl_->stop();
299	2x	}
300
301		//------------------------------------------------------------------------------
302
303		thread_pool::executor_type
304	11671x	thread_pool::
305		get_executor() const noexcept
306		{
307	11671x	return executor_type(
308	11671x	const_cast<thread_pool&>(*this));
309		}
310
311		void
312	448x	thread_pool::executor_type::
313		on_work_started() const noexcept
314		{
315	448x	pool_->impl_->on_work_started();
316	448x	}
317
318		void
319	448x	thread_pool::executor_type::
320		on_work_finished() const noexcept
321		{
322	448x	pool_->impl_->on_work_finished();
323	448x	}
324
325		void
326	16363x	thread_pool::executor_type::
327		post(continuation& c) const
328		{
329	16363x	pool_->impl_->post(c);
330	16363x	}
331
332		std::coroutine_handle<>
333	448x	thread_pool::executor_type::
334		dispatch(continuation& c) const
335		{
336	448x	if(pool_->impl_->running_in_this_thread())
337	5x	return c.h;
338	443x	pool_->impl_->post(c);
339	443x	return std::noop_coroutine();
340		}
341
342		} // capy
343		} // boost
344