src/ex/thread_pool.cpp

100.0% Lines (140 / 140) 100.0% 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 ([email protected])
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	18795x	void push(continuation* c) noexcept
72		{
73	18795x	c->reserved = nullptr;
74	18795x	if(tail_)
75	5161x	tail_->reserved = c;
76		else
77	13634x	head_ = c;
78	18795x	tail_ = c;
79	18795x	}
80
81	19185x	continuation* pop() noexcept
82		{
83	19185x	if(!head_)
84	390x	return nullptr;
85	18795x	continuation* c = head_;
86	18795x	head_ = static_cast<continuation*>(head_->reserved);
87	18795x	if(!head_)
88	13634x	tail_ = nullptr;
89	18795x	return c;
90		}
91
92	32694x	bool empty() const noexcept
93		{
94	32694x	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	390x	~impl() = default;
110
111		bool
112	678x	running_in_this_thread() const noexcept
113		{
114	678x	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	390x	drain_abandoned() noexcept
124		{
125	649x	while(auto* c = pop())
126		{
127	259x	auto h = c->h;
128	259x	if(h && h != std::noop_coroutine())
129	208x	h.destroy();
130	259x	}
131	390x	}
132
133	390x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
134	390x	: num_threads_(num_threads)
135		{
136	390x	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	390x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
142	390x	thread_name_prefix_[n] = '\0';
143	390x	}
144
145		void
146	18795x	post(continuation& c)
147		{
148	18795x	ensure_started();
149		{
150	18795x	std::lock_guard<std::mutex> lock(mutex_);
151	18795x	push(&c);
152		// Under the lock so the pool cannot drain, join, and
153		// destroy the condition variable mid-signal.
154	18795x	work_cv_.notify_one();
155	18795x	}
156	18795x	}
157
158		void
159	677x	on_work_started() noexcept
160		{
161	677x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
162	677x	}
163
164		void
165	677x	on_work_finished() noexcept
166		{
167	677x	if(outstanding_work_.fetch_sub(
168	677x	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	315x	std::lock_guard<std::mutex> lock(mutex_);
174	315x	if(outstanding_work_.load(
175	315x	std::memory_order_acquire) == 0 && joined_ && !stop_)
176		{
177	169x	stop_ = true;
178	169x	done_cv_.notify_all();
179	169x	work_cv_.notify_all();
180		}
181	315x	}
182	677x	}
183
184		void
185	646x	join() noexcept
186		{
187		{
188	646x	std::unique_lock<std::mutex> lock(mutex_);
189	646x	if(joined_)
190	256x	return;
191	390x	joined_ = true;
192
193	390x	if(outstanding_work_.load(
194	390x	std::memory_order_acquire) == 0)
195		{
196	166x	stop_ = true;
197	166x	work_cv_.notify_all();
198		}
199		else
200		{
201	224x	done_cv_.wait(lock, [this]{
202	394x	return stop_;
203		});
204		}
205	646x	}
206
207	834x	for(auto& t : threads_)
208	444x	if(t.joinable())
209	444x	t.join();
210		}
211
212		void
213	392x	stop() noexcept
214		{
215		{
216	392x	std::lock_guard<std::mutex> lock(mutex_);
217	392x	stop_ = true;
218	392x	}
219	392x	work_cv_.notify_all();
220	392x	done_cv_.notify_all();
221	392x	}
222
223		private:
224		void
225	18795x	ensure_started()
226		{
227	18795x	std::call_once(start_flag_, [this]{
228	340x	threads_.reserve(num_threads_);
229	784x	for(std::size_t i = 0; i < num_threads_; ++i)
230	888x	threads_.emplace_back([this, i]{ run(i); });
231	340x	});
232	18795x	}
233
234		void
235	444x	run(std::size_t index)
236		{
237		// Build name; set_current_thread_name truncates to platform limits.
238		char name[16];
239	444x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
240	444x	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	444x	scoped_pool(impl const* p) noexcept { current_.set(p); }
247	444x	~scoped_pool() noexcept { current_.set(nullptr); }
248	444x	} guard(this);
249
250		for(;;)
251		{
252	18980x	continuation* c = nullptr;
253		{
254	18980x	std::unique_lock<std::mutex> lock(mutex_);
255	18980x	work_cv_.wait(lock, [this]{
256	46752x	return !empty() \|\|
257	46752x	stop_;
258		});
259	18980x	if(stop_)
260	888x	return;
261	18536x	c = pop();
262	18980x	}
263	18536x	if(c)
264	18536x	safe_resume(c->h);
265	18536x	}
266	444x	}
267		};
268
269		//------------------------------------------------------------------------------
270
271	390x	thread_pool::
272		~thread_pool()
273		{
274	390x	impl_->stop();
275	390x	impl_->join();
276	390x	impl_->drain_abandoned();
277	390x	shutdown();
278	390x	destroy();
279	390x	delete impl_;
280	390x	}
281
282	390x	thread_pool::
283	390x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
284	390x	: impl_(new impl(num_threads, thread_name_prefix))
285		{
286	390x	this->set_frame_allocator(std::allocator<void>{});
287	390x	}
288
289		void
290	256x	thread_pool::
291		join() noexcept
292		{
293	256x	impl_->join();
294	256x	}
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	11802x	thread_pool::
307		get_executor() const noexcept
308		{
309	11802x	return executor_type(
310	11802x	const_cast<thread_pool&>(*this));
311		}
312
313		void
314	677x	thread_pool::executor_type::
315		on_work_started() const noexcept
316		{
317	677x	pool_->impl_->on_work_started();
318	677x	}
319
320		void
321	677x	thread_pool::executor_type::
322		on_work_finished() const noexcept
323		{
324	677x	pool_->impl_->on_work_finished();
325	677x	}
326
327		void
328	18124x	thread_pool::executor_type::
329		post(continuation& c) const
330		{
331	18124x	pool_->impl_->post(c);
332	18124x	}
333
334		std::coroutine_handle<>
335	678x	thread_pool::executor_type::
336		dispatch(continuation& c) const
337		{
338	678x	if(pool_->impl_->running_in_this_thread())
339	7x	return c.h;
340	671x	pool_->impl_->post(c);
341	671x	return std::noop_coroutine();
342		}
343
344		} // capy
345		} // boost
346