src/ex/thread_pool.cpp

99.3% Lines (150 / 151) 100.0% Functions (28 / 28)

[ ] prev/next file n p prev/next uncovered line

thread_pool.cpp

f(x) Functions (28)

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/abandon.hpp>
15		#include <boost/capy/ex/frame_allocator.hpp>
16		#include <boost/capy/test/thread_name.hpp>
17		#include <algorithm>
18		#include <atomic>
19		#include <condition_variable>
20		#include <cstdio>
21		#include <mutex>
22		#include <thread>
23		#include <vector>
24
25		/*
26		Thread pool implementation using a shared work queue.
27
28		Work items are continuations linked via their intrusive next pointer,
29		stored in a single queue protected by a mutex. No per-post heap
30		allocation: the continuation is owned by the caller and linked
31		directly. Worker threads wait on a condition_variable until work
32		is available or stop is requested.
33
34		Threads are started lazily on first post(), under mutex_, to avoid
35		spawning threads for pools that are constructed but never used. A
36		stopped pool starts none. Each thread is named
37		with a configurable prefix plus index for debugger visibility.
38
39		Work tracking: on_work_started/on_work_finished maintain the atomic
40		outstanding_work_ counter. on_work_started is lock-free, and so is
41		on_work_finished while the count stays above one. The release that
42		may take it to zero is made under mutex_, so join() can't see zero,
43		return and let the pool be destroyed while that call still uses it.
44		join() blocks until this counter reaches zero, then signals workers
45		to stop and joins threads.
46
47		Two shutdown paths:
48		- join(): waits for outstanding work to drain, then stops workers.
49		- stop(): immediately signals workers to exit; queued work stays queued.
50		- Destructor: stop() then join(), then queued work is abandoned.
51		*/
52
53		namespace boost {
54		namespace capy {
55
56		//------------------------------------------------------------------------------
57
58		class thread_pool::impl
59		{
60		// Identifies the pool owning the current worker thread, or
61		// nullptr if the calling thread is not a pool worker. Checked
62		// by dispatch() to decide between symmetric transfer (inline
63		// resume) and post.
64		static inline detail::thread_local_ptr<impl const> current_;
65
66		// Intrusive queue of continuations: the next link is stored in
67		// continuation::reserved (typed continuation* round-tripped through
68		// void*). No per-post allocation: the continuation is owned by the caller.
69		continuation* head_ = nullptr;
70		continuation* tail_ = nullptr;
71
72	23688x	void push(continuation* c) noexcept
73		{
74	23688x	c->reserved = nullptr;
75	23688x	if(tail_)
76	4868x	tail_->reserved = c;
77		else
78	18820x	head_ = c;
79	23688x	tail_ = c;
80	23688x	}
81
82	26530x	continuation* pop() noexcept
83		{
84	26530x	if(!head_)
85	2842x	return nullptr;
86	23688x	continuation* c = head_;
87	23688x	head_ = static_cast<continuation*>(head_->reserved);
88	23688x	if(!head_)
89	18820x	tail_ = nullptr;
90	23688x	return c;
91		}
92
93	42169x	bool empty() const noexcept
94		{
95	42169x	return head_ == nullptr;
96		}
97
98		std::mutex mutex_;
99		std::condition_variable work_cv_;
100		std::condition_variable done_cv_;
101		std::vector<std::thread> threads_;
102		std::atomic<std::size_t> outstanding_work_{0};
103		bool stop_{false};
104		bool joined_{false};
105		std::size_t num_threads_;
106		char thread_name_prefix_[13]{}; // 12 chars max + null terminator
107		std::atomic<bool> started_{false};
108
109		public:
110	1421x	~impl() = default;
111
112		bool
113	3199x	running_in_this_thread() const noexcept
114		{
115	3199x	return current_.get() == this;
116		}
117
118		// Abandon queued coroutines. Each is taken under the lock, as other
119		// threads may still post, and abandoned outside it, as abandoning
120		// can post.
121		void
122	2842x	drain_abandoned() noexcept
123		{
124		for(;;)
125		{
126		continuation* c;
127		{
128	3385x	std::lock_guard<std::mutex> lock(mutex_);
129	3385x	c = pop();
130	3385x	}
131	3385x	if(!c)
132	2842x	return;
133	543x	if(c->h && c->h != std::noop_coroutine())
134	290x	capy::abandon(*c);
135	543x	}
136		}
137
138	1421x	impl(std::size_t num_threads, std::string_view thread_name_prefix)
139	1421x	: num_threads_(num_threads)
140		{
141	1421x	if(num_threads_ == 0)
142	8x	num_threads_ = std::max(
143	4x	std::thread::hardware_concurrency(), 1u);
144
145		// Truncate prefix to 12 chars, leaving room for up to 3-digit index.
146	1421x	auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
147	1421x	thread_name_prefix_[n] = '\0';
148	1421x	}
149
150		void
151	23690x	post(continuation& c)
152		{
153	23690x	if(!started_.load(std::memory_order_acquire))
154		{
155		{
156	586x	std::lock_guard<std::mutex> lock(mutex_);
157	586x	if(stop_)
158		{
159		// Stopped before any thread started: nothing will
160		// run it, and the destructor's drain abandons it.
161	220x	push(&c);
162	220x	return;
163		}
164	586x	}
165	366x	ensure_started();
166		}
167	23468x	std::lock_guard<std::mutex> lock(mutex_);
168	23468x	push(&c);
169		// Under the lock so the pool cannot drain, join, and
170		// destroy the condition variable mid-signal.
171	23468x	work_cv_.notify_one();
172	23468x	}
173
174		void
175	4079x	on_work_started() noexcept
176		{
177	4079x	outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
178	4079x	}
179
180		void
181	3779x	on_work_finished() noexcept
182		{
183	3779x	auto n = outstanding_work_.load(std::memory_order_relaxed);
184	3787x	while(n > 1)
185		{
186	908x	if(outstanding_work_.compare_exchange_weak(
187		n, n - 1, std::memory_order_acq_rel))
188	446x	return;
189		}
190	3333x	std::lock_guard<std::mutex> lock(mutex_);
191	3333x	outstanding_work_.fetch_sub(1, std::memory_order_acq_rel);
192		// Re-read: a concurrent on_work_started() may have raised it.
193	3333x	if(outstanding_work_.load(
194	3333x	std::memory_order_acquire) == 0 && joined_ && !stop_)
195		{
196	708x	stop_ = true;
197	708x	done_cv_.notify_all();
198	708x	work_cv_.notify_all();
199		}
200	3333x	}
201
202		void
203	2703x	join() noexcept
204		{
205		{
206	2703x	std::unique_lock<std::mutex> lock(mutex_);
207	2703x	if(joined_)
208	1282x	return;
209	1421x	joined_ = true;
210
211	1421x	if(outstanding_work_.load(
212	1421x	std::memory_order_acquire) == 0)
213		{
214	359x	stop_ = true;
215	359x	work_cv_.notify_all();
216		}
217		else
218		{
219	1062x	done_cv_.wait(lock, [this]{
220	2071x	return stop_;
221		});
222		}
223	2703x	}
224
225	1909x	for(auto& t : threads_)
226	488x	if(t.joinable())
227	488x	t.join();
228		}
229
230		void
231	1724x	stop() noexcept
232		{
233		// Under the lock: the join() this releases may return and let
234		// the pool be destroyed as soon as the lock is free.
235	1724x	std::lock_guard<std::mutex> lock(mutex_);
236	1724x	stop_ = true;
237	1724x	work_cv_.notify_all();
238	1724x	done_cv_.notify_all();
239	1724x	}
240
241		private:
242		void
243	366x	ensure_started()
244		{
245		// Under the lock, so join() either sees every thread started
246		// here or makes this start none. A joined pool still starts them:
247		// join() may be waiting on work that needs them.
248	366x	std::lock_guard<std::mutex> lock(mutex_);
249	366x	if(started_.load(std::memory_order_relaxed) \|\| stop_)
250	✗	return;
251		// Start at the threads already running: if an earlier call
252		// threw partway, this one adds only the missing ones.
253	366x	threads_.reserve(num_threads_);
254	854x	for(std::size_t i = threads_.size(); i < num_threads_; ++i)
255	978x	threads_.emplace_back([this, i]{ run(i); });
256	364x	started_.store(true, std::memory_order_release);
257	366x	}
258
259		void
260	488x	run(std::size_t index)
261		{
262		// Build name; set_current_thread_name truncates to platform limits.
263		char name[16];
264	488x	std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
265	488x	set_current_thread_name(name);
266
267		// Mark this thread as a worker of this pool so dispatch()
268		// can symmetric-transfer when called from within pool work.
269		struct scoped_pool
270		{
271	488x	scoped_pool(impl const* p) noexcept { current_.set(p); }
272	488x	~scoped_pool() noexcept { current_.set(nullptr); }
273	488x	} guard(this);
274
275		for(;;)
276		{
277	23633x	continuation* c = nullptr;
278		{
279	23633x	std::unique_lock<std::mutex> lock(mutex_);
280	23633x	work_cv_.wait(lock, [this]{
281	61092x	return !empty() \|\|
282	61092x	stop_;
283		});
284	23633x	if(stop_)
285	976x	return;
286	23145x	c = pop();
287	23633x	}
288	23145x	if(c)
289	23145x	safe_resume(c->h);
290	23145x	}
291	488x	}
292		};
293
294		//------------------------------------------------------------------------------
295
296	1421x	thread_pool::
297		~thread_pool()
298		{
299	1421x	impl_->stop();
300	1421x	impl_->join();
301	1421x	shutdown();
302		// Takes what the unwinds post too, and the work of any service they
303		// create, which is shut down as it is created.
304	1421x	impl_->drain_abandoned();
305	1421x	destroy();
306		// Services give up their work in shutdown(), but anything a
307		// destructor posts anyway is still destroyed.
308	1421x	impl_->drain_abandoned();
309	1421x	delete impl_;
310	1421x	}
311
312	1421x	thread_pool::
313	1421x	thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
314	1421x	: impl_(new impl(num_threads, thread_name_prefix))
315		{
316	1421x	this->set_frame_allocator(std::allocator<void>{});
317	1421x	}
318
319		void
320	1282x	thread_pool::
321		join() noexcept
322		{
323	1282x	impl_->join();
324	1282x	}
325
326		void
327	303x	thread_pool::
328		stop() noexcept
329		{
330	303x	impl_->stop();
331	303x	}
332
333		//------------------------------------------------------------------------------
334
335		thread_pool::executor_type
336	15340x	thread_pool::
337		get_executor() const noexcept
338		{
339	15340x	return executor_type(
340	15340x	const_cast<thread_pool&>(*this));
341		}
342
343		void
344	4079x	thread_pool::executor_type::
345		on_work_started() const noexcept
346		{
347	4079x	pool_->impl_->on_work_started();
348	4079x	}
349
350		void
351	3779x	thread_pool::executor_type::
352		on_work_finished() const noexcept
353		{
354	3779x	pool_->impl_->on_work_finished();
355	3779x	}
356
357		void
358	20495x	thread_pool::executor_type::
359		post(continuation& c) const
360		{
361	20495x	pool_->impl_->post(c);
362	20493x	}
363
364		std::coroutine_handle<>
365	3199x	thread_pool::executor_type::
366		dispatch(continuation& c) const
367		{
368	3199x	if(pool_->impl_->running_in_this_thread())
369	4x	return c.h;
370	3195x	pool_->impl_->post(c);
371	3195x	return std::noop_coroutine();
372		}
373
374		} // capy
375		} // boost
376