. /src /ex /thread_pool.cpp
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1   // 1   //
2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com) 2   // Copyright (c) 2025 Vinnie Falco (vinnie.falco@gmail.com)
3   // Copyright (c) 2026 Michael Vandeberg 3   // Copyright (c) 2026 Michael Vandeberg
4   // 4   //
5   // Distributed under the Boost Software License, Version 1.0. (See accompanying 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) 6   // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7   // 7   //
8   // Official repository: https://github.com/boostorg/capy 8   // Official repository: https://github.com/boostorg/capy
9   // 9   //
10   10  
11   #include <boost/capy/ex/thread_pool.hpp> 11   #include <boost/capy/ex/thread_pool.hpp>
12   #include <boost/capy/continuation.hpp> 12   #include <boost/capy/continuation.hpp>
13   #include <boost/capy/detail/thread_local_ptr.hpp> 13   #include <boost/capy/detail/thread_local_ptr.hpp>
14   #include <boost/capy/ex/frame_allocator.hpp> 14   #include <boost/capy/ex/frame_allocator.hpp>
15   #include <boost/capy/test/thread_name.hpp> 15   #include <boost/capy/test/thread_name.hpp>
16   #include <algorithm> 16   #include <algorithm>
17   #include <atomic> 17   #include <atomic>
18   #include <condition_variable> 18   #include <condition_variable>
19   #include <cstdio> 19   #include <cstdio>
20   #include <mutex> 20   #include <mutex>
21   #include <thread> 21   #include <thread>
22   #include <vector> 22   #include <vector>
23   23  
24   /* 24   /*
25   Thread pool implementation using a shared work queue. 25   Thread pool implementation using a shared work queue.
26   26  
27   Work items are continuations linked via their intrusive next pointer, 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 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 29   allocation: the continuation is owned by the caller and linked
30   directly. Worker threads wait on a condition_variable until work 30   directly. Worker threads wait on a condition_variable until work
31   is available or stop is requested. 31   is available or stop is requested.
32   32  
33   Threads are started lazily on first post() via std::call_once to avoid 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 34   spawning threads for pools that are constructed but never used. Each
35   thread is named with a configurable prefix plus index for debugger 35   thread is named with a configurable prefix plus index for debugger
36   visibility. 36   visibility.
37   37  
38   Work tracking: on_work_started/on_work_finished maintain the atomic 38   Work tracking: on_work_started/on_work_finished maintain the atomic
39   outstanding_work_ counter. on_work_started is lock-free; the worker 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 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 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 42   consistent even if work is started in between. join() blocks until
43   this counter reaches zero, then signals workers to stop and joins 43   this counter reaches zero, then signals workers to stop and joins
44   threads. 44   threads.
45   45  
46   Two shutdown paths: 46   Two shutdown paths:
47   - join(): waits for outstanding work to drain, then stops workers. 47   - join(): waits for outstanding work to drain, then stops workers.
48   - stop(): immediately signals workers to exit; queued work is abandoned. 48   - stop(): immediately signals workers to exit; queued work is abandoned.
49   - Destructor: stop() then join() (abandon + wait for threads). 49   - Destructor: stop() then join() (abandon + wait for threads).
50   */ 50   */
51   51  
52   namespace boost { 52   namespace boost {
53   namespace capy { 53   namespace capy {
54   54  
55   //------------------------------------------------------------------------------ 55   //------------------------------------------------------------------------------
56   56  
57   class thread_pool::impl 57   class thread_pool::impl
58   { 58   {
59   // Identifies the pool owning the current worker thread, or 59   // Identifies the pool owning the current worker thread, or
60   // nullptr if the calling thread is not a pool worker. Checked 60   // nullptr if the calling thread is not a pool worker. Checked
61   // by dispatch() to decide between symmetric transfer (inline 61   // by dispatch() to decide between symmetric transfer (inline
62   // resume) and post. 62   // resume) and post.
63   static inline detail::thread_local_ptr<impl const> current_; 63   static inline detail::thread_local_ptr<impl const> current_;
64   64  
65   // Intrusive queue of continuations: the next link is stored in 65   // Intrusive queue of continuations: the next link is stored in
66   // continuation::reserved (typed continuation* round-tripped through 66   // continuation::reserved (typed continuation* round-tripped through
67   // void*). No per-post allocation: the continuation is owned by the caller. 67   // void*). No per-post allocation: the continuation is owned by the caller.
68   continuation* head_ = nullptr; 68   continuation* head_ = nullptr;
69   continuation* tail_ = nullptr; 69   continuation* tail_ = nullptr;
70   70  
HITCBC 71   18482 void push(continuation* c) noexcept 71   19973 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   18482 c->reserved = nullptr; 73   19973 c->reserved = nullptr;
HITCBC 74   18482 if(tail_) 74   19973 if(tail_)
HITCBC 75   7073 tail_->reserved = c; 75   4821 tail_->reserved = c;
76   else 76   else
HITCBC 77   11409 head_ = c; 77   15152 head_ = c;
HITCBC 78   18482 tail_ = c; 78   19973 tail_ = c;
HITCBC 79   18482 } 79   19973 }
80   80  
HITCBC 81   18881 continuation* pop() noexcept 81   20367 continuation* pop() noexcept
82   { 82   {
HITCBC 83   18881 if(!head_) 83   20367 if(!head_)
HITCBC 84   399 return nullptr; 84   394 return nullptr;
HITCBC 85   18482 continuation* c = head_; 85   19973 continuation* c = head_;
HITCBC 86   18482 head_ = static_cast<continuation*>(head_->reserved); 86   19973 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   18482 if(!head_) 87   19973 if(!head_)
HITCBC 88   11409 tail_ = nullptr; 88   15152 tail_ = nullptr;
HITCBC 89   18482 return c; 89   19973 return c;
90   } 90   }
91   91  
HITCBC 92   30234 bool empty() const noexcept 92   35573 bool empty() const noexcept
93   { 93   {
HITCBC 94   30234 return head_ == nullptr; 94   35573 return head_ == nullptr;
95   } 95   }
96   96  
97   std::mutex mutex_; 97   std::mutex mutex_;
98   std::condition_variable work_cv_; 98   std::condition_variable work_cv_;
99   std::condition_variable done_cv_; 99   std::condition_variable done_cv_;
100   std::vector<std::thread> threads_; 100   std::vector<std::thread> threads_;
101   std::atomic<std::size_t> outstanding_work_{0}; 101   std::atomic<std::size_t> outstanding_work_{0};
102   bool stop_{false}; 102   bool stop_{false};
103   bool joined_{false}; 103   bool joined_{false};
104   std::size_t num_threads_; 104   std::size_t num_threads_;
105   char thread_name_prefix_[13]{}; // 12 chars max + null terminator 105   char thread_name_prefix_[13]{}; // 12 chars max + null terminator
106   std::once_flag start_flag_; 106   std::once_flag start_flag_;
107   107  
108   public: 108   public:
HITCBC 109   399 ~impl() = default; 109   394 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   687 running_in_this_thread() const noexcept 112   682 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   687 return current_.get() == this; 114   682 return current_.get() == this;
115   } 115   }
116   116  
117   // Destroy abandoned coroutine frames. Must be called 117   // Destroy abandoned coroutine frames. Must be called
118   // before execution_context::shutdown()/destroy() so 118   // before execution_context::shutdown()/destroy() so
119   // that suspended-frame destructors touching services 119   // that suspended-frame destructors touching services
120   // (e.g. cancelling registrations) run while those 120   // (e.g. cancelling registrations) run while those
121   // services are still valid. 121   // services are still valid.
122   void 122   void
HITCBC 123   399 drain_abandoned() noexcept 123   394 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   580 while(auto* c = pop()) 125   553 while(auto* c = pop())
126   { 126   {
HITCBC 127   181 auto h = c->h; 127   159 auto h = c->h;
HITCBC 128   181 if(h && h != std::noop_coroutine()) 128   159 if(h && h != std::noop_coroutine())
HITCBC 129   130 h.destroy(); 129   108 h.destroy();
HITCBC 130   181 } 130   159 }
HITCBC 131   399 } 131   394 }
132   132  
HITCBC 133   399 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   394 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   399 : num_threads_(num_threads) 134   394 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   399 if(num_threads_ == 0) 136   394 if(num_threads_ == 0)
HITCBC 137   8 num_threads_ = std::max( 137   8 num_threads_ = std::max(
HITCBC 138   4 std::thread::hardware_concurrency(), 1u); 138   4 std::thread::hardware_concurrency(), 1u);
139   139  
140   // Truncate prefix to 12 chars, leaving room for up to 3-digit index. 140   // Truncate prefix to 12 chars, leaving room for up to 3-digit index.
HITCBC 141   399 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   394 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   399 thread_name_prefix_[n] = '\0'; 142   394 thread_name_prefix_[n] = '\0';
HITCBC 143   399 } 143   394 }
144   144  
145   void 145   void
HITCBC 146   18482 post(continuation& c) 146   19973 post(continuation& c)
147   { 147   {
HITCBC 148   18482 ensure_started(); 148   19973 ensure_started();
149   { 149   {
HITCBC 150   18482 std::lock_guard<std::mutex> lock(mutex_); 150   19973 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   18482 push(&c); 151   19973 push(&c);
152   // Under the lock so the pool cannot drain, join, and 152   // Under the lock so the pool cannot drain, join, and
153   // destroy the condition variable mid-signal. 153   // destroy the condition variable mid-signal.
HITCBC 154   18482 work_cv_.notify_one(); 154   19973 work_cv_.notify_one();
HITCBC 155   18482 } 155   19973 }
HITCBC 156   18482 } 156   19973 }
157   157  
158   void 158   void
HITCBC 159   686 on_work_started() noexcept 159   681 on_work_started() noexcept
160   { 160   {
HITCBC 161   686 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   681 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   686 } 162   681 }
163   163  
164   void 164   void
HITCBC 165   686 on_work_finished() noexcept 165   681 on_work_finished() noexcept
166   { 166   {
HITCBC 167   686 if(outstanding_work_.fetch_sub( 167   681 if(outstanding_work_.fetch_sub(
HITCBC 168   686 1, std::memory_order_acq_rel) == 1) 168   681 1, std::memory_order_acq_rel) == 1)
169   { 169   {
170   // fetch_sub's result can be stale: a concurrent 170   // fetch_sub's result can be stale: a concurrent
171   // on_work_started() may raise the count before we take the 171   // on_work_started() may raise the count before we take the
172   // lock, so re-read it here rather than trust the decrement. 172   // lock, so re-read it here rather than trust the decrement.
HITCBC 173   323 std::lock_guard<std::mutex> lock(mutex_); 173   318 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   323 if(outstanding_work_.load( 174   318 if(outstanding_work_.load(
HITCBC 175   323 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   318 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   178 stop_ = true; 177   180 stop_ = true;
HITCBC 178   178 done_cv_.notify_all(); 178   180 done_cv_.notify_all();
HITCBC 179   178 work_cv_.notify_all(); 179   180 work_cv_.notify_all();
180   } 180   }
HITCBC 181   323 } 181   318 }
HITCBC 182   686 } 182   681 }
183   183  
184   void 184   void
HITCBC 185   664 join() noexcept 185   654 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   664 std::unique_lock<std::mutex> lock(mutex_); 188   654 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   664 if(joined_) 189   654 if(joined_)
HITCBC 190   265 return; 190   260 return;
HITCBC 191   399 joined_ = true; 191   394 joined_ = true;
192   192  
HITCBC 193   399 if(outstanding_work_.load( 193   394 if(outstanding_work_.load(
HITCBC 194   399 std::memory_order_acquire) == 0) 194   394 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   171 stop_ = true; 196   164 stop_ = true;
HITCBC 197   171 work_cv_.notify_all(); 197   164 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   228 done_cv_.wait(lock, [this]{ 201   230 done_cv_.wait(lock, [this]{
HITCBC 202   407 return stop_; 202   411 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   664 } 205   654 }
206   206  
HITCBC 207   852 for(auto& t : threads_) 207   842 for(auto& t : threads_)
HITCBC 208   453 if(t.joinable()) 208   448 if(t.joinable())
HITCBC 209   453 t.join(); 209   448 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   401 stop() noexcept 213   396 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   401 std::lock_guard<std::mutex> lock(mutex_); 216   396 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   401 stop_ = true; 217   396 stop_ = true;
HITCBC 218   401 } 218   396 }
HITCBC 219   401 work_cv_.notify_all(); 219   396 work_cv_.notify_all();
HITCBC 220   401 done_cv_.notify_all(); 220   396 done_cv_.notify_all();
HITCBC 221   401 } 221   396 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   18482 ensure_started() 225   19973 ensure_started()
226   { 226   {
HITCBC 227   18482 std::call_once(start_flag_, [this]{ 227   19973 std::call_once(start_flag_, [this]{
HITCBC 228   349 threads_.reserve(num_threads_); 228   344 threads_.reserve(num_threads_);
HITCBC 229   802 for(std::size_t i = 0; i < num_threads_; ++i) 229   792 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   906 threads_.emplace_back([this, i]{ run(i); }); 230   896 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   349 }); 231   344 });
HITCBC 232   18482 } 232   19973 }
233   233  
234   void 234   void
HITCBC 235   453 run(std::size_t index) 235   448 run(std::size_t index)
236   { 236   {
237   // Build name; set_current_thread_name truncates to platform limits. 237   // Build name; set_current_thread_name truncates to platform limits.
238   char name[16]; 238   char name[16];
HITCBC 239   453 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   448 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   453 set_current_thread_name(name); 240   448 set_current_thread_name(name);
241   241  
242   // Mark this thread as a worker of this pool so dispatch() 242   // Mark this thread as a worker of this pool so dispatch()
243   // can symmetric-transfer when called from within pool work. 243   // can symmetric-transfer when called from within pool work.
244   struct scoped_pool 244   struct scoped_pool
245   { 245   {
HITCBC 246   453 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   448 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   453 ~scoped_pool() noexcept { current_.set(nullptr); } 247   448 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   453 } guard(this); 248   448 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   18754 continuation* c = nullptr; 252   20262 continuation* c = nullptr;
253   { 253   {
HITCBC 254   18754 std::unique_lock<std::mutex> lock(mutex_); 254   20262 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   18754 work_cv_.wait(lock, [this]{ 255   20262 work_cv_.wait(lock, [this]{
HITCBC 256   42076 return !empty() || 256   51253 return !empty() ||
HITCBC 257   42076 stop_; 257   51253 stop_;
258   }); 258   });
HITCBC 259   18754 if(stop_) 259   20262 if(stop_)
HITCBC 260   906 return; 260   896 return;
HITCBC 261   18301 c = pop(); 261   19814 c = pop();
HITCBC 262   18754 } 262   20262 }
HITCBC 263   18301 if(c) 263   19814 if(c)
HITCBC 264   18301 safe_resume(c->h); 264   19814 safe_resume(c->h);
HITCBC 265   18301 } 265   19814 }
HITCBC 266   453 } 266   448 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   399 thread_pool:: 271   394 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   399 impl_->stop(); 274   394 impl_->stop();
HITCBC 275   399 impl_->join(); 275   394 impl_->join();
HITCBC 276   399 impl_->drain_abandoned(); 276   394 impl_->drain_abandoned();
HITCBC 277   399 shutdown(); 277   394 shutdown();
HITCBC 278   399 destroy(); 278   394 destroy();
HITCBC 279   399 delete impl_; 279   394 delete impl_;
HITCBC 280   399 } 280   394 }
281   281  
HITCBC 282   399 thread_pool:: 282   394 thread_pool::
HITCBC 283   399 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   394 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   399 : impl_(new impl(num_threads, thread_name_prefix)) 284   394 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   399 this->set_frame_allocator(std::allocator<void>{}); 286   394 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   399 } 287   394 }
288   288  
289   void 289   void
HITCBC 290   265 thread_pool:: 290   260 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   265 impl_->join(); 293   260 impl_->join();
HITCBC 294   265 } 294   260 }
295   295  
296   void 296   void
HITCBC 297   2 thread_pool:: 297   2 thread_pool::
298   stop() noexcept 298   stop() noexcept
299   { 299   {
HITCBC 300   2 impl_->stop(); 300   2 impl_->stop();
HITCBC 301   2 } 301   2 }
302   302  
303   //------------------------------------------------------------------------------ 303   //------------------------------------------------------------------------------
304   304  
305   thread_pool::executor_type 305   thread_pool::executor_type
HITCBC 306   11811 thread_pool:: 306   11806 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11811 return executor_type( 309   11806 return executor_type(
HITCBC 310   11811 const_cast<thread_pool&>(*this)); 310   11806 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   686 thread_pool::executor_type:: 314   681 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   686 pool_->impl_->on_work_started(); 317   681 pool_->impl_->on_work_started();
HITCBC 318   686 } 318   681 }
319   319  
320   void 320   void
HITCBC 321   686 thread_pool::executor_type:: 321   681 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   686 pool_->impl_->on_work_finished(); 324   681 pool_->impl_->on_work_finished();
HITCBC 325   686 } 325   681 }
326   326  
327   void 327   void
HITCBC 328   17802 thread_pool::executor_type:: 328   19298 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   17802 pool_->impl_->post(c); 331   19298 pool_->impl_->post(c);
HITCBC 332   17802 } 332   19298 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   687 thread_pool::executor_type:: 335   682 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   687 if(pool_->impl_->running_in_this_thread()) 338   682 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   680 pool_->impl_->post(c); 340   675 pool_->impl_->post(c);
HITCBC 341   680 return std::noop_coroutine(); 341   675 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost