. /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   19895 void push(continuation* c) noexcept 71   18041 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19895 c->reserved = nullptr; 73   18041 c->reserved = nullptr;
HITCBC 74   19895 if(tail_) 74   18041 if(tail_)
HITCBC 75   3212 tail_->reserved = c; 75   5656 tail_->reserved = c;
76   else 76   else
HITCBC 77   16683 head_ = c; 77   12385 head_ = c;
HITCBC 78   19895 tail_ = c; 78   18041 tail_ = c;
HITCBC 79   19895 } 79   18041 }
80   80  
HITCBC 81   20279 continuation* pop() noexcept 81   18444 continuation* pop() noexcept
82   { 82   {
HITCBC 83   20279 if(!head_) 83   18444 if(!head_)
HITCBC 84   384 return nullptr; 84   403 return nullptr;
HITCBC 85   19895 continuation* c = head_; 85   18041 continuation* c = head_;
HITCBC 86   19895 head_ = static_cast<continuation*>(head_->reserved); 86   18041 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19895 if(!head_) 87   18041 if(!head_)
HITCBC 88   16683 tail_ = nullptr; 88   12385 tail_ = nullptr;
HITCBC 89   19895 return c; 89   18041 return c;
90   } 90   }
91   91  
HITCBC 92   37149 bool empty() const noexcept 92   30921 bool empty() const noexcept
93   { 93   {
HITCBC 94   37149 return head_ == nullptr; 94   30921 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   384 ~impl() = default; 109   403 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   672 running_in_this_thread() const noexcept 112   691 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   672 return current_.get() == this; 114   691 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   384 drain_abandoned() noexcept 123   403 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   569 while(auto* c = pop()) 125   595 while(auto* c = pop())
126   { 126   {
HITCBC 127   185 auto h = c->h; 127   192 auto h = c->h;
HITCBC 128   185 if(h && h != std::noop_coroutine()) 128   192 if(h && h != std::noop_coroutine())
HITCBC 129   135 h.destroy(); 129   141 h.destroy();
HITCBC 130   185 } 130   192 }
HITCBC 131   384 } 131   403 }
132   132  
HITCBC 133   384 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   403 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   384 : num_threads_(num_threads) 134   403 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   384 if(num_threads_ == 0) 136   403 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   384 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   403 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   384 thread_name_prefix_[n] = '\0'; 142   403 thread_name_prefix_[n] = '\0';
HITCBC 143   384 } 143   403 }
144   144  
145   void 145   void
HITCBC 146   19895 post(continuation& c) 146   18041 post(continuation& c)
147   { 147   {
HITCBC 148   19895 ensure_started(); 148   18041 ensure_started();
149   { 149   {
HITCBC 150   19895 std::lock_guard<std::mutex> lock(mutex_); 150   18041 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19895 push(&c); 151   18041 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   19895 work_cv_.notify_one(); 154   18041 work_cv_.notify_one();
HITCBC 155   19895 } 155   18041 }
HITCBC 156   19895 } 156   18041 }
157   157  
158   void 158   void
HITCBC 159   671 on_work_started() noexcept 159   690 on_work_started() noexcept
160   { 160   {
HITCBC 161   671 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   690 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   671 } 162   690 }
163   163  
164   void 164   void
HITCBC 165   671 on_work_finished() noexcept 165   690 on_work_finished() noexcept
166   { 166   {
HITCBC 167   671 if(outstanding_work_.fetch_sub( 167   690 if(outstanding_work_.fetch_sub(
HITCBC 168   671 1, std::memory_order_acq_rel) == 1) 168   690 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   308 std::lock_guard<std::mutex> lock(mutex_); 173   327 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   308 if(outstanding_work_.load( 174   327 if(outstanding_work_.load(
HITCBC 175   308 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   327 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   176 stop_ = true; 177   172 stop_ = true;
HITCBC 178   176 done_cv_.notify_all(); 178   172 done_cv_.notify_all();
HITCBC 179   176 work_cv_.notify_all(); 179   172 work_cv_.notify_all();
180   } 180   }
HITCBC 181   308 } 181   327 }
HITCBC 182   671 } 182   690 }
183   183  
184   void 184   void
HITCBC 185   634 join() noexcept 185   672 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   634 std::unique_lock<std::mutex> lock(mutex_); 188   672 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   634 if(joined_) 189   672 if(joined_)
HITCBC 190   250 return; 190   269 return;
HITCBC 191   384 joined_ = true; 191   403 joined_ = true;
192   192  
HITCBC 193   384 if(outstanding_work_.load( 193   403 if(outstanding_work_.load(
HITCBC 194   384 std::memory_order_acquire) == 0) 194   403 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   157 stop_ = true; 196   180 stop_ = true;
HITCBC 197   157 work_cv_.notify_all(); 197   180 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   227 done_cv_.wait(lock, [this]{ 201   223 done_cv_.wait(lock, [this]{
HITCBC 202   404 return stop_; 202   396 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   634 } 205   672 }
206   206  
HITCBC 207   822 for(auto& t : threads_) 207   860 for(auto& t : threads_)
HITCBC 208   438 if(t.joinable()) 208   457 if(t.joinable())
HITCBC 209   438 t.join(); 209   457 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   386 stop() noexcept 213   405 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   386 std::lock_guard<std::mutex> lock(mutex_); 216   405 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   386 stop_ = true; 217   405 stop_ = true;
HITCBC 218   386 } 218   405 }
HITCBC 219   386 work_cv_.notify_all(); 219   405 work_cv_.notify_all();
HITCBC 220   386 done_cv_.notify_all(); 220   405 done_cv_.notify_all();
HITCBC 221   386 } 221   405 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19895 ensure_started() 225   18041 ensure_started()
226   { 226   {
HITCBC 227   19895 std::call_once(start_flag_, [this]{ 227   18041 std::call_once(start_flag_, [this]{
HITCBC 228   334 threads_.reserve(num_threads_); 228   353 threads_.reserve(num_threads_);
HITCBC 229   772 for(std::size_t i = 0; i < num_threads_; ++i) 229   810 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   876 threads_.emplace_back([this, i]{ run(i); }); 230   914 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   334 }); 231   353 });
HITCBC 232   19895 } 232   18041 }
233   233  
234   void 234   void
HITCBC 235   438 run(std::size_t index) 235   457 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   438 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   457 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   438 set_current_thread_name(name); 240   457 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   438 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   457 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   438 ~scoped_pool() noexcept { current_.set(nullptr); } 247   457 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   438 } guard(this); 248   457 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   20148 continuation* c = nullptr; 252   18306 continuation* c = nullptr;
253   { 253   {
HITCBC 254   20148 std::unique_lock<std::mutex> lock(mutex_); 254   18306 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   20148 work_cv_.wait(lock, [this]{ 255   18306 work_cv_.wait(lock, [this]{
HITCBC 256   54500 return !empty() || 256   43904 return !empty() ||
HITCBC 257   54500 stop_; 257   43904 stop_;
258   }); 258   });
HITCBC 259   20148 if(stop_) 259   18306 if(stop_)
HITCBC 260   876 return; 260   914 return;
HITCBC 261   19710 c = pop(); 261   17849 c = pop();
HITCBC 262   20148 } 262   18306 }
HITCBC 263   19710 if(c) 263   17849 if(c)
HITCBC 264   19710 safe_resume(c->h); 264   17849 safe_resume(c->h);
HITCBC 265   19710 } 265   17849 }
HITCBC 266   438 } 266   457 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   384 thread_pool:: 271   403 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   384 impl_->stop(); 274   403 impl_->stop();
HITCBC 275   384 impl_->join(); 275   403 impl_->join();
HITCBC 276   384 impl_->drain_abandoned(); 276   403 impl_->drain_abandoned();
HITCBC 277   384 shutdown(); 277   403 shutdown();
HITCBC 278   384 destroy(); 278   403 destroy();
HITCBC 279   384 delete impl_; 279   403 delete impl_;
HITCBC 280   384 } 280   403 }
281   281  
HITCBC 282   384 thread_pool:: 282   403 thread_pool::
HITCBC 283   384 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   403 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   384 : impl_(new impl(num_threads, thread_name_prefix)) 284   403 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   384 this->set_frame_allocator(std::allocator<void>{}); 286   403 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   384 } 287   403 }
288   288  
289   void 289   void
HITCBC 290   250 thread_pool:: 290   269 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   250 impl_->join(); 293   269 impl_->join();
HITCBC 294   250 } 294   269 }
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   11796 thread_pool:: 306   11815 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11796 return executor_type( 309   11815 return executor_type(
HITCBC 310   11796 const_cast<thread_pool&>(*this)); 310   11815 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   671 thread_pool::executor_type:: 314   690 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   671 pool_->impl_->on_work_started(); 317   690 pool_->impl_->on_work_started();
HITCBC 318   671 } 318   690 }
319   319  
320   void 320   void
HITCBC 321   671 thread_pool::executor_type:: 321   690 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   671 pool_->impl_->on_work_finished(); 324   690 pool_->impl_->on_work_finished();
HITCBC 325   671 } 325   690 }
326   326  
327   void 327   void
HITCBC 328   19230 thread_pool::executor_type:: 328   17357 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   19230 pool_->impl_->post(c); 331   17357 pool_->impl_->post(c);
HITCBC 332   19230 } 332   17357 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   672 thread_pool::executor_type:: 335   691 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   672 if(pool_->impl_->running_in_this_thread()) 338   691 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   665 pool_->impl_->post(c); 340   684 pool_->impl_->post(c);
HITCBC 341   665 return std::noop_coroutine(); 341   684 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost