. /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   20902 void push(continuation* c) noexcept 71   16637 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   20902 c->reserved = nullptr; 73   16637 c->reserved = nullptr;
HITCBC 74   20902 if(tail_) 74   16637 if(tail_)
HITCBC 75   3245 tail_->reserved = c; 75   7747 tail_->reserved = c;
76   else 76   else
HITCBC 77   17657 head_ = c; 77   8890 head_ = c;
HITCBC 78   20902 tail_ = c; 78   16637 tail_ = c;
HITCBC 79   20902 } 79   16637 }
80   80  
HITCBC 81   21221 continuation* pop() noexcept 81   17005 continuation* pop() noexcept
82   { 82   {
HITCBC 83   21221 if(!head_) 83   17005 if(!head_)
HITCBC 84   319 return nullptr; 84   368 return nullptr;
HITCBC 85   20902 continuation* c = head_; 85   16637 continuation* c = head_;
HITCBC 86   20902 head_ = static_cast<continuation*>(head_->reserved); 86   16637 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   20902 if(!head_) 87   16637 if(!head_)
HITCBC 88   17657 tail_ = nullptr; 88   8890 tail_ = nullptr;
HITCBC 89   20902 return c; 89   16637 return c;
90   } 90   }
91   91  
HITCBC 92   38768 bool empty() const noexcept 92   25742 bool empty() const noexcept
93   { 93   {
HITCBC 94   38768 return head_ == nullptr; 94   25742 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   319 ~impl() = default; 109   368 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   501 running_in_this_thread() const noexcept 112   656 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   501 return current_.get() == this; 114   656 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   319 drain_abandoned() noexcept 123   368 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   496 while(auto* c = pop()) 125   667 while(auto* c = pop())
126   { 126   {
HITCBC 127   177 auto h = c->h; 127   299 auto h = c->h;
HITCBC 128   177 if(h && h != std::noop_coroutine()) 128   299 if(h && h != std::noop_coroutine())
HITCBC 129   127 h.destroy(); 129   236 h.destroy();
HITCBC 130   177 } 130   299 }
HITCBC 131   319 } 131   368 }
132   132  
HITCBC 133   319 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   368 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   319 : num_threads_(num_threads) 134   368 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   319 if(num_threads_ == 0) 136   368 if(num_threads_ == 0)
HITCBC 137   4 num_threads_ = std::max( 137   8 num_threads_ = std::max(
HITCBC 138   2 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   319 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   368 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   319 thread_name_prefix_[n] = '\0'; 142   368 thread_name_prefix_[n] = '\0';
HITCBC 143   319 } 143   368 }
144   144  
145   void 145   void
HITCBC 146   20902 post(continuation& c) 146   16637 post(continuation& c)
147   { 147   {
HITCBC 148   20902 ensure_started(); 148   16637 ensure_started();
149   { 149   {
HITCBC 150   20902 std::lock_guard<std::mutex> lock(mutex_); 150   16637 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   20902 push(&c); 151   16637 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   20902 work_cv_.notify_one(); 154   16637 work_cv_.notify_one();
HITCBC 155   20902 } 155   16637 }
HITCBC 156   20902 } 156   16637 }
157   157  
158   void 158   void
HITCBC 159   501 on_work_started() noexcept 159   655 on_work_started() noexcept
160   { 160   {
HITCBC 161   501 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   655 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   501 } 162   655 }
163   163  
164   void 164   void
HITCBC 165   501 on_work_finished() noexcept 165   655 on_work_finished() noexcept
166   { 166   {
HITCBC 167   501 if(outstanding_work_.fetch_sub( 167   655 if(outstanding_work_.fetch_sub(
HITCBC 168   501 1, std::memory_order_acq_rel) == 1) 168   655 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   249 std::lock_guard<std::mutex> lock(mutex_); 173   306 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   249 if(outstanding_work_.load( 174   306 if(outstanding_work_.load(
HITCBC 175   249 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   306 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   127 stop_ = true; 177   121 stop_ = true;
HITCBC 178   127 done_cv_.notify_all(); 178   121 done_cv_.notify_all();
HITCBC 179   127 work_cv_.notify_all(); 179   121 work_cv_.notify_all();
180   } 180   }
HITCBC 181   249 } 181   306 }
HITCBC 182   501 } 182   655 }
183   183  
184   void 184   void
HITCBC 185   508 join() noexcept 185   602 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   508 std::unique_lock<std::mutex> lock(mutex_); 188   602 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   508 if(joined_) 189   602 if(joined_)
HITCBC 190   189 return; 190   234 return;
HITCBC 191   319 joined_ = true; 191   368 joined_ = true;
192   192  
HITCBC 193   319 if(outstanding_work_.load( 193   368 if(outstanding_work_.load(
HITCBC 194   319 std::memory_order_acquire) == 0) 194   368 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   142 stop_ = true; 196   192 stop_ = true;
HITCBC 197   142 work_cv_.notify_all(); 197   192 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   177 done_cv_.wait(lock, [this]{ 201   176 done_cv_.wait(lock, [this]{
HITCBC 202   305 return stop_; 202   298 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   508 } 205   602 }
206   206  
HITCBC 207   672 for(auto& t : threads_) 207   790 for(auto& t : threads_)
HITCBC 208   353 if(t.joinable()) 208   422 if(t.joinable())
HITCBC 209   353 t.join(); 209   422 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   321 stop() noexcept 213   370 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   321 std::lock_guard<std::mutex> lock(mutex_); 216   370 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   321 stop_ = true; 217   370 stop_ = true;
HITCBC 218   321 } 218   370 }
HITCBC 219   321 work_cv_.notify_all(); 219   370 work_cv_.notify_all();
HITCBC 220   321 done_cv_.notify_all(); 220   370 done_cv_.notify_all();
HITCBC 221   321 } 221   370 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   20902 ensure_started() 225   16637 ensure_started()
226   { 226   {
HITCBC 227   20902 std::call_once(start_flag_, [this]{ 227   16637 std::call_once(start_flag_, [this]{
HITCBC 228   272 threads_.reserve(num_threads_); 228   318 threads_.reserve(num_threads_);
HITCBC 229   625 for(std::size_t i = 0; i < num_threads_; ++i) 229   740 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   706 threads_.emplace_back([this, i]{ run(i); }); 230   844 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   272 }); 231   318 });
HITCBC 232   20902 } 232   16637 }
233   233  
234   void 234   void
HITCBC 235   353 run(std::size_t index) 235   422 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   353 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   422 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   353 set_current_thread_name(name); 240   422 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   353 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   422 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   353 ~scoped_pool() noexcept { current_.set(nullptr); } 247   422 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   353 } guard(this); 248   422 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   21078 continuation* c = nullptr; 252   16760 continuation* c = nullptr;
253   { 253   {
HITCBC 254   21078 std::unique_lock<std::mutex> lock(mutex_); 254   16760 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   21078 work_cv_.wait(lock, [this]{ 255   16760 work_cv_.wait(lock, [this]{
HITCBC 256   56723 return !empty() || 256   35040 return !empty() ||
HITCBC 257   56723 stop_; 257   35040 stop_;
258   }); 258   });
HITCBC 259   21078 if(stop_) 259   16760 if(stop_)
HITCBC 260   706 return; 260   844 return;
HITCBC 261   20725 c = pop(); 261   16338 c = pop();
HITCBC 262   21078 } 262   16760 }
HITCBC 263   20725 if(c) 263   16338 if(c)
HITCBC 264   20725 safe_resume(c->h); 264   16338 safe_resume(c->h);
HITCBC 265   20725 } 265   16338 }
HITCBC 266   353 } 266   422 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   319 thread_pool:: 271   368 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   319 impl_->stop(); 274   368 impl_->stop();
HITCBC 275   319 impl_->join(); 275   368 impl_->join();
HITCBC 276   319 impl_->drain_abandoned(); 276   368 impl_->drain_abandoned();
HITCBC 277   319 shutdown(); 277   368 shutdown();
HITCBC 278   319 destroy(); 278   368 destroy();
HITCBC 279   319 delete impl_; 279   368 delete impl_;
HITCBC 280   319 } 280   368 }
281   281  
HITCBC 282   319 thread_pool:: 282   368 thread_pool::
HITCBC 283   319 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   368 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   319 : impl_(new impl(num_threads, thread_name_prefix)) 284   368 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   319 this->set_frame_allocator(std::allocator<void>{}); 286   368 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   319 } 287   368 }
288   288  
289   void 289   void
HITCBC 290   189 thread_pool:: 290   234 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   189 impl_->join(); 293   234 impl_->join();
HITCBC 294   189 } 294   234 }
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   11725 thread_pool:: 306   11780 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11725 return executor_type( 309   11780 return executor_type(
HITCBC 310   11725 const_cast<thread_pool&>(*this)); 310   11780 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   501 thread_pool::executor_type:: 314   655 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   501 pool_->impl_->on_work_started(); 317   655 pool_->impl_->on_work_started();
HITCBC 318   501 } 318   655 }
319   319  
320   void 320   void
HITCBC 321   501 thread_pool::executor_type:: 321   655 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   501 pool_->impl_->on_work_finished(); 324   655 pool_->impl_->on_work_finished();
HITCBC 325   501 } 325   655 }
326   326  
327   void 327   void
HITCBC 328   20406 thread_pool::executor_type:: 328   15988 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   20406 pool_->impl_->post(c); 331   15988 pool_->impl_->post(c);
HITCBC 332   20406 } 332   15988 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   501 thread_pool::executor_type:: 335   656 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   501 if(pool_->impl_->running_in_this_thread()) 338   656 if(pool_->impl_->running_in_this_thread())
HITCBC 339   5 return c.h; 339   7 return c.h;
HITCBC 340   496 pool_->impl_->post(c); 340   649 pool_->impl_->post(c);
HITCBC 341   496 return std::noop_coroutine(); 341   649 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost