. /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   18018 void push(continuation* c) noexcept 71   20426 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   18018 c->reserved = nullptr; 73   20426 c->reserved = nullptr;
HITCBC 74   18018 if(tail_) 74   20426 if(tail_)
HITCBC 75   5347 tail_->reserved = c; 75   1694 tail_->reserved = c;
76   else 76   else
HITCBC 77   12671 head_ = c; 77   18732 head_ = c;
HITCBC 78   18018 tail_ = c; 78   20426 tail_ = c;
HITCBC 79   18018 } 79   20426 }
80   80  
HITCBC 81   18414 continuation* pop() noexcept 81   20824 continuation* pop() noexcept
82   { 82   {
HITCBC 83   18414 if(!head_) 83   20824 if(!head_)
HITCBC 84   396 return nullptr; 84   398 return nullptr;
HITCBC 85   18018 continuation* c = head_; 85   20426 continuation* c = head_;
HITCBC 86   18018 head_ = static_cast<continuation*>(head_->reserved); 86   20426 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   18018 if(!head_) 87   20426 if(!head_)
HITCBC 88   12671 tail_ = nullptr; 88   18732 tail_ = nullptr;
HITCBC 89   18018 return c; 89   20426 return c;
90   } 90   }
91   91  
HITCBC 92   30692 bool empty() const noexcept 92   39582 bool empty() const noexcept
93   { 93   {
HITCBC 94   30692 return head_ == nullptr; 94   39582 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   396 ~impl() = default; 109   398 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   684 running_in_this_thread() const noexcept 112   686 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   684 return current_.get() == this; 114   686 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   396 drain_abandoned() noexcept 123   398 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   598 while(auto* c = pop()) 125   573 while(auto* c = pop())
126   { 126   {
HITCBC 127   202 auto h = c->h; 127   175 auto h = c->h;
HITCBC 128   202 if(h && h != std::noop_coroutine()) 128   175 if(h && h != std::noop_coroutine())
HITCBC 129   151 h.destroy(); 129   125 h.destroy();
HITCBC 130   202 } 130   175 }
HITCBC 131   396 } 131   398 }
132   132  
HITCBC 133   396 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   398 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   396 : num_threads_(num_threads) 134   398 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   396 if(num_threads_ == 0) 136   398 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   396 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   398 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   396 thread_name_prefix_[n] = '\0'; 142   398 thread_name_prefix_[n] = '\0';
HITCBC 143   396 } 143   398 }
144   144  
145   void 145   void
HITCBC 146   18018 post(continuation& c) 146   20426 post(continuation& c)
147   { 147   {
HITCBC 148   18018 ensure_started(); 148   20426 ensure_started();
149   { 149   {
HITCBC 150   18018 std::lock_guard<std::mutex> lock(mutex_); 150   20426 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   18018 push(&c); 151   20426 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   18018 work_cv_.notify_one(); 154   20426 work_cv_.notify_one();
HITCBC 155   18018 } 155   20426 }
HITCBC 156   18018 } 156   20426 }
157   157  
158   void 158   void
HITCBC 159   683 on_work_started() noexcept 159   685 on_work_started() noexcept
160   { 160   {
HITCBC 161   683 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   685 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   683 } 162   685 }
163   163  
164   void 164   void
HITCBC 165   683 on_work_finished() noexcept 165   685 on_work_finished() noexcept
166   { 166   {
HITCBC 167   683 if(outstanding_work_.fetch_sub( 167   685 if(outstanding_work_.fetch_sub(
HITCBC 168   683 1, std::memory_order_acq_rel) == 1) 168   685 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   321 std::lock_guard<std::mutex> lock(mutex_); 173   338 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   321 if(outstanding_work_.load( 174   338 if(outstanding_work_.load(
HITCBC 175   321 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   338 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   188 stop_ = true; 177   183 stop_ = true;
HITCBC 178   188 done_cv_.notify_all(); 178   183 done_cv_.notify_all();
HITCBC 179   188 work_cv_.notify_all(); 179   183 work_cv_.notify_all();
180   } 180   }
HITCBC 181   321 } 181   338 }
HITCBC 182   683 } 182   685 }
183   183  
184   void 184   void
HITCBC 185   658 join() noexcept 185   662 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   658 std::unique_lock<std::mutex> lock(mutex_); 188   662 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   658 if(joined_) 189   662 if(joined_)
HITCBC 190   262 return; 190   264 return;
HITCBC 191   396 joined_ = true; 191   398 joined_ = true;
192   192  
HITCBC 193   396 if(outstanding_work_.load( 193   398 if(outstanding_work_.load(
HITCBC 194   396 std::memory_order_acquire) == 0) 194   398 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   156 stop_ = true; 196   164 stop_ = true;
HITCBC 197   156 work_cv_.notify_all(); 197   164 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   240 done_cv_.wait(lock, [this]{ 201   234 done_cv_.wait(lock, [this]{
HITCBC 202   429 return stop_; 202   418 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   658 } 205   662 }
206   206  
HITCBC 207   846 for(auto& t : threads_) 207   850 for(auto& t : threads_)
HITCBC 208   450 if(t.joinable()) 208   452 if(t.joinable())
HITCBC 209   450 t.join(); 209   452 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   398 stop() noexcept 213   400 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   398 std::lock_guard<std::mutex> lock(mutex_); 216   400 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   398 stop_ = true; 217   400 stop_ = true;
HITCBC 218   398 } 218   400 }
HITCBC 219   398 work_cv_.notify_all(); 219   400 work_cv_.notify_all();
HITCBC 220   398 done_cv_.notify_all(); 220   400 done_cv_.notify_all();
HITCBC 221   398 } 221   400 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   18018 ensure_started() 225   20426 ensure_started()
226   { 226   {
HITCBC 227   18018 std::call_once(start_flag_, [this]{ 227   20426 std::call_once(start_flag_, [this]{
HITCBC 228   346 threads_.reserve(num_threads_); 228   348 threads_.reserve(num_threads_);
HITCBC 229   796 for(std::size_t i = 0; i < num_threads_; ++i) 229   800 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   900 threads_.emplace_back([this, i]{ run(i); }); 230   904 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   346 }); 231   348 });
HITCBC 232   18018 } 232   20426 }
233   233  
234   void 234   void
HITCBC 235   450 run(std::size_t index) 235   452 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   450 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   452 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   450 set_current_thread_name(name); 240   452 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   450 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   452 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   450 ~scoped_pool() noexcept { current_.set(nullptr); } 247   452 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   450 } guard(this); 248   452 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   18266 continuation* c = nullptr; 252   20703 continuation* c = nullptr;
253   { 253   {
HITCBC 254   18266 std::unique_lock<std::mutex> lock(mutex_); 254   20703 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   18266 work_cv_.wait(lock, [this]{ 255   20703 work_cv_.wait(lock, [this]{
HITCBC 256   43473 return !empty() || 256   58837 return !empty() ||
HITCBC 257   43473 stop_; 257   58837 stop_;
258   }); 258   });
HITCBC 259   18266 if(stop_) 259   20703 if(stop_)
HITCBC 260   900 return; 260   904 return;
HITCBC 261   17816 c = pop(); 261   20251 c = pop();
HITCBC 262   18266 } 262   20703 }
HITCBC 263   17816 if(c) 263   20251 if(c)
HITCBC 264   17816 safe_resume(c->h); 264   20251 safe_resume(c->h);
HITCBC 265   17816 } 265   20251 }
HITCBC 266   450 } 266   452 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   396 thread_pool:: 271   398 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   396 impl_->stop(); 274   398 impl_->stop();
HITCBC 275   396 impl_->join(); 275   398 impl_->join();
HITCBC 276   396 impl_->drain_abandoned(); 276   398 impl_->drain_abandoned();
HITCBC 277   396 shutdown(); 277   398 shutdown();
HITCBC 278   396 destroy(); 278   398 destroy();
HITCBC 279   396 delete impl_; 279   398 delete impl_;
HITCBC 280   396 } 280   398 }
281   281  
HITCBC 282   396 thread_pool:: 282   398 thread_pool::
HITCBC 283   396 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   398 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   396 : impl_(new impl(num_threads, thread_name_prefix)) 284   398 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   396 this->set_frame_allocator(std::allocator<void>{}); 286   398 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   396 } 287   398 }
288   288  
289   void 289   void
HITCBC 290   262 thread_pool:: 290   264 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   262 impl_->join(); 293   264 impl_->join();
HITCBC 294   262 } 294   264 }
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   11808 thread_pool:: 306   11810 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11808 return executor_type( 309   11810 return executor_type(
HITCBC 310   11808 const_cast<thread_pool&>(*this)); 310   11810 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   683 thread_pool::executor_type:: 314   685 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   683 pool_->impl_->on_work_started(); 317   685 pool_->impl_->on_work_started();
HITCBC 318   683 } 318   685 }
319   319  
320   void 320   void
HITCBC 321   683 thread_pool::executor_type:: 321   685 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   683 pool_->impl_->on_work_finished(); 324   685 pool_->impl_->on_work_finished();
HITCBC 325   683 } 325   685 }
326   326  
327   void 327   void
HITCBC 328   17341 thread_pool::executor_type:: 328   19747 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   17341 pool_->impl_->post(c); 331   19747 pool_->impl_->post(c);
HITCBC 332   17341 } 332   19747 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   684 thread_pool::executor_type:: 335   686 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   684 if(pool_->impl_->running_in_this_thread()) 338   686 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   677 pool_->impl_->post(c); 340   679 pool_->impl_->post(c);
HITCBC 341   677 return std::noop_coroutine(); 341   679 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost