. /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   19026 void push(continuation* c) noexcept 71   18839 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19026 c->reserved = nullptr; 73   18839 c->reserved = nullptr;
HITCBC 74   19026 if(tail_) 74   18839 if(tail_)
HITCBC 75   2576 tail_->reserved = c; 75   3213 tail_->reserved = c;
76   else 76   else
HITCBC 77   16450 head_ = c; 77   15626 head_ = c;
HITCBC 78   19026 tail_ = c; 78   18839 tail_ = c;
HITCBC 79   19026 } 79   18839 }
80   80  
HITCBC 81   19416 continuation* pop() noexcept 81   19243 continuation* pop() noexcept
82   { 82   {
HITCBC 83   19416 if(!head_) 83   19243 if(!head_)
HITCBC 84   390 return nullptr; 84   404 return nullptr;
HITCBC 85   19026 continuation* c = head_; 85   18839 continuation* c = head_;
HITCBC 86   19026 head_ = static_cast<continuation*>(head_->reserved); 86   18839 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19026 if(!head_) 87   18839 if(!head_)
HITCBC 88   16450 tail_ = nullptr; 88   15626 tail_ = nullptr;
HITCBC 89   19026 return c; 89   18839 return c;
90   } 90   }
91   91  
HITCBC 92   35737 bool empty() const noexcept 92   35031 bool empty() const noexcept
93   { 93   {
HITCBC 94   35737 return head_ == nullptr; 94   35031 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   390 ~impl() = default; 109   404 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   678 running_in_this_thread() const noexcept 112   692 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   678 return current_.get() == this; 114   692 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   390 drain_abandoned() noexcept 123   404 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   588 while(auto* c = pop()) 125   607 while(auto* c = pop())
126   { 126   {
HITCBC 127   198 auto h = c->h; 127   203 auto h = c->h;
HITCBC 128   198 if(h && h != std::noop_coroutine()) 128   203 if(h && h != std::noop_coroutine())
HITCBC 129   147 h.destroy(); 129   152 h.destroy();
HITCBC 130   198 } 130   203 }
HITCBC 131   390 } 131   404 }
132   132  
HITCBC 133   390 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   404 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   390 : num_threads_(num_threads) 134   404 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   390 if(num_threads_ == 0) 136   404 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   390 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   404 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   390 thread_name_prefix_[n] = '\0'; 142   404 thread_name_prefix_[n] = '\0';
HITCBC 143   390 } 143   404 }
144   144  
145   void 145   void
HITCBC 146   19026 post(continuation& c) 146   18839 post(continuation& c)
147   { 147   {
HITCBC 148   19026 ensure_started(); 148   18839 ensure_started();
149   { 149   {
HITCBC 150   19026 std::lock_guard<std::mutex> lock(mutex_); 150   18839 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19026 push(&c); 151   18839 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   19026 work_cv_.notify_one(); 154   18839 work_cv_.notify_one();
HITCBC 155   19026 } 155   18839 }
HITCBC 156   19026 } 156   18839 }
157   157  
158   void 158   void
HITCBC 159   677 on_work_started() noexcept 159   691 on_work_started() noexcept
160   { 160   {
HITCBC 161   677 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   691 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   677 } 162   691 }
163   163  
164   void 164   void
HITCBC 165   677 on_work_finished() noexcept 165   691 on_work_finished() noexcept
166   { 166   {
HITCBC 167   677 if(outstanding_work_.fetch_sub( 167   691 if(outstanding_work_.fetch_sub(
HITCBC 168   677 1, std::memory_order_acq_rel) == 1) 168   691 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   314 std::lock_guard<std::mutex> lock(mutex_); 173   328 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   314 if(outstanding_work_.load( 174   328 if(outstanding_work_.load(
HITCBC 175   314 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   328 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   165 stop_ = true; 177   196 stop_ = true;
HITCBC 178   165 done_cv_.notify_all(); 178   196 done_cv_.notify_all();
HITCBC 179   165 work_cv_.notify_all(); 179   196 work_cv_.notify_all();
180   } 180   }
HITCBC 181   314 } 181   328 }
HITCBC 182   677 } 182   691 }
183   183  
184   void 184   void
HITCBC 185   646 join() noexcept 185   674 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   646 std::unique_lock<std::mutex> lock(mutex_); 188   674 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   646 if(joined_) 189   674 if(joined_)
HITCBC 190   256 return; 190   270 return;
HITCBC 191   390 joined_ = true; 191   404 joined_ = true;
192   192  
HITCBC 193   390 if(outstanding_work_.load( 193   404 if(outstanding_work_.load(
HITCBC 194   390 std::memory_order_acquire) == 0) 194   404 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   174 stop_ = true; 196   152 stop_ = true;
HITCBC 197   174 work_cv_.notify_all(); 197   152 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   216 done_cv_.wait(lock, [this]{ 201   252 done_cv_.wait(lock, [this]{
HITCBC 202   382 return stop_; 202   449 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   646 } 205   674 }
206   206  
HITCBC 207   834 for(auto& t : threads_) 207   862 for(auto& t : threads_)
HITCBC 208   444 if(t.joinable()) 208   458 if(t.joinable())
HITCBC 209   444 t.join(); 209   458 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   392 stop() noexcept 213   406 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   392 std::lock_guard<std::mutex> lock(mutex_); 216   406 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   392 stop_ = true; 217   406 stop_ = true;
HITCBC 218   392 } 218   406 }
HITCBC 219   392 work_cv_.notify_all(); 219   406 work_cv_.notify_all();
HITCBC 220   392 done_cv_.notify_all(); 220   406 done_cv_.notify_all();
HITCBC 221   392 } 221   406 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19026 ensure_started() 225   18839 ensure_started()
226   { 226   {
HITCBC 227   19026 std::call_once(start_flag_, [this]{ 227   18839 std::call_once(start_flag_, [this]{
HITCBC 228   340 threads_.reserve(num_threads_); 228   354 threads_.reserve(num_threads_);
HITCBC 229   784 for(std::size_t i = 0; i < num_threads_; ++i) 229   812 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   888 threads_.emplace_back([this, i]{ run(i); }); 230   916 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   340 }); 231   354 });
HITCBC 232   19026 } 232   18839 }
233   233  
234   void 234   void
HITCBC 235   444 run(std::size_t index) 235   458 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   444 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   458 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   444 set_current_thread_name(name); 240   458 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   444 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   458 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   444 ~scoped_pool() noexcept { current_.set(nullptr); } 247   458 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   444 } guard(this); 248   458 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   19272 continuation* c = nullptr; 252   19094 continuation* c = nullptr;
253   { 253   {
HITCBC 254   19272 std::unique_lock<std::mutex> lock(mutex_); 254   19094 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   19272 work_cv_.wait(lock, [this]{ 255   19094 work_cv_.wait(lock, [this]{
HITCBC 256   52557 return !empty() || 256   51325 return !empty() ||
HITCBC 257   52557 stop_; 257   51325 stop_;
258   }); 258   });
HITCBC 259   19272 if(stop_) 259   19094 if(stop_)
HITCBC 260   888 return; 260   916 return;
HITCBC 261   18828 c = pop(); 261   18636 c = pop();
HITCBC 262   19272 } 262   19094 }
HITCBC 263   18828 if(c) 263   18636 if(c)
HITCBC 264   18828 safe_resume(c->h); 264   18636 safe_resume(c->h);
HITCBC 265   18828 } 265   18636 }
HITCBC 266   444 } 266   458 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   390 thread_pool:: 271   404 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   390 impl_->stop(); 274   404 impl_->stop();
HITCBC 275   390 impl_->join(); 275   404 impl_->join();
HITCBC 276   390 impl_->drain_abandoned(); 276   404 impl_->drain_abandoned();
HITCBC 277   390 shutdown(); 277   404 shutdown();
HITCBC 278   390 destroy(); 278   404 destroy();
HITCBC 279   390 delete impl_; 279   404 delete impl_;
HITCBC 280   390 } 280   404 }
281   281  
HITCBC 282   390 thread_pool:: 282   404 thread_pool::
HITCBC 283   390 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   404 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   390 : impl_(new impl(num_threads, thread_name_prefix)) 284   404 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   390 this->set_frame_allocator(std::allocator<void>{}); 286   404 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   390 } 287   404 }
288   288  
289   void 289   void
HITCBC 290   256 thread_pool:: 290   270 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   256 impl_->join(); 293   270 impl_->join();
HITCBC 294   256 } 294   270 }
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   11802 thread_pool:: 306   11816 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11802 return executor_type( 309   11816 return executor_type(
HITCBC 310   11802 const_cast<thread_pool&>(*this)); 310   11816 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   677 thread_pool::executor_type:: 314   691 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   677 pool_->impl_->on_work_started(); 317   691 pool_->impl_->on_work_started();
HITCBC 318   677 } 318   691 }
319   319  
320   void 320   void
HITCBC 321   677 thread_pool::executor_type:: 321   691 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   677 pool_->impl_->on_work_finished(); 324   691 pool_->impl_->on_work_finished();
HITCBC 325   677 } 325   691 }
326   326  
327   void 327   void
HITCBC 328   18355 thread_pool::executor_type:: 328   18154 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   18355 pool_->impl_->post(c); 331   18154 pool_->impl_->post(c);
HITCBC 332   18355 } 332   18154 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   678 thread_pool::executor_type:: 335   692 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   678 if(pool_->impl_->running_in_this_thread()) 338   692 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   671 pool_->impl_->post(c); 340   685 pool_->impl_->post(c);
HITCBC 341   671 return std::noop_coroutine(); 341   685 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost