. /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   18053 void push(continuation* c) noexcept 71   20136 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   18053 c->reserved = nullptr; 73   20136 c->reserved = nullptr;
HITCBC 74   18053 if(tail_) 74   20136 if(tail_)
HITCBC 75   3537 tail_->reserved = c; 75   1528 tail_->reserved = c;
76   else 76   else
HITCBC 77   14516 head_ = c; 77   18608 head_ = c;
HITCBC 78   18053 tail_ = c; 78   20136 tail_ = c;
HITCBC 79   18053 } 79   20136 }
80   80  
HITCBC 81   18450 continuation* pop() noexcept 81   20540 continuation* pop() noexcept
82   { 82   {
HITCBC 83   18450 if(!head_) 83   20540 if(!head_)
HITCBC 84   397 return nullptr; 84   404 return nullptr;
HITCBC 85   18053 continuation* c = head_; 85   20136 continuation* c = head_;
HITCBC 86   18053 head_ = static_cast<continuation*>(head_->reserved); 86   20136 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   18053 if(!head_) 87   20136 if(!head_)
HITCBC 88   14516 tail_ = nullptr; 88   18608 tail_ = nullptr;
HITCBC 89   18053 return c; 89   20136 return c;
90   } 90   }
91   91  
HITCBC 92   32914 bool empty() const noexcept 92   39383 bool empty() const noexcept
93   { 93   {
HITCBC 94   32914 return head_ == nullptr; 94   39383 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   397 ~impl() = default; 109   404 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   685 running_in_this_thread() const noexcept 112   692 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   685 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   397 drain_abandoned() noexcept 123   404 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   585 while(auto* c = pop()) 125   614 while(auto* c = pop())
126   { 126   {
HITCBC 127   188 auto h = c->h; 127   210 auto h = c->h;
HITCBC 128   188 if(h && h != std::noop_coroutine()) 128   210 if(h && h != std::noop_coroutine())
HITCBC 129   137 h.destroy(); 129   159 h.destroy();
HITCBC 130   188 } 130   210 }
HITCBC 131   397 } 131   404 }
132   132  
HITCBC 133   397 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   397 : num_threads_(num_threads) 134   404 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   397 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   397 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   404 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   397 thread_name_prefix_[n] = '\0'; 142   404 thread_name_prefix_[n] = '\0';
HITCBC 143   397 } 143   404 }
144   144  
145   void 145   void
HITCBC 146   18053 post(continuation& c) 146   20136 post(continuation& c)
147   { 147   {
HITCBC 148   18053 ensure_started(); 148   20136 ensure_started();
149   { 149   {
HITCBC 150   18053 std::lock_guard<std::mutex> lock(mutex_); 150   20136 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   18053 push(&c); 151   20136 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   18053 work_cv_.notify_one(); 154   20136 work_cv_.notify_one();
HITCBC 155   18053 } 155   20136 }
HITCBC 156   18053 } 156   20136 }
157   157  
158   void 158   void
HITCBC 159   684 on_work_started() noexcept 159   691 on_work_started() noexcept
160   { 160   {
HITCBC 161   684 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   691 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   684 } 162   691 }
163   163  
164   void 164   void
HITCBC 165   684 on_work_finished() noexcept 165   691 on_work_finished() noexcept
166   { 166   {
HITCBC 167   684 if(outstanding_work_.fetch_sub( 167   691 if(outstanding_work_.fetch_sub(
HITCBC 168   684 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   321 std::lock_guard<std::mutex> lock(mutex_); 173   328 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   321 if(outstanding_work_.load( 174   328 if(outstanding_work_.load(
HITCBC 175   321 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   328 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   188 stop_ = true; 177   186 stop_ = true;
HITCBC 178   188 done_cv_.notify_all(); 178   186 done_cv_.notify_all();
HITCBC 179   188 work_cv_.notify_all(); 179   186 work_cv_.notify_all();
180   } 180   }
HITCBC 181   321 } 181   328 }
HITCBC 182   684 } 182   691 }
183   183  
184   void 184   void
HITCBC 185   660 join() noexcept 185   674 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   660 std::unique_lock<std::mutex> lock(mutex_); 188   674 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   660 if(joined_) 189   674 if(joined_)
HITCBC 190   263 return; 190   270 return;
HITCBC 191   397 joined_ = true; 191   404 joined_ = true;
192   192  
HITCBC 193   397 if(outstanding_work_.load( 193   404 if(outstanding_work_.load(
HITCBC 194   397 std::memory_order_acquire) == 0) 194   404 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   156 stop_ = true; 196   166 stop_ = true;
HITCBC 197   156 work_cv_.notify_all(); 197   166 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   241 done_cv_.wait(lock, [this]{ 201   238 done_cv_.wait(lock, [this]{
HITCBC 202   430 return stop_; 202   425 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   660 } 205   674 }
206   206  
HITCBC 207   848 for(auto& t : threads_) 207   862 for(auto& t : threads_)
HITCBC 208   451 if(t.joinable()) 208   458 if(t.joinable())
HITCBC 209   451 t.join(); 209   458 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   399 stop() noexcept 213   406 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   399 std::lock_guard<std::mutex> lock(mutex_); 216   406 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   399 stop_ = true; 217   406 stop_ = true;
HITCBC 218   399 } 218   406 }
HITCBC 219   399 work_cv_.notify_all(); 219   406 work_cv_.notify_all();
HITCBC 220   399 done_cv_.notify_all(); 220   406 done_cv_.notify_all();
HITCBC 221   399 } 221   406 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   18053 ensure_started() 225   20136 ensure_started()
226   { 226   {
HITCBC 227   18053 std::call_once(start_flag_, [this]{ 227   20136 std::call_once(start_flag_, [this]{
HITCBC 228   347 threads_.reserve(num_threads_); 228   354 threads_.reserve(num_threads_);
HITCBC 229   798 for(std::size_t i = 0; i < num_threads_; ++i) 229   812 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   902 threads_.emplace_back([this, i]{ run(i); }); 230   916 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   347 }); 231   354 });
HITCBC 232   18053 } 232   20136 }
233   233  
234   void 234   void
HITCBC 235   451 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   451 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   451 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   451 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   458 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   451 ~scoped_pool() noexcept { current_.set(nullptr); } 247   458 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   451 } guard(this); 248   458 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   18316 continuation* c = nullptr; 252   20384 continuation* c = nullptr;
253   { 253   {
HITCBC 254   18316 std::unique_lock<std::mutex> lock(mutex_); 254   20384 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   18316 work_cv_.wait(lock, [this]{ 255   20384 work_cv_.wait(lock, [this]{
HITCBC 256   47868 return !empty() || 256   58749 return !empty() ||
HITCBC 257   47868 stop_; 257   58749 stop_;
258   }); 258   });
HITCBC 259   18316 if(stop_) 259   20384 if(stop_)
HITCBC 260   902 return; 260   916 return;
HITCBC 261   17865 c = pop(); 261   19926 c = pop();
HITCBC 262   18316 } 262   20384 }
HITCBC 263   17865 if(c) 263   19926 if(c)
HITCBC 264   17865 safe_resume(c->h); 264   19926 safe_resume(c->h);
HITCBC 265   17865 } 265   19926 }
HITCBC 266   451 } 266   458 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   397 thread_pool:: 271   404 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   397 impl_->stop(); 274   404 impl_->stop();
HITCBC 275   397 impl_->join(); 275   404 impl_->join();
HITCBC 276   397 impl_->drain_abandoned(); 276   404 impl_->drain_abandoned();
HITCBC 277   397 shutdown(); 277   404 shutdown();
HITCBC 278   397 destroy(); 278   404 destroy();
HITCBC 279   397 delete impl_; 279   404 delete impl_;
HITCBC 280   397 } 280   404 }
281   281  
HITCBC 282   397 thread_pool:: 282   404 thread_pool::
HITCBC 283   397 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   397 : impl_(new impl(num_threads, thread_name_prefix)) 284   404 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   397 this->set_frame_allocator(std::allocator<void>{}); 286   404 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   397 } 287   404 }
288   288  
289   void 289   void
HITCBC 290   263 thread_pool:: 290   270 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   263 impl_->join(); 293   270 impl_->join();
HITCBC 294   263 } 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   11809 thread_pool:: 306   11816 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11809 return executor_type( 309   11816 return executor_type(
HITCBC 310   11809 const_cast<thread_pool&>(*this)); 310   11816 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   684 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   684 pool_->impl_->on_work_started(); 317   691 pool_->impl_->on_work_started();
HITCBC 318   684 } 318   691 }
319   319  
320   void 320   void
HITCBC 321   684 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   684 pool_->impl_->on_work_finished(); 324   691 pool_->impl_->on_work_finished();
HITCBC 325   684 } 325   691 }
326   326  
327   void 327   void
HITCBC 328   17375 thread_pool::executor_type:: 328   19451 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   17375 pool_->impl_->post(c); 331   19451 pool_->impl_->post(c);
HITCBC 332   17375 } 332   19451 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   685 thread_pool::executor_type:: 335   692 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   685 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   678 pool_->impl_->post(c); 340   685 pool_->impl_->post(c);
HITCBC 341   678 return std::noop_coroutine(); 341   685 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost