. /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 an atomic 38   Work tracking: on_work_started/on_work_finished maintain an atomic
39   outstanding_work_ counter. join() blocks until this counter reaches 39   outstanding_work_ counter. join() blocks until this counter reaches
40   zero, then signals workers to stop and joins threads. 40   zero, then signals workers to stop and joins threads.
41   41  
42   Two shutdown paths: 42   Two shutdown paths:
43   - join(): waits for outstanding work to drain, then stops workers. 43   - join(): waits for outstanding work to drain, then stops workers.
44   - stop(): immediately signals workers to exit; queued work is abandoned. 44   - stop(): immediately signals workers to exit; queued work is abandoned.
45   - Destructor: stop() then join() (abandon + wait for threads). 45   - Destructor: stop() then join() (abandon + wait for threads).
46   */ 46   */
47   47  
48   namespace boost { 48   namespace boost {
49   namespace capy { 49   namespace capy {
50   50  
51   //------------------------------------------------------------------------------ 51   //------------------------------------------------------------------------------
52   52  
53   class thread_pool::impl 53   class thread_pool::impl
54   { 54   {
55   // Identifies the pool owning the current worker thread, or 55   // Identifies the pool owning the current worker thread, or
56   // nullptr if the calling thread is not a pool worker. Checked 56   // nullptr if the calling thread is not a pool worker. Checked
57   // by dispatch() to decide between symmetric transfer (inline 57   // by dispatch() to decide between symmetric transfer (inline
58   // resume) and post. 58   // resume) and post.
59   static inline detail::thread_local_ptr<impl const> current_; 59   static inline detail::thread_local_ptr<impl const> current_;
60   60  
61   // Intrusive queue of continuations via continuation::next. 61   // Intrusive queue of continuations via continuation::next.
62   // No per-post allocation: the continuation is owned by the caller. 62   // No per-post allocation: the continuation is owned by the caller.
63   continuation* head_ = nullptr; 63   continuation* head_ = nullptr;
64   continuation* tail_ = nullptr; 64   continuation* tail_ = nullptr;
65   65  
HITCBC 66   19822 void push(continuation* c) noexcept 66   19790 void push(continuation* c) noexcept
67   { 67   {
HITCBC 68   19822 c->next = nullptr; 68   19790 c->next = nullptr;
HITCBC 69   19822 if(tail_) 69   19790 if(tail_)
HITCBC 70   3842 tail_->next = c; 70   4439 tail_->next = c;
71   else 71   else
HITCBC 72   15980 head_ = c; 72   15351 head_ = c;
HITCBC 73   19822 tail_ = c; 73   19790 tail_ = c;
HITCBC 74   19822 } 74   19790 }
75   75  
HITCBC 76   19991 continuation* pop() noexcept 76   19958 continuation* pop() noexcept
77   { 77   {
HITCBC 78   19991 if(!head_) 78   19958 if(!head_)
HITCBC 79   169 return nullptr; 79   168 return nullptr;
HITCBC 80   19822 continuation* c = head_; 80   19790 continuation* c = head_;
HITCBC 81   19822 head_ = head_->next; 81   19790 head_ = head_->next;
HITCBC 82   19822 if(!head_) 82   19790 if(!head_)
HITCBC 83   15980 tail_ = nullptr; 83   15351 tail_ = nullptr;
HITCBC 84   19822 return c; 84   19790 return c;
85   } 85   }
86   86  
HITCBC 87   38186 bool empty() const noexcept 87   37024 bool empty() const noexcept
88   { 88   {
HITCBC 89   38186 return head_ == nullptr; 89   37024 return head_ == nullptr;
90   } 90   }
91   91  
92   std::mutex mutex_; 92   std::mutex mutex_;
93   std::condition_variable work_cv_; 93   std::condition_variable work_cv_;
94   std::condition_variable done_cv_; 94   std::condition_variable done_cv_;
95   std::vector<std::thread> threads_; 95   std::vector<std::thread> threads_;
96   std::atomic<std::size_t> outstanding_work_{0}; 96   std::atomic<std::size_t> outstanding_work_{0};
97   bool stop_{false}; 97   bool stop_{false};
98   bool joined_{false}; 98   bool joined_{false};
99   std::size_t num_threads_; 99   std::size_t num_threads_;
100   char thread_name_prefix_[13]{}; // 12 chars max + null terminator 100   char thread_name_prefix_[13]{}; // 12 chars max + null terminator
101   std::once_flag start_flag_; 101   std::once_flag start_flag_;
102   102  
103   public: 103   public:
HITCBC 104   169 ~impl() = default; 104   168 ~impl() = default;
105   105  
106   bool 106   bool
HITCBC 107   358 running_in_this_thread() const noexcept 107   357 running_in_this_thread() const noexcept
108   { 108   {
HITCBC 109   358 return current_.get() == this; 109   357 return current_.get() == this;
110   } 110   }
111   111  
112   // Destroy abandoned coroutine frames. Must be called 112   // Destroy abandoned coroutine frames. Must be called
113   // before execution_context::shutdown()/destroy() so 113   // before execution_context::shutdown()/destroy() so
114   // that suspended-frame destructors (e.g. delay_awaitable 114   // that suspended-frame destructors (e.g. delay_awaitable
115   // calling timer_service::cancel()) run while services 115   // calling timer_service::cancel()) run while services
116   // are still valid. 116   // are still valid.
117   void 117   void
HITCBC 118   169 drain_abandoned() noexcept 118   168 drain_abandoned() noexcept
119   { 119   {
HITCBC 120   384 while(auto* c = pop()) 120   399 while(auto* c = pop())
121   { 121   {
HITCBC 122   215 auto h = c->h; 122   231 auto h = c->h;
HITCBC 123   215 if(h && h != std::noop_coroutine()) 123   231 if(h && h != std::noop_coroutine())
HITCBC 124   164 h.destroy(); 124   180 h.destroy();
HITCBC 125   215 } 125   231 }
HITCBC 126   169 } 126   168 }
127   127  
HITCBC 128   169 impl(std::size_t num_threads, std::string_view thread_name_prefix) 128   168 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 129   169 : num_threads_(num_threads) 129   168 : num_threads_(num_threads)
130   { 130   {
HITCBC 131   169 if(num_threads_ == 0) 131   168 if(num_threads_ == 0)
HITCBC 132   4 num_threads_ = std::max( 132   4 num_threads_ = std::max(
HITCBC 133   2 std::thread::hardware_concurrency(), 1u); 133   2 std::thread::hardware_concurrency(), 1u);
134   134  
135   // Truncate prefix to 12 chars, leaving room for up to 3-digit index. 135   // Truncate prefix to 12 chars, leaving room for up to 3-digit index.
HITCBC 136   169 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 136   168 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 137   169 thread_name_prefix_[n] = '\0'; 137   168 thread_name_prefix_[n] = '\0';
HITCBC 138   169 } 138   168 }
139   139  
140   void 140   void
HITCBC 141   19822 post(continuation& c) 141   19790 post(continuation& c)
142   { 142   {
HITCBC 143   19822 ensure_started(); 143   19790 ensure_started();
144   { 144   {
HITCBC 145   19822 std::lock_guard<std::mutex> lock(mutex_); 145   19790 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 146   19822 push(&c); 146   19790 push(&c);
HITCBC 147   19822 } 147   19790 }
HITCBC 148   19822 work_cv_.notify_one(); 148   19790 work_cv_.notify_one();
HITCBC 149   19822 } 149   19790 }
150   150  
151   void 151   void
HITCBC 152   349 on_work_started() noexcept 152   348 on_work_started() noexcept
153   { 153   {
HITCBC 154   349 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 154   348 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 155   349 } 155   348 }
156   156  
157   void 157   void
HITCBC 158   349 on_work_finished() noexcept 158   348 on_work_finished() noexcept
159   { 159   {
HITCBC 160   349 if(outstanding_work_.fetch_sub( 160   348 if(outstanding_work_.fetch_sub(
HITCBC 161   349 1, std::memory_order_acq_rel) == 1) 161   348 1, std::memory_order_acq_rel) == 1)
162   { 162   {
HITCBC 163   89 std::lock_guard<std::mutex> lock(mutex_); 163   88 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 164   89 if(joined_ && !stop_) 164   88 if(joined_ && !stop_)
HITCBC 165   4 stop_ = true; 165   5 stop_ = true;
HITCBC 166   89 done_cv_.notify_all(); 166   88 done_cv_.notify_all();
HITCBC 167   89 work_cv_.notify_all(); 167   88 work_cv_.notify_all();
HITCBC 168   89 } 168   88 }
HITCBC 169   349 } 169   348 }
170   170  
171   void 171   void
HITCBC 172   181 join() noexcept 172   180 join() noexcept
173   { 173   {
174   { 174   {
HITCBC 175   181 std::unique_lock<std::mutex> lock(mutex_); 175   180 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 176   181 if(joined_) 176   180 if(joined_)
HITCBC 177   12 return; 177   12 return;
HITCBC 178   169 joined_ = true; 178   168 joined_ = true;
179   179  
HITCBC 180   169 if(outstanding_work_.load( 180   168 if(outstanding_work_.load(
HITCBC 181   169 std::memory_order_acquire) == 0) 181   168 std::memory_order_acquire) == 0)
182   { 182   {
HITCBC 183   109 stop_ = true; 183   107 stop_ = true;
HITCBC 184   109 work_cv_.notify_all(); 184   107 work_cv_.notify_all();
185   } 185   }
186   else 186   else
187   { 187   {
HITCBC 188   60 done_cv_.wait(lock, [this]{ 188   61 done_cv_.wait(lock, [this]{
HITCBC 189   65 return stop_; 189   67 return stop_;
190   }); 190   });
191   } 191   }
HITCBC 192   181 } 192   180 }
193   193  
HITCBC 194   364 for(auto& t : threads_) 194   362 for(auto& t : threads_)
HITCBC 195   195 if(t.joinable()) 195   194 if(t.joinable())
HITCBC 196   195 t.join(); 196   194 t.join();
197   } 197   }
198   198  
199   void 199   void
HITCBC 200   171 stop() noexcept 200   170 stop() noexcept
201   { 201   {
202   { 202   {
HITCBC 203   171 std::lock_guard<std::mutex> lock(mutex_); 203   170 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 204   171 stop_ = true; 204   170 stop_ = true;
HITCBC 205   171 } 205   170 }
HITCBC 206   171 work_cv_.notify_all(); 206   170 work_cv_.notify_all();
HITCBC 207   171 done_cv_.notify_all(); 207   170 done_cv_.notify_all();
HITCBC 208   171 } 208   170 }
209   209  
210   private: 210   private:
211   void 211   void
HITCBC 212   19822 ensure_started() 212   19790 ensure_started()
213   { 213   {
HITCBC 214   19822 std::call_once(start_flag_, [this]{ 214   19790 std::call_once(start_flag_, [this]{
HITCBC 215   111 threads_.reserve(num_threads_); 215   110 threads_.reserve(num_threads_);
HITCBC 216   306 for(std::size_t i = 0; i < num_threads_; ++i) 216   304 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 217   390 threads_.emplace_back([this, i]{ run(i); }); 217   388 threads_.emplace_back([this, i]{ run(i); });
HITCBC 218   111 }); 218   110 });
HITCBC 219   19822 } 219   19790 }
220   220  
221   void 221   void
HITCBC 222   195 run(std::size_t index) 222   194 run(std::size_t index)
223   { 223   {
224   // Build name; set_current_thread_name truncates to platform limits. 224   // Build name; set_current_thread_name truncates to platform limits.
225   char name[16]; 225   char name[16];
HITCBC 226   195 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 226   194 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 227   195 set_current_thread_name(name); 227   194 set_current_thread_name(name);
228   228  
229   // Mark this thread as a worker of this pool so dispatch() 229   // Mark this thread as a worker of this pool so dispatch()
230   // can symmetric-transfer when called from within pool work. 230   // can symmetric-transfer when called from within pool work.
231   struct scoped_pool 231   struct scoped_pool
232   { 232   {
HITCBC 233   195 scoped_pool(impl const* p) noexcept { current_.set(p); } 233   194 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 234   195 ~scoped_pool() noexcept { current_.set(nullptr); } 234   194 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 235   195 } guard(this); 235   194 } guard(this);
236   236  
237   for(;;) 237   for(;;)
238   { 238   {
HITCBC 239   19802 continuation* c = nullptr; 239   19753 continuation* c = nullptr;
240   { 240   {
HITCBC 241   19802 std::unique_lock<std::mutex> lock(mutex_); 241   19753 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 242   19802 work_cv_.wait(lock, [this]{ 242   19753 work_cv_.wait(lock, [this]{
HITCBC 243   56664 return !empty() || 243   54386 return !empty() ||
HITCBC 244   56664 stop_; 244   54386 stop_;
245   }); 245   });
HITCBC 246   19802 if(stop_) 246   19753 if(stop_)
HITCBC 247   390 return; 247   388 return;
HITCBC 248   19607 c = pop(); 248   19559 c = pop();
HITCBC 249   19802 } 249   19753 }
HITCBC 250   19607 if(c) 250   19559 if(c)
HITCBC 251   19607 safe_resume(c->h); 251   19559 safe_resume(c->h);
HITCBC 252   19607 } 252   19559 }
HITCBC 253   195 } 253   194 }
254   }; 254   };
255   255  
256   //------------------------------------------------------------------------------ 256   //------------------------------------------------------------------------------
257   257  
HITCBC 258   169 thread_pool:: 258   168 thread_pool::
259   ~thread_pool() 259   ~thread_pool()
260   { 260   {
HITCBC 261   169 impl_->stop(); 261   168 impl_->stop();
HITCBC 262   169 impl_->join(); 262   168 impl_->join();
HITCBC 263   169 impl_->drain_abandoned(); 263   168 impl_->drain_abandoned();
HITCBC 264   169 shutdown(); 264   168 shutdown();
HITCBC 265   169 destroy(); 265   168 destroy();
HITCBC 266   169 delete impl_; 266   168 delete impl_;
HITCBC 267   169 } 267   168 }
268   268  
HITCBC 269   169 thread_pool:: 269   168 thread_pool::
HITCBC 270   169 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 270   168 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 271   169 : impl_(new impl(num_threads, thread_name_prefix)) 271   168 : impl_(new impl(num_threads, thread_name_prefix))
272   { 272   {
HITCBC 273   169 this->set_frame_allocator(std::allocator<void>{}); 273   168 this->set_frame_allocator(std::allocator<void>{});
HITCBC 274   169 } 274   168 }
275   275  
276   void 276   void
HITCBC 277   12 thread_pool:: 277   12 thread_pool::
278   join() noexcept 278   join() noexcept
279   { 279   {
HITCBC 280   12 impl_->join(); 280   12 impl_->join();
HITCBC 281   12 } 281   12 }
282   282  
283   void 283   void
HITCBC 284   2 thread_pool:: 284   2 thread_pool::
285   stop() noexcept 285   stop() noexcept
286   { 286   {
HITCBC 287   2 impl_->stop(); 287   2 impl_->stop();
HITCBC 288   2 } 288   2 }
289   289  
290   //------------------------------------------------------------------------------ 290   //------------------------------------------------------------------------------
291   291  
292   thread_pool::executor_type 292   thread_pool::executor_type
HITCBC 293   11583 thread_pool:: 293   11582 thread_pool::
294   get_executor() const noexcept 294   get_executor() const noexcept
295   { 295   {
HITCBC 296   11583 return executor_type( 296   11582 return executor_type(
HITCBC 297   11583 const_cast<thread_pool&>(*this)); 297   11582 const_cast<thread_pool&>(*this));
298   } 298   }
299   299  
300   void 300   void
HITCBC 301   349 thread_pool::executor_type:: 301   348 thread_pool::executor_type::
302   on_work_started() const noexcept 302   on_work_started() const noexcept
303   { 303   {
HITCBC 304   349 pool_->impl_->on_work_started(); 304   348 pool_->impl_->on_work_started();
HITCBC 305   349 } 305   348 }
306   306  
307   void 307   void
HITCBC 308   349 thread_pool::executor_type:: 308   348 thread_pool::executor_type::
309   on_work_finished() const noexcept 309   on_work_finished() const noexcept
310   { 310   {
HITCBC 311   349 pool_->impl_->on_work_finished(); 311   348 pool_->impl_->on_work_finished();
HITCBC 312   349 } 312   348 }
313   313  
314   void 314   void
HITCBC 315   19478 thread_pool::executor_type:: 315   19447 thread_pool::executor_type::
316   post(continuation& c) const 316   post(continuation& c) const
317   { 317   {
HITCBC 318   19478 pool_->impl_->post(c); 318   19447 pool_->impl_->post(c);
HITCBC 319   19478 } 319   19447 }
320   320  
321   std::coroutine_handle<> 321   std::coroutine_handle<>
HITCBC 322   358 thread_pool::executor_type:: 322   357 thread_pool::executor_type::
323   dispatch(continuation& c) const 323   dispatch(continuation& c) const
324   { 324   {
HITCBC 325   358 if(pool_->impl_->running_in_this_thread()) 325   357 if(pool_->impl_->running_in_this_thread())
HITCBC 326   14 return c.h; 326   14 return c.h;
HITCBC 327   344 pool_->impl_->post(c); 327   343 pool_->impl_->post(c);
HITCBC 328   344 return std::noop_coroutine(); 328   343 return std::noop_coroutine();
329   } 329   }
330   330  
331   } // capy 331   } // capy
332   } // boost 332   } // boost