. /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   19730 void push(continuation* c) noexcept 71   18864 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19730 c->reserved = nullptr; 73   18864 c->reserved = nullptr;
HITCBC 74   19730 if(tail_) 74   18864 if(tail_)
HITCBC 75   1757 tail_->reserved = c; 75   1834 tail_->reserved = c;
76   else 76   else
HITCBC 77   17973 head_ = c; 77   17030 head_ = c;
HITCBC 78   19730 tail_ = c; 78   18864 tail_ = c;
HITCBC 79   19730 } 79   18864 }
80   80  
HITCBC 81   20109 continuation* pop() noexcept 81   19260 continuation* pop() noexcept
82   { 82   {
HITCBC 83   20109 if(!head_) 83   19260 if(!head_)
HITCBC 84   379 return nullptr; 84   396 return nullptr;
HITCBC 85   19730 continuation* c = head_; 85   18864 continuation* c = head_;
HITCBC 86   19730 head_ = static_cast<continuation*>(head_->reserved); 86   18864 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19730 if(!head_) 87   18864 if(!head_)
HITCBC 88   17973 tail_ = nullptr; 88   17030 tail_ = nullptr;
HITCBC 89   19730 return c; 89   18864 return c;
90   } 90   }
91   91  
HITCBC 92   38092 bool empty() const noexcept 92   36738 bool empty() const noexcept
93   { 93   {
HITCBC 94   38092 return head_ == nullptr; 94   36738 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   379 ~impl() = default; 109   396 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   667 running_in_this_thread() const noexcept 112   684 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   667 return current_.get() == this; 114   684 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   379 drain_abandoned() noexcept 123   396 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   560 while(auto* c = pop()) 125   558 while(auto* c = pop())
126   { 126   {
HITCBC 127   181 auto h = c->h; 127   162 auto h = c->h;
HITCBC 128   181 if(h && h != std::noop_coroutine()) 128   162 if(h && h != std::noop_coroutine())
HITCBC 129   130 h.destroy(); 129   111 h.destroy();
HITCBC 130   181 } 130   162 }
HITCBC 131   379 } 131   396 }
132   132  
HITCBC 133   379 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   396 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   379 : num_threads_(num_threads) 134   396 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   379 if(num_threads_ == 0) 136   396 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   379 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   396 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   379 thread_name_prefix_[n] = '\0'; 142   396 thread_name_prefix_[n] = '\0';
HITCBC 143   379 } 143   396 }
144   144  
145   void 145   void
HITCBC 146   19730 post(continuation& c) 146   18864 post(continuation& c)
147   { 147   {
HITCBC 148   19730 ensure_started(); 148   18864 ensure_started();
149   { 149   {
HITCBC 150   19730 std::lock_guard<std::mutex> lock(mutex_); 150   18864 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19730 push(&c); 151   18864 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   19730 work_cv_.notify_one(); 154   18864 work_cv_.notify_one();
HITCBC 155   19730 } 155   18864 }
HITCBC 156   19730 } 156   18864 }
157   157  
158   void 158   void
HITCBC 159   666 on_work_started() noexcept 159   683 on_work_started() noexcept
160   { 160   {
HITCBC 161   666 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   683 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   666 } 162   683 }
163   163  
164   void 164   void
HITCBC 165   666 on_work_finished() noexcept 165   683 on_work_finished() noexcept
166   { 166   {
HITCBC 167   666 if(outstanding_work_.fetch_sub( 167   683 if(outstanding_work_.fetch_sub(
HITCBC 168   666 1, std::memory_order_acq_rel) == 1) 168   683 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   303 std::lock_guard<std::mutex> lock(mutex_); 173   321 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   303 if(outstanding_work_.load( 174   321 if(outstanding_work_.load(
HITCBC 175   303 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   321 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   180 stop_ = true; 177   181 stop_ = true;
HITCBC 178   180 done_cv_.notify_all(); 178   181 done_cv_.notify_all();
HITCBC 179   180 work_cv_.notify_all(); 179   181 work_cv_.notify_all();
180   } 180   }
HITCBC 181   303 } 181   321 }
HITCBC 182   666 } 182   683 }
183   183  
184   void 184   void
HITCBC 185   624 join() noexcept 185   658 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   624 std::unique_lock<std::mutex> lock(mutex_); 188   658 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   624 if(joined_) 189   658 if(joined_)
HITCBC 190   245 return; 190   262 return;
HITCBC 191   379 joined_ = true; 191   396 joined_ = true;
192   192  
HITCBC 193   379 if(outstanding_work_.load( 193   396 if(outstanding_work_.load(
HITCBC 194   379 std::memory_order_acquire) == 0) 194   396 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   149 stop_ = true; 196   163 stop_ = true;
HITCBC 197   149 work_cv_.notify_all(); 197   163 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   230 done_cv_.wait(lock, [this]{ 201   233 done_cv_.wait(lock, [this]{
HITCBC 202   411 return stop_; 202   415 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   624 } 205   658 }
206   206  
HITCBC 207   812 for(auto& t : threads_) 207   846 for(auto& t : threads_)
HITCBC 208   433 if(t.joinable()) 208   450 if(t.joinable())
HITCBC 209   433 t.join(); 209   450 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   381 stop() noexcept 213   398 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   381 std::lock_guard<std::mutex> lock(mutex_); 216   398 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   381 stop_ = true; 217   398 stop_ = true;
HITCBC 218   381 } 218   398 }
HITCBC 219   381 work_cv_.notify_all(); 219   398 work_cv_.notify_all();
HITCBC 220   381 done_cv_.notify_all(); 220   398 done_cv_.notify_all();
HITCBC 221   381 } 221   398 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19730 ensure_started() 225   18864 ensure_started()
226   { 226   {
HITCBC 227   19730 std::call_once(start_flag_, [this]{ 227   18864 std::call_once(start_flag_, [this]{
HITCBC 228   329 threads_.reserve(num_threads_); 228   346 threads_.reserve(num_threads_);
HITCBC 229   762 for(std::size_t i = 0; i < num_threads_; ++i) 229   796 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   866 threads_.emplace_back([this, i]{ run(i); }); 230   900 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   329 }); 231   346 });
HITCBC 232   19730 } 232   18864 }
233   233  
234   void 234   void
HITCBC 235   433 run(std::size_t index) 235   450 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   433 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   450 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   433 set_current_thread_name(name); 240   450 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   433 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   450 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   433 ~scoped_pool() noexcept { current_.set(nullptr); } 247   450 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   433 } guard(this); 248   450 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   19982 continuation* c = nullptr; 252   19152 continuation* c = nullptr;
253   { 253   {
HITCBC 254   19982 std::unique_lock<std::mutex> lock(mutex_); 254   19152 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   19982 work_cv_.wait(lock, [this]{ 255   19152 work_cv_.wait(lock, [this]{
HITCBC 256   56548 return !empty() || 256   54692 return !empty() ||
HITCBC 257   56548 stop_; 257   54692 stop_;
258   }); 258   });
HITCBC 259   19982 if(stop_) 259   19152 if(stop_)
HITCBC 260   866 return; 260   900 return;
HITCBC 261   19549 c = pop(); 261   18702 c = pop();
HITCBC 262   19982 } 262   19152 }
HITCBC 263   19549 if(c) 263   18702 if(c)
HITCBC 264   19549 safe_resume(c->h); 264   18702 safe_resume(c->h);
HITCBC 265   19549 } 265   18702 }
HITCBC 266   433 } 266   450 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   379 thread_pool:: 271   396 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   379 impl_->stop(); 274   396 impl_->stop();
HITCBC 275   379 impl_->join(); 275   396 impl_->join();
HITCBC 276   379 impl_->drain_abandoned(); 276   396 impl_->drain_abandoned();
HITCBC 277   379 shutdown(); 277   396 shutdown();
HITCBC 278   379 destroy(); 278   396 destroy();
HITCBC 279   379 delete impl_; 279   396 delete impl_;
HITCBC 280   379 } 280   396 }
281   281  
HITCBC 282   379 thread_pool:: 282   396 thread_pool::
HITCBC 283   379 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   396 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   379 : impl_(new impl(num_threads, thread_name_prefix)) 284   396 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   379 this->set_frame_allocator(std::allocator<void>{}); 286   396 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   379 } 287   396 }
288   288  
289   void 289   void
HITCBC 290   245 thread_pool:: 290   262 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   245 impl_->join(); 293   262 impl_->join();
HITCBC 294   245 } 294   262 }
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   11791 thread_pool:: 306   11808 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11791 return executor_type( 309   11808 return executor_type(
HITCBC 310   11791 const_cast<thread_pool&>(*this)); 310   11808 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   666 thread_pool::executor_type:: 314   683 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   666 pool_->impl_->on_work_started(); 317   683 pool_->impl_->on_work_started();
HITCBC 318   666 } 318   683 }
319   319  
320   void 320   void
HITCBC 321   666 thread_pool::executor_type:: 321   683 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   666 pool_->impl_->on_work_finished(); 324   683 pool_->impl_->on_work_finished();
HITCBC 325   666 } 325   683 }
326   326  
327   void 327   void
HITCBC 328   19070 thread_pool::executor_type:: 328   18187 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   19070 pool_->impl_->post(c); 331   18187 pool_->impl_->post(c);
HITCBC 332   19070 } 332   18187 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   667 thread_pool::executor_type:: 335   684 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   667 if(pool_->impl_->running_in_this_thread()) 338   684 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   660 pool_->impl_->post(c); 340   677 pool_->impl_->post(c);
HITCBC 341   660 return std::noop_coroutine(); 341   677 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost