. /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   19925 void push(continuation* c) noexcept 71   18574 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19925 c->reserved = nullptr; 73   18574 c->reserved = nullptr;
HITCBC 74   19925 if(tail_) 74   18574 if(tail_)
HITCBC 75   1277 tail_->reserved = c; 75   3962 tail_->reserved = c;
76   else 76   else
HITCBC 77   18648 head_ = c; 77   14612 head_ = c;
HITCBC 78   19925 tail_ = c; 78   18574 tail_ = c;
HITCBC 79   19925 } 79   18574 }
80   80  
HITCBC 81   20319 continuation* pop() noexcept 81   18972 continuation* pop() noexcept
82   { 82   {
HITCBC 83   20319 if(!head_) 83   18972 if(!head_)
HITCBC 84   394 return nullptr; 84   398 return nullptr;
HITCBC 85   19925 continuation* c = head_; 85   18574 continuation* c = head_;
HITCBC 86   19925 head_ = static_cast<continuation*>(head_->reserved); 86   18574 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19925 if(!head_) 87   18574 if(!head_)
HITCBC 88   18648 tail_ = nullptr; 88   14612 tail_ = nullptr;
HITCBC 89   19925 return c; 89   18574 return c;
90   } 90   }
91   91  
HITCBC 92   39008 bool empty() const noexcept 92   33576 bool empty() const noexcept
93   { 93   {
HITCBC 94   39008 return head_ == nullptr; 94   33576 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   394 ~impl() = default; 109   398 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   682 running_in_this_thread() const noexcept 112   686 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   682 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   394 drain_abandoned() noexcept 123   398 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   597 while(auto* c = pop()) 125   594 while(auto* c = pop())
126   { 126   {
HITCBC 127   203 auto h = c->h; 127   196 auto h = c->h;
HITCBC 128   203 if(h && h != std::noop_coroutine()) 128   196 if(h && h != std::noop_coroutine())
HITCBC 129   152 h.destroy(); 129   145 h.destroy();
HITCBC 130   203 } 130   196 }
HITCBC 131   394 } 131   398 }
132   132  
HITCBC 133   394 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   394 : num_threads_(num_threads) 134   398 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   394 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   394 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   398 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   394 thread_name_prefix_[n] = '\0'; 142   398 thread_name_prefix_[n] = '\0';
HITCBC 143   394 } 143   398 }
144   144  
145   void 145   void
HITCBC 146   19925 post(continuation& c) 146   18574 post(continuation& c)
147   { 147   {
HITCBC 148   19925 ensure_started(); 148   18574 ensure_started();
149   { 149   {
HITCBC 150   19925 std::lock_guard<std::mutex> lock(mutex_); 150   18574 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19925 push(&c); 151   18574 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   19925 work_cv_.notify_one(); 154   18574 work_cv_.notify_one();
HITCBC 155   19925 } 155   18574 }
HITCBC 156   19925 } 156   18574 }
157   157  
158   void 158   void
HITCBC 159   681 on_work_started() noexcept 159   685 on_work_started() noexcept
160   { 160   {
HITCBC 161   681 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   685 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   681 } 162   685 }
163   163  
164   void 164   void
HITCBC 165   681 on_work_finished() noexcept 165   685 on_work_finished() noexcept
166   { 166   {
HITCBC 167   681 if(outstanding_work_.fetch_sub( 167   685 if(outstanding_work_.fetch_sub(
HITCBC 168   681 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   318 std::lock_guard<std::mutex> lock(mutex_); 173   322 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   318 if(outstanding_work_.load( 174   322 if(outstanding_work_.load(
HITCBC 175   318 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   322 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   181 stop_ = true; 177   178 stop_ = true;
HITCBC 178   181 done_cv_.notify_all(); 178   178 done_cv_.notify_all();
HITCBC 179   181 work_cv_.notify_all(); 179   178 work_cv_.notify_all();
180   } 180   }
HITCBC 181   318 } 181   322 }
HITCBC 182   681 } 182   685 }
183   183  
184   void 184   void
HITCBC 185   654 join() noexcept 185   662 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   654 std::unique_lock<std::mutex> lock(mutex_); 188   662 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   654 if(joined_) 189   662 if(joined_)
HITCBC 190   260 return; 190   264 return;
HITCBC 191   394 joined_ = true; 191   398 joined_ = true;
192   192  
HITCBC 193   394 if(outstanding_work_.load( 193   398 if(outstanding_work_.load(
HITCBC 194   394 std::memory_order_acquire) == 0) 194   398 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   159 stop_ = true; 196   170 stop_ = true;
HITCBC 197   159 work_cv_.notify_all(); 197   170 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   235 done_cv_.wait(lock, [this]{ 201   228 done_cv_.wait(lock, [this]{
HITCBC 202   417 return stop_; 202   407 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   654 } 205   662 }
206   206  
HITCBC 207   842 for(auto& t : threads_) 207   850 for(auto& t : threads_)
HITCBC 208   448 if(t.joinable()) 208   452 if(t.joinable())
HITCBC 209   448 t.join(); 209   452 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   396 stop() noexcept 213   400 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   396 std::lock_guard<std::mutex> lock(mutex_); 216   400 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   396 stop_ = true; 217   400 stop_ = true;
HITCBC 218   396 } 218   400 }
HITCBC 219   396 work_cv_.notify_all(); 219   400 work_cv_.notify_all();
HITCBC 220   396 done_cv_.notify_all(); 220   400 done_cv_.notify_all();
HITCBC 221   396 } 221   400 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19925 ensure_started() 225   18574 ensure_started()
226   { 226   {
HITCBC 227   19925 std::call_once(start_flag_, [this]{ 227   18574 std::call_once(start_flag_, [this]{
HITCBC 228   344 threads_.reserve(num_threads_); 228   348 threads_.reserve(num_threads_);
HITCBC 229   792 for(std::size_t i = 0; i < num_threads_; ++i) 229   800 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   896 threads_.emplace_back([this, i]{ run(i); }); 230   904 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   344 }); 231   348 });
HITCBC 232   19925 } 232   18574 }
233   233  
234   void 234   void
HITCBC 235   448 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   448 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   448 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   448 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   452 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   448 ~scoped_pool() noexcept { current_.set(nullptr); } 247   452 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   448 } guard(this); 248   452 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   20170 continuation* c = nullptr; 252   18830 continuation* c = nullptr;
253   { 253   {
HITCBC 254   20170 std::unique_lock<std::mutex> lock(mutex_); 254   18830 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   20170 work_cv_.wait(lock, [this]{ 255   18830 work_cv_.wait(lock, [this]{
HITCBC 256   58197 return !empty() || 256   48687 return !empty() ||
HITCBC 257   58197 stop_; 257   48687 stop_;
258   }); 258   });
HITCBC 259   20170 if(stop_) 259   18830 if(stop_)
HITCBC 260   896 return; 260   904 return;
HITCBC 261   19722 c = pop(); 261   18378 c = pop();
HITCBC 262   20170 } 262   18830 }
HITCBC 263   19722 if(c) 263   18378 if(c)
HITCBC 264   19722 safe_resume(c->h); 264   18378 safe_resume(c->h);
HITCBC 265   19722 } 265   18378 }
HITCBC 266   448 } 266   452 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   394 thread_pool:: 271   398 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   394 impl_->stop(); 274   398 impl_->stop();
HITCBC 275   394 impl_->join(); 275   398 impl_->join();
HITCBC 276   394 impl_->drain_abandoned(); 276   398 impl_->drain_abandoned();
HITCBC 277   394 shutdown(); 277   398 shutdown();
HITCBC 278   394 destroy(); 278   398 destroy();
HITCBC 279   394 delete impl_; 279   398 delete impl_;
HITCBC 280   394 } 280   398 }
281   281  
HITCBC 282   394 thread_pool:: 282   398 thread_pool::
HITCBC 283   394 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   394 : impl_(new impl(num_threads, thread_name_prefix)) 284   398 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   394 this->set_frame_allocator(std::allocator<void>{}); 286   398 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   394 } 287   398 }
288   288  
289   void 289   void
HITCBC 290   260 thread_pool:: 290   264 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   260 impl_->join(); 293   264 impl_->join();
HITCBC 294   260 } 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   11806 thread_pool:: 306   11810 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11806 return executor_type( 309   11810 return executor_type(
HITCBC 310   11806 const_cast<thread_pool&>(*this)); 310   11810 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   681 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   681 pool_->impl_->on_work_started(); 317   685 pool_->impl_->on_work_started();
HITCBC 318   681 } 318   685 }
319   319  
320   void 320   void
HITCBC 321   681 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   681 pool_->impl_->on_work_finished(); 324   685 pool_->impl_->on_work_finished();
HITCBC 325   681 } 325   685 }
326   326  
327   void 327   void
HITCBC 328   19250 thread_pool::executor_type:: 328   17895 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   19250 pool_->impl_->post(c); 331   17895 pool_->impl_->post(c);
HITCBC 332   19250 } 332   17895 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   682 thread_pool::executor_type:: 335   686 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   682 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   675 pool_->impl_->post(c); 340   679 pool_->impl_->post(c);
HITCBC 341   675 return std::noop_coroutine(); 341   679 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost