. /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   19090 void push(continuation* c) noexcept 71   18731 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19090 c->reserved = nullptr; 73   18731 c->reserved = nullptr;
HITCBC 74   19090 if(tail_) 74   18731 if(tail_)
HITCBC 75   1993 tail_->reserved = c; 75   2879 tail_->reserved = c;
76   else 76   else
HITCBC 77   17097 head_ = c; 77   15852 head_ = c;
HITCBC 78   19090 tail_ = c; 78   18731 tail_ = c;
HITCBC 79   19090 } 79   18731 }
80   80  
HITCBC 81   19479 continuation* pop() noexcept 81   19117 continuation* pop() noexcept
82   { 82   {
HITCBC 83   19479 if(!head_) 83   19117 if(!head_)
HITCBC 84   389 return nullptr; 84   386 return nullptr;
HITCBC 85   19090 continuation* c = head_; 85   18731 continuation* c = head_;
HITCBC 86   19090 head_ = static_cast<continuation*>(head_->reserved); 86   18731 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19090 if(!head_) 87   18731 if(!head_)
HITCBC 88   17097 tail_ = nullptr; 88   15852 tail_ = nullptr;
HITCBC 89   19090 return c; 89   18731 return c;
90   } 90   }
91   91  
HITCBC 92   36645 bool empty() const noexcept 92   35338 bool empty() const noexcept
93   { 93   {
HITCBC 94   36645 return head_ == nullptr; 94   35338 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   389 ~impl() = default; 109   386 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   677 running_in_this_thread() const noexcept 112   674 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   677 return current_.get() == this; 114   674 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   389 drain_abandoned() noexcept 123   386 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   568 while(auto* c = pop()) 125   546 while(auto* c = pop())
126   { 126   {
HITCBC 127   179 auto h = c->h; 127   160 auto h = c->h;
HITCBC 128   179 if(h && h != std::noop_coroutine()) 128   160 if(h && h != std::noop_coroutine())
HITCBC 129   128 h.destroy(); 129   109 h.destroy();
HITCBC 130   179 } 130   160 }
HITCBC 131   389 } 131   386 }
132   132  
HITCBC 133   389 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   386 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   389 : num_threads_(num_threads) 134   386 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   389 if(num_threads_ == 0) 136   386 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   389 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   386 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   389 thread_name_prefix_[n] = '\0'; 142   386 thread_name_prefix_[n] = '\0';
HITCBC 143   389 } 143   386 }
144   144  
145   void 145   void
HITCBC 146   19090 post(continuation& c) 146   18731 post(continuation& c)
147   { 147   {
HITCBC 148   19090 ensure_started(); 148   18731 ensure_started();
149   { 149   {
HITCBC 150   19090 std::lock_guard<std::mutex> lock(mutex_); 150   18731 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19090 push(&c); 151   18731 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   19090 work_cv_.notify_one(); 154   18731 work_cv_.notify_one();
HITCBC 155   19090 } 155   18731 }
HITCBC 156   19090 } 156   18731 }
157   157  
158   void 158   void
HITCBC 159   676 on_work_started() noexcept 159   673 on_work_started() noexcept
160   { 160   {
HITCBC 161   676 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   673 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   676 } 162   673 }
163   163  
164   void 164   void
HITCBC 165   676 on_work_finished() noexcept 165   673 on_work_finished() noexcept
166   { 166   {
HITCBC 167   676 if(outstanding_work_.fetch_sub( 167   673 if(outstanding_work_.fetch_sub(
HITCBC 168   676 1, std::memory_order_acq_rel) == 1) 168   673 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   313 std::lock_guard<std::mutex> lock(mutex_); 173   311 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   313 if(outstanding_work_.load( 174   311 if(outstanding_work_.load(
HITCBC 175   313 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   311 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   195 stop_ = true; 177   159 stop_ = true;
HITCBC 178   195 done_cv_.notify_all(); 178   159 done_cv_.notify_all();
HITCBC 179   195 work_cv_.notify_all(); 179   159 work_cv_.notify_all();
180   } 180   }
HITCBC 181   313 } 181   311 }
HITCBC 182   676 } 182   673 }
183   183  
184   void 184   void
HITCBC 185   644 join() noexcept 185   638 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   644 std::unique_lock<std::mutex> lock(mutex_); 188   638 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   644 if(joined_) 189   638 if(joined_)
HITCBC 190   255 return; 190   252 return;
HITCBC 191   389 joined_ = true; 191   386 joined_ = true;
192   192  
HITCBC 193   389 if(outstanding_work_.load( 193   386 if(outstanding_work_.load(
HITCBC 194   389 std::memory_order_acquire) == 0) 194   386 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   140 stop_ = true; 196   177 stop_ = true;
HITCBC 197   140 work_cv_.notify_all(); 197   177 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   249 done_cv_.wait(lock, [this]{ 201   209 done_cv_.wait(lock, [this]{
HITCBC 202   445 return stop_; 202   369 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   644 } 205   638 }
206   206  
HITCBC 207   832 for(auto& t : threads_) 207   826 for(auto& t : threads_)
HITCBC 208   443 if(t.joinable()) 208   440 if(t.joinable())
HITCBC 209   443 t.join(); 209   440 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   391 stop() noexcept 213   388 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   391 std::lock_guard<std::mutex> lock(mutex_); 216   388 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   391 stop_ = true; 217   388 stop_ = true;
HITCBC 218   391 } 218   388 }
HITCBC 219   391 work_cv_.notify_all(); 219   388 work_cv_.notify_all();
HITCBC 220   391 done_cv_.notify_all(); 220   388 done_cv_.notify_all();
HITCBC 221   391 } 221   388 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19090 ensure_started() 225   18731 ensure_started()
226   { 226   {
HITCBC 227   19090 std::call_once(start_flag_, [this]{ 227   18731 std::call_once(start_flag_, [this]{
HITCBC 228   339 threads_.reserve(num_threads_); 228   336 threads_.reserve(num_threads_);
HITCBC 229   782 for(std::size_t i = 0; i < num_threads_; ++i) 229   776 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   886 threads_.emplace_back([this, i]{ run(i); }); 230   880 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   339 }); 231   336 });
HITCBC 232   19090 } 232   18731 }
233   233  
234   void 234   void
HITCBC 235   443 run(std::size_t index) 235   440 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   443 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   440 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   443 set_current_thread_name(name); 240   440 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   443 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   440 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   443 ~scoped_pool() noexcept { current_.set(nullptr); } 247   440 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   443 } guard(this); 248   440 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   19354 continuation* c = nullptr; 252   19011 continuation* c = nullptr;
253   { 253   {
HITCBC 254   19354 std::unique_lock<std::mutex> lock(mutex_); 254   19011 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   19354 work_cv_.wait(lock, [this]{ 255   19011 work_cv_.wait(lock, [this]{
HITCBC 256   54284 return !empty() || 256   52020 return !empty() ||
HITCBC 257   54284 stop_; 257   52020 stop_;
258   }); 258   });
HITCBC 259   19354 if(stop_) 259   19011 if(stop_)
HITCBC 260   886 return; 260   880 return;
HITCBC 261   18911 c = pop(); 261   18571 c = pop();
HITCBC 262   19354 } 262   19011 }
HITCBC 263   18911 if(c) 263   18571 if(c)
HITCBC 264   18911 safe_resume(c->h); 264   18571 safe_resume(c->h);
HITCBC 265   18911 } 265   18571 }
HITCBC 266   443 } 266   440 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   389 thread_pool:: 271   386 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   389 impl_->stop(); 274   386 impl_->stop();
HITCBC 275   389 impl_->join(); 275   386 impl_->join();
HITCBC 276   389 impl_->drain_abandoned(); 276   386 impl_->drain_abandoned();
HITCBC 277   389 shutdown(); 277   386 shutdown();
HITCBC 278   389 destroy(); 278   386 destroy();
HITCBC 279   389 delete impl_; 279   386 delete impl_;
HITCBC 280   389 } 280   386 }
281   281  
HITCBC 282   389 thread_pool:: 282   386 thread_pool::
HITCBC 283   389 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   386 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   389 : impl_(new impl(num_threads, thread_name_prefix)) 284   386 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   389 this->set_frame_allocator(std::allocator<void>{}); 286   386 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   389 } 287   386 }
288   288  
289   void 289   void
HITCBC 290   255 thread_pool:: 290   252 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   255 impl_->join(); 293   252 impl_->join();
HITCBC 294   255 } 294   252 }
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   11801 thread_pool:: 306   11798 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11801 return executor_type( 309   11798 return executor_type(
HITCBC 310   11801 const_cast<thread_pool&>(*this)); 310   11798 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   676 thread_pool::executor_type:: 314   673 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   676 pool_->impl_->on_work_started(); 317   673 pool_->impl_->on_work_started();
HITCBC 318   676 } 318   673 }
319   319  
320   void 320   void
HITCBC 321   676 thread_pool::executor_type:: 321   673 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   676 pool_->impl_->on_work_finished(); 324   673 pool_->impl_->on_work_finished();
HITCBC 325   676 } 325   673 }
326   326  
327   void 327   void
HITCBC 328   18420 thread_pool::executor_type:: 328   18064 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   18420 pool_->impl_->post(c); 331   18064 pool_->impl_->post(c);
HITCBC 332   18420 } 332   18064 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   677 thread_pool::executor_type:: 335   674 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   677 if(pool_->impl_->running_in_this_thread()) 338   674 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   670 pool_->impl_->post(c); 340   667 pool_->impl_->post(c);
HITCBC 341   670 return std::noop_coroutine(); 341   667 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost