. /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   19223 void push(continuation* c) noexcept 71   20272 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19223 c->reserved = nullptr; 73   20272 c->reserved = nullptr;
HITCBC 74   19223 if(tail_) 74   20272 if(tail_)
HITCBC 75   1734 tail_->reserved = c; 75   6108 tail_->reserved = c;
76   else 76   else
HITCBC 77   17489 head_ = c; 77   14164 head_ = c;
HITCBC 78   19223 tail_ = c; 78   20272 tail_ = c;
HITCBC 79   19223 } 79   20272 }
80   80  
HITCBC 81   19604 continuation* pop() noexcept 81   20631 continuation* pop() noexcept
82   { 82   {
HITCBC 83   19604 if(!head_) 83   20631 if(!head_)
HITCBC 84   381 return nullptr; 84   359 return nullptr;
HITCBC 85   19223 continuation* c = head_; 85   20272 continuation* c = head_;
HITCBC 86   19223 head_ = static_cast<continuation*>(head_->reserved); 86   20272 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19223 if(!head_) 87   20272 if(!head_)
HITCBC 88   17489 tail_ = nullptr; 88   14164 tail_ = nullptr;
HITCBC 89   19223 return c; 89   20272 return c;
90   } 90   }
91   91  
HITCBC 92   37383 bool empty() const noexcept 92   34728 bool empty() const noexcept
93   { 93   {
HITCBC 94   37383 return head_ == nullptr; 94   34728 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   381 ~impl() = default; 109   359 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   669 running_in_this_thread() const noexcept 112   647 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   669 return current_.get() == this; 114   647 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   381 drain_abandoned() noexcept 123   359 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   572 while(auto* c = pop()) 125   702 while(auto* c = pop())
126   { 126   {
HITCBC 127   191 auto h = c->h; 127   343 auto h = c->h;
HITCBC 128   191 if(h && h != std::noop_coroutine()) 128   343 if(h && h != std::noop_coroutine())
HITCBC 129   140 h.destroy(); 129   212 h.destroy();
HITCBC 130   191 } 130   343 }
HITCBC 131   381 } 131   359 }
132   132  
HITCBC 133   381 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   359 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   381 : num_threads_(num_threads) 134   359 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   381 if(num_threads_ == 0) 136   359 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   381 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   359 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   381 thread_name_prefix_[n] = '\0'; 142   359 thread_name_prefix_[n] = '\0';
HITCBC 143   381 } 143   359 }
144   144  
145   void 145   void
HITCBC 146   19223 post(continuation& c) 146   20272 post(continuation& c)
147   { 147   {
HITCBC 148   19223 ensure_started(); 148   20272 ensure_started();
149   { 149   {
HITCBC 150   19223 std::lock_guard<std::mutex> lock(mutex_); 150   20272 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19223 push(&c); 151   20272 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   19223 work_cv_.notify_one(); 154   20272 work_cv_.notify_one();
HITCBC 155   19223 } 155   20272 }
HITCBC 156   19223 } 156   20272 }
157   157  
158   void 158   void
HITCBC 159   668 on_work_started() noexcept 159   646 on_work_started() noexcept
160   { 160   {
HITCBC 161   668 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   646 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   668 } 162   646 }
163   163  
164   void 164   void
HITCBC 165   668 on_work_finished() noexcept 165   646 on_work_finished() noexcept
166   { 166   {
HITCBC 167   668 if(outstanding_work_.fetch_sub( 167   646 if(outstanding_work_.fetch_sub(
HITCBC 168   668 1, std::memory_order_acq_rel) == 1) 168   646 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   305 std::lock_guard<std::mutex> lock(mutex_); 173   283 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   305 if(outstanding_work_.load( 174   283 if(outstanding_work_.load(
HITCBC 175   305 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   283 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   171 stop_ = true; 177   111 stop_ = true;
HITCBC 178   171 done_cv_.notify_all(); 178   111 done_cv_.notify_all();
HITCBC 179   171 work_cv_.notify_all(); 179   111 work_cv_.notify_all();
180   } 180   }
HITCBC 181   305 } 181   283 }
HITCBC 182   668 } 182   646 }
183   183  
184   void 184   void
HITCBC 185   628 join() noexcept 185   584 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   628 std::unique_lock<std::mutex> lock(mutex_); 188   584 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   628 if(joined_) 189   584 if(joined_)
HITCBC 190   247 return; 190   225 return;
HITCBC 191   381 joined_ = true; 191   359 joined_ = true;
192   192  
HITCBC 193   381 if(outstanding_work_.load( 193   359 if(outstanding_work_.load(
HITCBC 194   381 std::memory_order_acquire) == 0) 194   359 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   157 stop_ = true; 196   196 stop_ = true;
HITCBC 197   157 work_cv_.notify_all(); 197   196 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   224 done_cv_.wait(lock, [this]{ 201   163 done_cv_.wait(lock, [this]{
HITCBC 202   396 return stop_; 202   275 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   628 } 205   584 }
206   206  
HITCBC 207   816 for(auto& t : threads_) 207   772 for(auto& t : threads_)
HITCBC 208   435 if(t.joinable()) 208   413 if(t.joinable())
HITCBC 209   435 t.join(); 209   413 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   383 stop() noexcept 213   361 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   383 std::lock_guard<std::mutex> lock(mutex_); 216   361 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   383 stop_ = true; 217   361 stop_ = true;
HITCBC 218   383 } 218   361 }
HITCBC 219   383 work_cv_.notify_all(); 219   361 work_cv_.notify_all();
HITCBC 220   383 done_cv_.notify_all(); 220   361 done_cv_.notify_all();
HITCBC 221   383 } 221   361 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19223 ensure_started() 225   20272 ensure_started()
226   { 226   {
HITCBC 227   19223 std::call_once(start_flag_, [this]{ 227   20272 std::call_once(start_flag_, [this]{
HITCBC 228   331 threads_.reserve(num_threads_); 228   309 threads_.reserve(num_threads_);
HITCBC 229   766 for(std::size_t i = 0; i < num_threads_; ++i) 229   722 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   870 threads_.emplace_back([this, i]{ run(i); }); 230   826 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   331 }); 231   309 });
HITCBC 232   19223 } 232   20272 }
233   233  
234   void 234   void
HITCBC 235   435 run(std::size_t index) 235   413 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   435 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   413 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   435 set_current_thread_name(name); 240   413 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   435 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   413 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   435 ~scoped_pool() noexcept { current_.set(nullptr); } 247   413 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   435 } guard(this); 248   413 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   19467 continuation* c = nullptr; 252   20342 continuation* c = nullptr;
253   { 253   {
HITCBC 254   19467 std::unique_lock<std::mutex> lock(mutex_); 254   20342 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   19467 work_cv_.wait(lock, [this]{ 255   20342 work_cv_.wait(lock, [this]{
HITCBC 256   55641 return !empty() || 256   49425 return !empty() ||
HITCBC 257   55641 stop_; 257   49425 stop_;
258   }); 258   });
HITCBC 259   19467 if(stop_) 259   20342 if(stop_)
HITCBC 260   870 return; 260   826 return;
HITCBC 261   19032 c = pop(); 261   19929 c = pop();
HITCBC 262   19467 } 262   20342 }
HITCBC 263   19032 if(c) 263   19929 if(c)
HITCBC 264   19032 safe_resume(c->h); 264   19929 safe_resume(c->h);
HITCBC 265   19032 } 265   19929 }
HITCBC 266   435 } 266   413 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   381 thread_pool:: 271   359 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   381 impl_->stop(); 274   359 impl_->stop();
HITCBC 275   381 impl_->join(); 275   359 impl_->join();
HITCBC 276   381 impl_->drain_abandoned(); 276   359 impl_->drain_abandoned();
HITCBC 277   381 shutdown(); 277   359 shutdown();
HITCBC 278   381 destroy(); 278   359 destroy();
HITCBC 279   381 delete impl_; 279   359 delete impl_;
HITCBC 280   381 } 280   359 }
281   281  
HITCBC 282   381 thread_pool:: 282   359 thread_pool::
HITCBC 283   381 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   359 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   381 : impl_(new impl(num_threads, thread_name_prefix)) 284   359 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   381 this->set_frame_allocator(std::allocator<void>{}); 286   359 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   381 } 287   359 }
288   288  
289   void 289   void
HITCBC 290   247 thread_pool:: 290   225 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   247 impl_->join(); 293   225 impl_->join();
HITCBC 294   247 } 294   225 }
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   11793 thread_pool:: 306   11771 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11793 return executor_type( 309   11771 return executor_type(
HITCBC 310   11793 const_cast<thread_pool&>(*this)); 310   11771 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   668 thread_pool::executor_type:: 314   646 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   668 pool_->impl_->on_work_started(); 317   646 pool_->impl_->on_work_started();
HITCBC 318   668 } 318   646 }
319   319  
320   void 320   void
HITCBC 321   668 thread_pool::executor_type:: 321   646 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   668 pool_->impl_->on_work_finished(); 324   646 pool_->impl_->on_work_finished();
HITCBC 325   668 } 325   646 }
326   326  
327   void 327   void
HITCBC 328   18561 thread_pool::executor_type:: 328   19632 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   18561 pool_->impl_->post(c); 331   19632 pool_->impl_->post(c);
HITCBC 332   18561 } 332   19632 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   669 thread_pool::executor_type:: 335   647 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   669 if(pool_->impl_->running_in_this_thread()) 338   647 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   662 pool_->impl_->post(c); 340   640 pool_->impl_->post(c);
HITCBC 341   662 return std::noop_coroutine(); 341   640 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost