. /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   19513 void push(continuation* c) noexcept 71   20511 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19513 c->reserved = nullptr; 73   20511 c->reserved = nullptr;
HITCBC 74   19513 if(tail_) 74   20511 if(tail_)
HITCBC 75   1480 tail_->reserved = c; 75   6144 tail_->reserved = c;
76   else 76   else
HITCBC 77   18033 head_ = c; 77   14367 head_ = c;
HITCBC 78   19513 tail_ = c; 78   20511 tail_ = c;
HITCBC 79   19513 } 79   20511 }
80   80  
HITCBC 81   19905 continuation* pop() noexcept 81   20900 continuation* pop() noexcept
82   { 82   {
HITCBC 83   19905 if(!head_) 83   20900 if(!head_)
HITCBC 84   392 return nullptr; 84   389 return nullptr;
HITCBC 85   19513 continuation* c = head_; 85   20511 continuation* c = head_;
HITCBC 86   19513 head_ = static_cast<continuation*>(head_->reserved); 86   20511 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19513 if(!head_) 87   20511 if(!head_)
HITCBC 88   18033 tail_ = nullptr; 88   14367 tail_ = nullptr;
HITCBC 89   19513 return c; 89   20511 return c;
90   } 90   }
91   91  
HITCBC 92   38341 bool empty() const noexcept 92   35237 bool empty() const noexcept
93   { 93   {
HITCBC 94   38341 return head_ == nullptr; 94   35237 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   392 ~impl() = default; 109   389 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   680 running_in_this_thread() const noexcept 112   677 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   680 return current_.get() == this; 114   677 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   392 drain_abandoned() noexcept 123   389 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   576 while(auto* c = pop()) 125   570 while(auto* c = pop())
126   { 126   {
HITCBC 127   184 auto h = c->h; 127   181 auto h = c->h;
HITCBC 128   184 if(h && h != std::noop_coroutine()) 128   181 if(h && h != std::noop_coroutine())
HITCBC 129   133 h.destroy(); 129   130 h.destroy();
HITCBC 130   184 } 130   181 }
HITCBC 131   392 } 131   389 }
132   132  
HITCBC 133   392 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   389 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   392 : num_threads_(num_threads) 134   389 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   392 if(num_threads_ == 0) 136   389 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   392 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   389 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   392 thread_name_prefix_[n] = '\0'; 142   389 thread_name_prefix_[n] = '\0';
HITCBC 143   392 } 143   389 }
144   144  
145   void 145   void
HITCBC 146   19513 post(continuation& c) 146   20511 post(continuation& c)
147   { 147   {
HITCBC 148   19513 ensure_started(); 148   20511 ensure_started();
149   { 149   {
HITCBC 150   19513 std::lock_guard<std::mutex> lock(mutex_); 150   20511 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19513 push(&c); 151   20511 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   19513 work_cv_.notify_one(); 154   20511 work_cv_.notify_one();
HITCBC 155   19513 } 155   20511 }
HITCBC 156   19513 } 156   20511 }
157   157  
158   void 158   void
HITCBC 159   679 on_work_started() noexcept 159   676 on_work_started() noexcept
160   { 160   {
HITCBC 161   679 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   676 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   679 } 162   676 }
163   163  
164   void 164   void
HITCBC 165   679 on_work_finished() noexcept 165   676 on_work_finished() noexcept
166   { 166   {
HITCBC 167   679 if(outstanding_work_.fetch_sub( 167   676 if(outstanding_work_.fetch_sub(
HITCBC 168   679 1, std::memory_order_acq_rel) == 1) 168   676 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   316 std::lock_guard<std::mutex> lock(mutex_); 173   313 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   316 if(outstanding_work_.load( 174   313 if(outstanding_work_.load(
HITCBC 175   316 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   313 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   178 stop_ = true; 177   189 stop_ = true;
HITCBC 178   178 done_cv_.notify_all(); 178   189 done_cv_.notify_all();
HITCBC 179   178 work_cv_.notify_all(); 179   189 work_cv_.notify_all();
180   } 180   }
HITCBC 181   316 } 181   313 }
HITCBC 182   679 } 182   676 }
183   183  
184   void 184   void
HITCBC 185   650 join() noexcept 185   644 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   650 std::unique_lock<std::mutex> lock(mutex_); 188   644 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   650 if(joined_) 189   644 if(joined_)
HITCBC 190   258 return; 190   255 return;
HITCBC 191   392 joined_ = true; 191   389 joined_ = true;
192   192  
HITCBC 193   392 if(outstanding_work_.load( 193   389 if(outstanding_work_.load(
HITCBC 194   392 std::memory_order_acquire) == 0) 194   389 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   159 stop_ = true; 196   146 stop_ = true;
HITCBC 197   159 work_cv_.notify_all(); 197   146 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   233 done_cv_.wait(lock, [this]{ 201   243 done_cv_.wait(lock, [this]{
HITCBC 202   412 return stop_; 202   433 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   650 } 205   644 }
206   206  
HITCBC 207   838 for(auto& t : threads_) 207   832 for(auto& t : threads_)
HITCBC 208   446 if(t.joinable()) 208   443 if(t.joinable())
HITCBC 209   446 t.join(); 209   443 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   394 stop() noexcept 213   391 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   394 std::lock_guard<std::mutex> lock(mutex_); 216   391 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   394 stop_ = true; 217   391 stop_ = true;
HITCBC 218   394 } 218   391 }
HITCBC 219   394 work_cv_.notify_all(); 219   391 work_cv_.notify_all();
HITCBC 220   394 done_cv_.notify_all(); 220   391 done_cv_.notify_all();
HITCBC 221   394 } 221   391 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19513 ensure_started() 225   20511 ensure_started()
226   { 226   {
HITCBC 227   19513 std::call_once(start_flag_, [this]{ 227   20511 std::call_once(start_flag_, [this]{
HITCBC 228   342 threads_.reserve(num_threads_); 228   339 threads_.reserve(num_threads_);
HITCBC 229   788 for(std::size_t i = 0; i < num_threads_; ++i) 229   782 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   892 threads_.emplace_back([this, i]{ run(i); }); 230   886 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   342 }); 231   339 });
HITCBC 232   19513 } 232   20511 }
233   233  
234   void 234   void
HITCBC 235   446 run(std::size_t index) 235   443 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   446 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   443 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   446 set_current_thread_name(name); 240   443 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   446 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   443 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   446 ~scoped_pool() noexcept { current_.set(nullptr); } 247   443 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   446 } guard(this); 248   443 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   19775 continuation* c = nullptr; 252   20773 continuation* c = nullptr;
253   { 253   {
HITCBC 254   19775 std::unique_lock<std::mutex> lock(mutex_); 254   20773 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   19775 work_cv_.wait(lock, [this]{ 255   20773 work_cv_.wait(lock, [this]{
HITCBC 256   57256 return !empty() || 256   50051 return !empty() ||
HITCBC 257   57256 stop_; 257   50051 stop_;
258   }); 258   });
HITCBC 259   19775 if(stop_) 259   20773 if(stop_)
HITCBC 260   892 return; 260   886 return;
HITCBC 261   19329 c = pop(); 261   20330 c = pop();
HITCBC 262   19775 } 262   20773 }
HITCBC 263   19329 if(c) 263   20330 if(c)
HITCBC 264   19329 safe_resume(c->h); 264   20330 safe_resume(c->h);
HITCBC 265   19329 } 265   20330 }
HITCBC 266   446 } 266   443 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   392 thread_pool:: 271   389 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   392 impl_->stop(); 274   389 impl_->stop();
HITCBC 275   392 impl_->join(); 275   389 impl_->join();
HITCBC 276   392 impl_->drain_abandoned(); 276   389 impl_->drain_abandoned();
HITCBC 277   392 shutdown(); 277   389 shutdown();
HITCBC 278   392 destroy(); 278   389 destroy();
HITCBC 279   392 delete impl_; 279   389 delete impl_;
HITCBC 280   392 } 280   389 }
281   281  
HITCBC 282   392 thread_pool:: 282   389 thread_pool::
HITCBC 283   392 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   389 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   392 : impl_(new impl(num_threads, thread_name_prefix)) 284   389 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   392 this->set_frame_allocator(std::allocator<void>{}); 286   389 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   392 } 287   389 }
288   288  
289   void 289   void
HITCBC 290   258 thread_pool:: 290   255 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   258 impl_->join(); 293   255 impl_->join();
HITCBC 294   258 } 294   255 }
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   11804 thread_pool:: 306   11801 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11804 return executor_type( 309   11801 return executor_type(
HITCBC 310   11804 const_cast<thread_pool&>(*this)); 310   11801 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   679 thread_pool::executor_type:: 314   676 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   679 pool_->impl_->on_work_started(); 317   676 pool_->impl_->on_work_started();
HITCBC 318   679 } 318   676 }
319   319  
320   void 320   void
HITCBC 321   679 thread_pool::executor_type:: 321   676 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   679 pool_->impl_->on_work_finished(); 324   676 pool_->impl_->on_work_finished();
HITCBC 325   679 } 325   676 }
326   326  
327   void 327   void
HITCBC 328   18840 thread_pool::executor_type:: 328   19841 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   18840 pool_->impl_->post(c); 331   19841 pool_->impl_->post(c);
HITCBC 332   18840 } 332   19841 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   680 thread_pool::executor_type:: 335   677 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   680 if(pool_->impl_->running_in_this_thread()) 338   677 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   673 pool_->impl_->post(c); 340   670 pool_->impl_->post(c);
HITCBC 341   673 return std::noop_coroutine(); 341   670 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost