. /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   19533 void push(continuation* c) noexcept 71   18489 void push(continuation* c) noexcept
72   { 72   {
HITCBC 73   19533 c->reserved = nullptr; 73   18489 c->reserved = nullptr;
HITCBC 74   19533 if(tail_) 74   18489 if(tail_)
HITCBC 75   1744 tail_->reserved = c; 75   2708 tail_->reserved = c;
76   else 76   else
HITCBC 77   17789 head_ = c; 77   15781 head_ = c;
HITCBC 78   19533 tail_ = c; 78   18489 tail_ = c;
HITCBC 79   19533 } 79   18489 }
80   80  
HITCBC 81   19933 continuation* pop() noexcept 81   18867 continuation* pop() noexcept
82   { 82   {
HITCBC 83   19933 if(!head_) 83   18867 if(!head_)
HITCBC 84   400 return nullptr; 84   378 return nullptr;
HITCBC 85   19533 continuation* c = head_; 85   18489 continuation* c = head_;
HITCBC 86   19533 head_ = static_cast<continuation*>(head_->reserved); 86   18489 head_ = static_cast<continuation*>(head_->reserved);
HITCBC 87   19533 if(!head_) 87   18489 if(!head_)
HITCBC 88   17789 tail_ = nullptr; 88   15781 tail_ = nullptr;
HITCBC 89   19533 return c; 89   18489 return c;
90   } 90   }
91   91  
HITCBC 92   37847 bool empty() const noexcept 92   34915 bool empty() const noexcept
93   { 93   {
HITCBC 94   37847 return head_ == nullptr; 94   34915 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   400 ~impl() = default; 109   378 ~impl() = default;
110   110  
111   bool 111   bool
HITCBC 112   688 running_in_this_thread() const noexcept 112   666 running_in_this_thread() const noexcept
113   { 113   {
HITCBC 114   688 return current_.get() == this; 114   666 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   400 drain_abandoned() noexcept 123   378 drain_abandoned() noexcept
124   { 124   {
HITCBC 125   595 while(auto* c = pop()) 125   553 while(auto* c = pop())
126   { 126   {
HITCBC 127   195 auto h = c->h; 127   175 auto h = c->h;
HITCBC 128   195 if(h && h != std::noop_coroutine()) 128   175 if(h && h != std::noop_coroutine())
HITCBC 129   144 h.destroy(); 129   125 h.destroy();
HITCBC 130   195 } 130   175 }
HITCBC 131   400 } 131   378 }
132   132  
HITCBC 133   400 impl(std::size_t num_threads, std::string_view thread_name_prefix) 133   378 impl(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 134   400 : num_threads_(num_threads) 134   378 : num_threads_(num_threads)
135   { 135   {
HITCBC 136   400 if(num_threads_ == 0) 136   378 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   400 auto n = thread_name_prefix.copy(thread_name_prefix_, 12); 141   378 auto n = thread_name_prefix.copy(thread_name_prefix_, 12);
HITCBC 142   400 thread_name_prefix_[n] = '\0'; 142   378 thread_name_prefix_[n] = '\0';
HITCBC 143   400 } 143   378 }
144   144  
145   void 145   void
HITCBC 146   19533 post(continuation& c) 146   18489 post(continuation& c)
147   { 147   {
HITCBC 148   19533 ensure_started(); 148   18489 ensure_started();
149   { 149   {
HITCBC 150   19533 std::lock_guard<std::mutex> lock(mutex_); 150   18489 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 151   19533 push(&c); 151   18489 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   19533 work_cv_.notify_one(); 154   18489 work_cv_.notify_one();
HITCBC 155   19533 } 155   18489 }
HITCBC 156   19533 } 156   18489 }
157   157  
158   void 158   void
HITCBC 159   687 on_work_started() noexcept 159   665 on_work_started() noexcept
160   { 160   {
HITCBC 161   687 outstanding_work_.fetch_add(1, std::memory_order_acq_rel); 161   665 outstanding_work_.fetch_add(1, std::memory_order_acq_rel);
HITCBC 162   687 } 162   665 }
163   163  
164   void 164   void
HITCBC 165   687 on_work_finished() noexcept 165   665 on_work_finished() noexcept
166   { 166   {
HITCBC 167   687 if(outstanding_work_.fetch_sub( 167   665 if(outstanding_work_.fetch_sub(
HITCBC 168   687 1, std::memory_order_acq_rel) == 1) 168   665 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   324 std::lock_guard<std::mutex> lock(mutex_); 173   302 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 174   324 if(outstanding_work_.load( 174   302 if(outstanding_work_.load(
HITCBC 175   324 std::memory_order_acquire) == 0 && joined_ && !stop_) 175   302 std::memory_order_acquire) == 0 && joined_ && !stop_)
176   { 176   {
HITCBC 177   184 stop_ = true; 177   178 stop_ = true;
HITCBC 178   184 done_cv_.notify_all(); 178   178 done_cv_.notify_all();
HITCBC 179   184 work_cv_.notify_all(); 179   178 work_cv_.notify_all();
180   } 180   }
HITCBC 181   324 } 181   302 }
HITCBC 182   687 } 182   665 }
183   183  
184   void 184   void
HITCBC 185   666 join() noexcept 185   622 join() noexcept
186   { 186   {
187   { 187   {
HITCBC 188   666 std::unique_lock<std::mutex> lock(mutex_); 188   622 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 189   666 if(joined_) 189   622 if(joined_)
HITCBC 190   266 return; 190   244 return;
HITCBC 191   400 joined_ = true; 191   378 joined_ = true;
192   192  
HITCBC 193   400 if(outstanding_work_.load( 193   378 if(outstanding_work_.load(
HITCBC 194   400 std::memory_order_acquire) == 0) 194   378 std::memory_order_acquire) == 0)
195   { 195   {
HITCBC 196   166 stop_ = true; 196   150 stop_ = true;
HITCBC 197   166 work_cv_.notify_all(); 197   150 work_cv_.notify_all();
198   } 198   }
199   else 199   else
200   { 200   {
HITCBC 201   234 done_cv_.wait(lock, [this]{ 201   228 done_cv_.wait(lock, [this]{
HITCBC 202   419 return stop_; 202   407 return stop_;
203   }); 203   });
204   } 204   }
HITCBC 205   666 } 205   622 }
206   206  
HITCBC 207   854 for(auto& t : threads_) 207   810 for(auto& t : threads_)
HITCBC 208   454 if(t.joinable()) 208   432 if(t.joinable())
HITCBC 209   454 t.join(); 209   432 t.join();
210   } 210   }
211   211  
212   void 212   void
HITCBC 213   402 stop() noexcept 213   380 stop() noexcept
214   { 214   {
215   { 215   {
HITCBC 216   402 std::lock_guard<std::mutex> lock(mutex_); 216   380 std::lock_guard<std::mutex> lock(mutex_);
HITCBC 217   402 stop_ = true; 217   380 stop_ = true;
HITCBC 218   402 } 218   380 }
HITCBC 219   402 work_cv_.notify_all(); 219   380 work_cv_.notify_all();
HITCBC 220   402 done_cv_.notify_all(); 220   380 done_cv_.notify_all();
HITCBC 221   402 } 221   380 }
222   222  
223   private: 223   private:
224   void 224   void
HITCBC 225   19533 ensure_started() 225   18489 ensure_started()
226   { 226   {
HITCBC 227   19533 std::call_once(start_flag_, [this]{ 227   18489 std::call_once(start_flag_, [this]{
HITCBC 228   350 threads_.reserve(num_threads_); 228   328 threads_.reserve(num_threads_);
HITCBC 229   804 for(std::size_t i = 0; i < num_threads_; ++i) 229   760 for(std::size_t i = 0; i < num_threads_; ++i)
HITCBC 230   908 threads_.emplace_back([this, i]{ run(i); }); 230   864 threads_.emplace_back([this, i]{ run(i); });
HITCBC 231   350 }); 231   328 });
HITCBC 232   19533 } 232   18489 }
233   233  
234   void 234   void
HITCBC 235   454 run(std::size_t index) 235   432 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   454 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index); 239   432 std::snprintf(name, sizeof(name), "%s%zu", thread_name_prefix_, index);
HITCBC 240   454 set_current_thread_name(name); 240   432 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   454 scoped_pool(impl const* p) noexcept { current_.set(p); } 246   432 scoped_pool(impl const* p) noexcept { current_.set(p); }
HITCBC 247   454 ~scoped_pool() noexcept { current_.set(nullptr); } 247   432 ~scoped_pool() noexcept { current_.set(nullptr); }
HITCBC 248   454 } guard(this); 248   432 } guard(this);
249   249  
250   for(;;) 250   for(;;)
251   { 251   {
HITCBC 252   19792 continuation* c = nullptr; 252   18746 continuation* c = nullptr;
253   { 253   {
HITCBC 254   19792 std::unique_lock<std::mutex> lock(mutex_); 254   18746 std::unique_lock<std::mutex> lock(mutex_);
HITCBC 255   19792 work_cv_.wait(lock, [this]{ 255   18746 work_cv_.wait(lock, [this]{
HITCBC 256   56269 return !empty() || 256   51424 return !empty() ||
HITCBC 257   56269 stop_; 257   51424 stop_;
258   }); 258   });
HITCBC 259   19792 if(stop_) 259   18746 if(stop_)
HITCBC 260   908 return; 260   864 return;
HITCBC 261   19338 c = pop(); 261   18314 c = pop();
HITCBC 262   19792 } 262   18746 }
HITCBC 263   19338 if(c) 263   18314 if(c)
HITCBC 264   19338 safe_resume(c->h); 264   18314 safe_resume(c->h);
HITCBC 265   19338 } 265   18314 }
HITCBC 266   454 } 266   432 }
267   }; 267   };
268   268  
269   //------------------------------------------------------------------------------ 269   //------------------------------------------------------------------------------
270   270  
HITCBC 271   400 thread_pool:: 271   378 thread_pool::
272   ~thread_pool() 272   ~thread_pool()
273   { 273   {
HITCBC 274   400 impl_->stop(); 274   378 impl_->stop();
HITCBC 275   400 impl_->join(); 275   378 impl_->join();
HITCBC 276   400 impl_->drain_abandoned(); 276   378 impl_->drain_abandoned();
HITCBC 277   400 shutdown(); 277   378 shutdown();
HITCBC 278   400 destroy(); 278   378 destroy();
HITCBC 279   400 delete impl_; 279   378 delete impl_;
HITCBC 280   400 } 280   378 }
281   281  
HITCBC 282   400 thread_pool:: 282   378 thread_pool::
HITCBC 283   400 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix) 283   378 thread_pool(std::size_t num_threads, std::string_view thread_name_prefix)
HITCBC 284   400 : impl_(new impl(num_threads, thread_name_prefix)) 284   378 : impl_(new impl(num_threads, thread_name_prefix))
285   { 285   {
HITCBC 286   400 this->set_frame_allocator(std::allocator<void>{}); 286   378 this->set_frame_allocator(std::allocator<void>{});
HITCBC 287   400 } 287   378 }
288   288  
289   void 289   void
HITCBC 290   266 thread_pool:: 290   244 thread_pool::
291   join() noexcept 291   join() noexcept
292   { 292   {
HITCBC 293   266 impl_->join(); 293   244 impl_->join();
HITCBC 294   266 } 294   244 }
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   11812 thread_pool:: 306   11790 thread_pool::
307   get_executor() const noexcept 307   get_executor() const noexcept
308   { 308   {
HITCBC 309   11812 return executor_type( 309   11790 return executor_type(
HITCBC 310   11812 const_cast<thread_pool&>(*this)); 310   11790 const_cast<thread_pool&>(*this));
311   } 311   }
312   312  
313   void 313   void
HITCBC 314   687 thread_pool::executor_type:: 314   665 thread_pool::executor_type::
315   on_work_started() const noexcept 315   on_work_started() const noexcept
316   { 316   {
HITCBC 317   687 pool_->impl_->on_work_started(); 317   665 pool_->impl_->on_work_started();
HITCBC 318   687 } 318   665 }
319   319  
320   void 320   void
HITCBC 321   687 thread_pool::executor_type:: 321   665 thread_pool::executor_type::
322   on_work_finished() const noexcept 322   on_work_finished() const noexcept
323   { 323   {
HITCBC 324   687 pool_->impl_->on_work_finished(); 324   665 pool_->impl_->on_work_finished();
HITCBC 325   687 } 325   665 }
326   326  
327   void 327   void
HITCBC 328   18852 thread_pool::executor_type:: 328   17830 thread_pool::executor_type::
329   post(continuation& c) const 329   post(continuation& c) const
330   { 330   {
HITCBC 331   18852 pool_->impl_->post(c); 331   17830 pool_->impl_->post(c);
HITCBC 332   18852 } 332   17830 }
333   333  
334   std::coroutine_handle<> 334   std::coroutine_handle<>
HITCBC 335   688 thread_pool::executor_type:: 335   666 thread_pool::executor_type::
336   dispatch(continuation& c) const 336   dispatch(continuation& c) const
337   { 337   {
HITCBC 338   688 if(pool_->impl_->running_in_this_thread()) 338   666 if(pool_->impl_->running_in_this_thread())
HITCBC 339   7 return c.h; 339   7 return c.h;
HITCBC 340   681 pool_->impl_->post(c); 340   659 pool_->impl_->post(c);
HITCBC 341   681 return std::noop_coroutine(); 341   659 return std::noop_coroutine();
342   } 342   }
343   343  
344   } // capy 344   } // capy
345   } // boost 345   } // boost