. /include/boost/capy /ex /thread_pool.hpp
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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/cppalliance/capy 8   // Official repository: https://github.com/cppalliance/capy
9   // 9   //
10   10  
11   #ifndef BOOST_CAPY_EX_THREAD_POOL_HPP 11   #ifndef BOOST_CAPY_EX_THREAD_POOL_HPP
12   #define BOOST_CAPY_EX_THREAD_POOL_HPP 12   #define BOOST_CAPY_EX_THREAD_POOL_HPP
13   13  
14   #include <boost/capy/detail/config.hpp> 14   #include <boost/capy/detail/config.hpp>
15   #include <boost/capy/continuation.hpp> 15   #include <boost/capy/continuation.hpp>
16   #include <coroutine> 16   #include <coroutine>
17   #include <boost/capy/ex/execution_context.hpp> 17   #include <boost/capy/ex/execution_context.hpp>
18   #include <cstddef> 18   #include <cstddef>
19   #include <string_view> 19   #include <string_view>
20   20  
21   namespace boost { 21   namespace boost {
22   namespace capy { 22   namespace capy {
23   23  
24   /** Distributes posted work across a fixed group of worker threads via a shared queue. 24   /** Distributes posted work across a fixed group of worker threads via a shared queue.
25   25  
26   Use this when you need to run coroutines on multiple threads 26   Use this when you need to run coroutines on multiple threads
27   without the overhead of creating and destroying threads for 27   without the overhead of creating and destroying threads for
28   each task. Work items are distributed across the pool using 28   each task. Work items are distributed across the pool using
29   a shared queue. 29   a shared queue.
30   30  
31   @par Thread Safety 31   @par Thread Safety
32   Distinct objects: Safe. 32   Distinct objects: Safe.
33   Shared objects: Safe for @ref get_executor, @ref join, and 33   Shared objects: Safe for @ref get_executor, @ref join, and
34   @ref stop. Unsafe for construction and destruction. 34   @ref stop. Unsafe for construction and destruction.
35   35  
36   @par Example 36   @par Example
37   @par !example example 37   @par !example example
38   38  
39   39  
40   @note `join()` waits only for work that holds outstanding-work 40   @note `join()` waits only for work that holds outstanding-work
41   counting, which `run_async` (and `make_work_guard`) provide. A bare 41   counting, which `run_async` (and `make_work_guard`) provide. A bare
42   `executor_type::post()` does not register outstanding work, so 42   `executor_type::post()` does not register outstanding work, so
43   `join()` does not wait for it. 43   `join()` does not wait for it.
44   */ 44   */
45   class BOOST_CAPY_DECL 45   class BOOST_CAPY_DECL
46   thread_pool 46   thread_pool
47   : public execution_context 47   : public execution_context
48   { 48   {
49   class impl; 49   class impl;
50   impl* impl_; 50   impl* impl_;
51   51  
52   public: 52   public:
53   class executor_type; 53   class executor_type;
54   54  
55   /** Destroy the thread pool. 55   /** Destroy the thread pool.
56   56  
57   Signals all worker threads to stop, waits for them to 57   Signals all worker threads to stop, waits for them to
58   finish, and destroys any pending work items. 58   finish, and destroys any pending work items.
59   59  
60   @pre No thread outside this pool may post or dispatch work to it 60   @pre No thread outside this pool may post or dispatch work to it
61   (or to a strand built on it) concurrently with, or after, 61   (or to a strand built on it) concurrently with, or after,
62   destruction. Doing so is undefined behavior. Submit such work 62   destruction. Doing so is undefined behavior. Submit such work
63   through @ref run_async or @ref run and call @ref join before 63   through @ref run_async or @ref run and call @ref join before
64   the pool is destroyed, so it has completed first. 64   the pool is destroyed, so it has completed first.
65   */ 65   */
66   ~thread_pool(); 66   ~thread_pool();
67   67  
68   /** Construct a thread pool. 68   /** Construct a thread pool.
69   69  
70   Records the requested worker count; no threads are created 70   Records the requested worker count; no threads are created
71   yet. Threads start lazily on the executor's first `post()`. 71   yet. Threads start lazily on the executor's first `post()`.
72   If `num_threads` is zero, the number of threads is set to 72   If `num_threads` is zero, the number of threads is set to
73   the hardware concurrency, or one if that cannot be determined. 73   the hardware concurrency, or one if that cannot be determined.
74   74  
75   @param num_threads The number of worker threads, or zero 75   @param num_threads The number of worker threads, or zero
76   for automatic selection. 76   for automatic selection.
77   77  
78   @param thread_name_prefix The prefix for worker thread names. 78   @param thread_name_prefix The prefix for worker thread names.
79   Thread names appear as "{prefix}0", "{prefix}1", etc. 79   Thread names appear as "{prefix}0", "{prefix}1", etc.
80   The prefix is truncated to 12 characters. Defaults to 80   The prefix is truncated to 12 characters. Defaults to
81   "capy-pool-". 81   "capy-pool-".
82   */ 82   */
83   explicit 83   explicit
84   thread_pool( 84   thread_pool(
85   std::size_t num_threads = 0, 85   std::size_t num_threads = 0,
86   std::string_view thread_name_prefix = "capy-pool-"); 86   std::string_view thread_name_prefix = "capy-pool-");
87   87  
88   /** Copy construction is disabled; a pool owns its worker threads. 88   /** Copy construction is disabled; a pool owns its worker threads.
89   89  
90   @param other The pool that would be copied. 90   @param other The pool that would be copied.
91   */ 91   */
92   thread_pool(thread_pool const& other) = delete; 92   thread_pool(thread_pool const& other) = delete;
93   93  
94   /** Copy assignment is disabled; a pool owns its worker threads. 94   /** Copy assignment is disabled; a pool owns its worker threads.
95   95  
96   @param other The pool that would be assigned from. 96   @param other The pool that would be assigned from.
97   97  
98   @return A reference to `*this`. 98   @return A reference to `*this`.
99   */ 99   */
100   thread_pool& operator=(thread_pool const& other) = delete; 100   thread_pool& operator=(thread_pool const& other) = delete;
101   101  
102   /** Wait for all outstanding work to complete. 102   /** Wait for all outstanding work to complete.
103   103  
104   Releases the internal work guard, then blocks the calling 104   Releases the internal work guard, then blocks the calling
105   thread until all outstanding work tracked by 105   thread until all outstanding work tracked by
106   @ref executor_type::on_work_started and 106   @ref executor_type::on_work_started and
107   @ref executor_type::on_work_finished completes. After all 107   @ref executor_type::on_work_finished completes. After all
108   work finishes, joins the worker threads. 108   work finishes, joins the worker threads.
109   109  
110   If @ref stop is called while `join()` is blocking, the 110   If @ref stop is called while `join()` is blocking, the
111   pool stops without waiting for remaining work to 111   pool stops without waiting for remaining work to
112   complete. Worker threads finish their current item and 112   complete. Worker threads finish their current item and
113   exit; `join()` still waits for all threads to be joined 113   exit; `join()` still waits for all threads to be joined
114   before returning. 114   before returning.
115   115  
116   This function is idempotent. The first call performs the 116   This function is idempotent. The first call performs the
117   join; subsequent calls return immediately. 117   join; subsequent calls return immediately.
118   118  
119   @pre Must not be called from a thread in this pool (undefined 119   @pre Must not be called from a thread in this pool (undefined
120   behavior). 120   behavior).
121   121  
122   @par Postconditions 122   @par Postconditions
123   All worker threads have been joined. The pool cannot be 123   All worker threads have been joined. The pool cannot be
124   reused. 124   reused.
125   125  
126   @par Thread Safety 126   @par Thread Safety
127   May be called from any thread not in this pool. 127   May be called from any thread not in this pool.
128   */ 128   */
129   void 129   void
130   join() noexcept; 130   join() noexcept;
131   131  
132   /** Request all worker threads to stop. 132   /** Request all worker threads to stop.
133   133  
134   Signals all threads to exit after finishing their current 134   Signals all threads to exit after finishing their current
135   work item. Queued work that has not started stays queued 135   work item. Queued work that has not started stays queued
136   until the pool is destroyed, which abandons it. Does not 136   until the pool is destroyed, which abandons it. Does not
137   wait for threads to exit. 137   wait for threads to exit.
138   138  
139   If @ref join is blocking on another thread, calling 139   If @ref join is blocking on another thread, calling
140   `stop()` causes it to stop waiting for outstanding 140   `stop()` causes it to stop waiting for outstanding
141   work. The `join()` call still waits for worker threads 141   work. The `join()` call still waits for worker threads
142   to finish their current item and exit before returning. 142   to finish their current item and exit before returning.
143   143  
144   @par Thread Safety 144   @par Thread Safety
145   May be called concurrently from any thread, including a 145   May be called concurrently from any thread, including a
146   thread in this pool. 146   thread in this pool.
147   */ 147   */
148   void 148   void
149   stop() noexcept; 149   stop() noexcept;
150   150  
151   /** Return an executor for this thread pool. 151   /** Return an executor for this thread pool.
152   152  
153   @return An executor associated with this thread pool. 153   @return An executor associated with this thread pool.
154   */ 154   */
155   executor_type 155   executor_type
156   get_executor() const noexcept; 156   get_executor() const noexcept;
157   }; 157   };
158   158  
159   /** An executor that submits work to a thread_pool. 159   /** An executor that submits work to a thread_pool.
160   160  
161   Executors are lightweight handles that can be copied and stored. 161   Executors are lightweight handles that can be copied and stored.
162   All copies refer to the same underlying thread pool. 162   All copies refer to the same underlying thread pool.
163   163  
164   @par Thread Safety 164   @par Thread Safety
165   Distinct objects: Safe. 165   Distinct objects: Safe.
166   Shared objects: Safe. 166   Shared objects: Safe.
167   */ 167   */
168   class thread_pool::executor_type 168   class thread_pool::executor_type
169   { 169   {
170   friend class thread_pool; 170   friend class thread_pool;
171   171  
172   thread_pool* pool_ = nullptr; 172   thread_pool* pool_ = nullptr;
173   173  
174   explicit 174   explicit
HITCBC 175   14842 executor_type(thread_pool& pool) noexcept 175   15346 executor_type(thread_pool& pool) noexcept
HITCBC 176   14842 : pool_(&pool) 176   15346 : pool_(&pool)
177   { 177   {
HITCBC 178   14842 } 178   15346 }
179   179  
180   public: 180   public:
181   /** Construct a default null executor. 181   /** Construct a default null executor.
182   182  
183   The resulting executor is not associated with any pool. 183   The resulting executor is not associated with any pool.
184   `context()`, `dispatch()`, and `post()` require the 184   `context()`, `dispatch()`, and `post()` require the
185   executor to be associated with a pool before use. 185   executor to be associated with a pool before use.
186   */ 186   */
187   executor_type() = default; 187   executor_type() = default;
188   188  
189   /** Return the underlying thread pool. 189   /** Return the underlying thread pool.
190   190  
191   @return A reference to the associated pool. The behavior is 191   @return A reference to the associated pool. The behavior is
192   undefined if the executor is not associated with a pool. 192   undefined if the executor is not associated with a pool.
193   */ 193   */
194   thread_pool& 194   thread_pool&
HITCBC 195   12132 context() const noexcept 195   12636 context() const noexcept
196   { 196   {
HITCBC 197   12132 return *pool_; 197   12636 return *pool_;
198   } 198   }
199   199  
200   /** Notify that work has started. 200   /** Notify that work has started.
201   201  
202   Increments the outstanding work count. Must be paired 202   Increments the outstanding work count. Must be paired
203   with a subsequent call to @ref on_work_finished. 203   with a subsequent call to @ref on_work_finished.
204   204  
205   @see on_work_finished, work_guard 205   @see on_work_finished, work_guard
206   */ 206   */
207   BOOST_CAPY_DECL 207   BOOST_CAPY_DECL
208   void 208   void
209   on_work_started() const noexcept; 209   on_work_started() const noexcept;
210   210  
211   /** Notify that work has finished. 211   /** Notify that work has finished.
212   212  
213   Decrements the outstanding work count. When the count 213   Decrements the outstanding work count. When the count
214   reaches zero after @ref thread_pool::join is called, 214   reaches zero after @ref thread_pool::join is called,
215   the pool's worker threads are signaled to stop. 215   the pool's worker threads are signaled to stop.
216   216  
217   @pre A preceding call to @ref on_work_started was made. 217   @pre A preceding call to @ref on_work_started was made.
218   218  
219   @see on_work_started, work_guard 219   @see on_work_started, work_guard
220   */ 220   */
221   BOOST_CAPY_DECL 221   BOOST_CAPY_DECL
222   void 222   void
223   on_work_finished() const noexcept; 223   on_work_finished() const noexcept;
224   224  
225   /** Dispatch a continuation for execution. 225   /** Dispatch a continuation for execution.
226   226  
227   If the calling thread is a worker of this pool, returns 227   If the calling thread is a worker of this pool, returns
228   `c.h` for symmetric transfer so the caller can resume the 228   `c.h` for symmetric transfer so the caller can resume the
229   continuation inline. Otherwise, posts the continuation to 229   continuation inline. Otherwise, posts the continuation to
230   the pool for execution on a worker thread and returns 230   the pool for execution on a worker thread and returns
231   `std::noop_coroutine()`. 231   `std::noop_coroutine()`.
232   232  
233   @param c The continuation to execute. On the post path, 233   @param c The continuation to execute. On the post path,
234   must remain at a stable address until dequeued 234   must remain at a stable address until dequeued
235   and resumed. 235   and resumed.
236   236  
237   @return `c.h` when the calling thread is a pool worker; 237   @return `c.h` when the calling thread is a pool worker;
238   `std::noop_coroutine()` otherwise. 238   `std::noop_coroutine()` otherwise.
239   */ 239   */
240   BOOST_CAPY_DECL 240   BOOST_CAPY_DECL
241   std::coroutine_handle<> 241   std::coroutine_handle<>
242   dispatch(continuation& c) const; 242   dispatch(continuation& c) const;
243   243  
244   /** Post a continuation to the thread pool. 244   /** Post a continuation to the thread pool.
245   245  
246   The continuation is resumed on one of the pool's 246   The continuation is resumed on one of the pool's
247   worker threads. The continuation must remain at a stable 247   worker threads. The continuation must remain at a stable
248   address until it is dequeued and resumed. 248   address until it is dequeued and resumed.
249   249  
250   @param c The continuation to execute. 250   @param c The continuation to execute.
251   */ 251   */
252   BOOST_CAPY_DECL 252   BOOST_CAPY_DECL
253   void 253   void
254   post(continuation& c) const; 254   post(continuation& c) const;
255   255  
256   /** Return true if two executors refer to the same thread pool. 256   /** Return true if two executors refer to the same thread pool.
257   257  
258   @param other The executor to compare against. 258   @param other The executor to compare against.
259   259  
260   @return `true` if both executors refer to the same pool. 260   @return `true` if both executors refer to the same pool.
261   */ 261   */
262   bool 262   bool
HITCBC 263   15 operator==(executor_type const& other) const noexcept 263   15 operator==(executor_type const& other) const noexcept
264   { 264   {
HITCBC 265   15 return pool_ == other.pool_; 265   15 return pool_ == other.pool_;
266   } 266   }
267   }; 267   };
268   268  
269   } // capy 269   } // capy
270   } // boost 270   } // boost
271   271  
272   #endif 272   #endif