. /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. A thread
  65 + owned by one of this pool's services may post or dispatch
  66 + until that service's `shutdown()` returns.
65   */ 67   */
66   ~thread_pool(); 68   ~thread_pool();
67   69  
68   /** Construct a thread pool. 70   /** Construct a thread pool.
69   71  
70   Records the requested worker count; no threads are created 72   Records the requested worker count; no threads are created
71   yet. Threads start lazily on the executor's first `post()`. 73   yet. Threads start lazily on the executor's first `post()`.
72   If `num_threads` is zero, the number of threads is set to 74   If `num_threads` is zero, the number of threads is set to
73   the hardware concurrency, or one if that cannot be determined. 75   the hardware concurrency, or one if that cannot be determined.
74   76  
75   @param num_threads The number of worker threads, or zero 77   @param num_threads The number of worker threads, or zero
76   for automatic selection. 78   for automatic selection.
77   79  
78   @param thread_name_prefix The prefix for worker thread names. 80   @param thread_name_prefix The prefix for worker thread names.
79   Thread names appear as "{prefix}0", "{prefix}1", etc. 81   Thread names appear as "{prefix}0", "{prefix}1", etc.
80   The prefix is truncated to 12 characters. Defaults to 82   The prefix is truncated to 12 characters. Defaults to
81   "capy-pool-". 83   "capy-pool-".
82   */ 84   */
83   explicit 85   explicit
84   thread_pool( 86   thread_pool(
85   std::size_t num_threads = 0, 87   std::size_t num_threads = 0,
86   std::string_view thread_name_prefix = "capy-pool-"); 88   std::string_view thread_name_prefix = "capy-pool-");
87   89  
88   /** Copy construction is disabled; a pool owns its worker threads. 90   /** Copy construction is disabled; a pool owns its worker threads.
89   91  
90   @param other The pool that would be copied. 92   @param other The pool that would be copied.
91   */ 93   */
92   thread_pool(thread_pool const& other) = delete; 94   thread_pool(thread_pool const& other) = delete;
93   95  
94   /** Copy assignment is disabled; a pool owns its worker threads. 96   /** Copy assignment is disabled; a pool owns its worker threads.
95   97  
96   @param other The pool that would be assigned from. 98   @param other The pool that would be assigned from.
97   99  
98   @return A reference to `*this`. 100   @return A reference to `*this`.
99   */ 101   */
100   thread_pool& operator=(thread_pool const& other) = delete; 102   thread_pool& operator=(thread_pool const& other) = delete;
101   103  
102   /** Wait for all outstanding work to complete. 104   /** Wait for all outstanding work to complete.
103   105  
104   Releases the internal work guard, then blocks the calling 106   Releases the internal work guard, then blocks the calling
105   thread until all outstanding work tracked by 107   thread until all outstanding work tracked by
106   @ref executor_type::on_work_started and 108   @ref executor_type::on_work_started and
107   @ref executor_type::on_work_finished completes. After all 109   @ref executor_type::on_work_finished completes. After all
108   work finishes, joins the worker threads. 110   work finishes, joins the worker threads.
109   111  
110   If @ref stop is called while `join()` is blocking, the 112   If @ref stop is called while `join()` is blocking, the
111   pool stops without waiting for remaining work to 113   pool stops without waiting for remaining work to
112   complete. Worker threads finish their current item and 114   complete. Worker threads finish their current item and
113   exit; `join()` still waits for all threads to be joined 115   exit; `join()` still waits for all threads to be joined
114   before returning. 116   before returning.
115   117  
116   This function is idempotent. The first call performs the 118   This function is idempotent. The first call performs the
117   join; subsequent calls return immediately. 119   join; subsequent calls return immediately.
118   120  
119   @pre Must not be called from a thread in this pool (undefined 121   @pre Must not be called from a thread in this pool (undefined
120   behavior). 122   behavior).
121   123  
122   @par Postconditions 124   @par Postconditions
123   All worker threads have been joined. The pool cannot be 125   All worker threads have been joined. The pool cannot be
124   reused. 126   reused.
125   127  
126   @par Thread Safety 128   @par Thread Safety
127   May be called from any thread not in this pool. 129   May be called from any thread not in this pool.
128   */ 130   */
129   void 131   void
130   join() noexcept; 132   join() noexcept;
131   133  
132   /** Request all worker threads to stop. 134   /** Request all worker threads to stop.
133   135  
134   Signals all threads to exit after finishing their current 136   Signals all threads to exit after finishing their current
135   work item. Queued work that has not started stays queued 137   work item. Queued work that has not started stays queued
136   until the pool is destroyed, which abandons it. Does not 138   until the pool is destroyed, which abandons it. Does not
137   wait for threads to exit. 139   wait for threads to exit.
138   140  
139   If @ref join is blocking on another thread, calling 141   If @ref join is blocking on another thread, calling
140   `stop()` causes it to stop waiting for outstanding 142   `stop()` causes it to stop waiting for outstanding
141   work. The `join()` call still waits for worker threads 143   work. The `join()` call still waits for worker threads
142   to finish their current item and exit before returning. 144   to finish their current item and exit before returning.
143   145  
144   @par Thread Safety 146   @par Thread Safety
145   May be called concurrently from any thread, including a 147   May be called concurrently from any thread, including a
146   thread in this pool. 148   thread in this pool.
147   */ 149   */
148   void 150   void
149   stop() noexcept; 151   stop() noexcept;
150   152  
151   /** Return an executor for this thread pool. 153   /** Return an executor for this thread pool.
152   154  
153   @return An executor associated with this thread pool. 155   @return An executor associated with this thread pool.
154   */ 156   */
155   executor_type 157   executor_type
156   get_executor() const noexcept; 158   get_executor() const noexcept;
157   }; 159   };
158   160  
159   /** An executor that submits work to a thread_pool. 161   /** An executor that submits work to a thread_pool.
160   162  
161   Executors are lightweight handles that can be copied and stored. 163   Executors are lightweight handles that can be copied and stored.
162   All copies refer to the same underlying thread pool. 164   All copies refer to the same underlying thread pool.
163   165  
164   @par Thread Safety 166   @par Thread Safety
165   Distinct objects: Safe. 167   Distinct objects: Safe.
166   Shared objects: Safe. 168   Shared objects: Safe.
167   */ 169   */
168   class thread_pool::executor_type 170   class thread_pool::executor_type
169   { 171   {
170   friend class thread_pool; 172   friend class thread_pool;
171   173  
172   thread_pool* pool_ = nullptr; 174   thread_pool* pool_ = nullptr;
173   175  
174   explicit 176   explicit
HITCBC 175   15338 executor_type(thread_pool& pool) noexcept 177   14857 executor_type(thread_pool& pool) noexcept
HITCBC 176   15338 : pool_(&pool) 178   14857 : pool_(&pool)
177   { 179   {
HITCBC 178   15338 } 180   14857 }
179   181  
180   public: 182   public:
181   /** Construct a default null executor. 183   /** Construct a default null executor.
182   184  
183   The resulting executor is not associated with any pool. 185   The resulting executor is not associated with any pool.
184   `context()`, `dispatch()`, and `post()` require the 186   `context()`, `dispatch()`, and `post()` require the
185   executor to be associated with a pool before use. 187   executor to be associated with a pool before use.
186   */ 188   */
187   executor_type() = default; 189   executor_type() = default;
188   190  
189   /** Return the underlying thread pool. 191   /** Return the underlying thread pool.
190   192  
191   @return A reference to the associated pool. The behavior is 193   @return A reference to the associated pool. The behavior is
192   undefined if the executor is not associated with a pool. 194   undefined if the executor is not associated with a pool.
193   */ 195   */
194   thread_pool& 196   thread_pool&
HITCBC 195   12628 context() const noexcept 197   12147 context() const noexcept
196   { 198   {
HITCBC 197   12628 return *pool_; 199   12147 return *pool_;
198   } 200   }
199   201  
200   /** Notify that work has started. 202   /** Notify that work has started.
201   203  
202   Increments the outstanding work count. Must be paired 204   Increments the outstanding work count. Must be paired
203   with a subsequent call to @ref on_work_finished. 205   with a subsequent call to @ref on_work_finished.
204   206  
205   @see on_work_finished, work_guard 207   @see on_work_finished, work_guard
206   */ 208   */
207   BOOST_CAPY_DECL 209   BOOST_CAPY_DECL
208   void 210   void
209   on_work_started() const noexcept; 211   on_work_started() const noexcept;
210   212  
211   /** Notify that work has finished. 213   /** Notify that work has finished.
212   214  
213   Decrements the outstanding work count. When the count 215   Decrements the outstanding work count. When the count
214   reaches zero after @ref thread_pool::join is called, 216   reaches zero after @ref thread_pool::join is called,
215   the pool's worker threads are signaled to stop. 217   the pool's worker threads are signaled to stop.
216   218  
217   @pre A preceding call to @ref on_work_started was made. 219   @pre A preceding call to @ref on_work_started was made.
218   220  
219   @see on_work_started, work_guard 221   @see on_work_started, work_guard
220   */ 222   */
221   BOOST_CAPY_DECL 223   BOOST_CAPY_DECL
222   void 224   void
223   on_work_finished() const noexcept; 225   on_work_finished() const noexcept;
224   226  
225   /** Dispatch a continuation for execution. 227   /** Dispatch a continuation for execution.
226   228  
227   If the calling thread is a worker of this pool, returns 229   If the calling thread is a worker of this pool, returns
228   `c.h` for symmetric transfer so the caller can resume the 230   `c.h` for symmetric transfer so the caller can resume the
229   continuation inline. Otherwise, posts the continuation to 231   continuation inline. Otherwise, posts the continuation to
230   the pool for execution on a worker thread and returns 232   the pool for execution on a worker thread and returns
231   `std::noop_coroutine()`. 233   `std::noop_coroutine()`.
232   234  
233   @param c The continuation to execute. On the post path, 235   @param c The continuation to execute. On the post path,
234   must remain at a stable address until dequeued 236   must remain at a stable address until dequeued
235   and resumed. 237   and resumed.
236   238  
237   @return `c.h` when the calling thread is a pool worker; 239   @return `c.h` when the calling thread is a pool worker;
238   `std::noop_coroutine()` otherwise. 240   `std::noop_coroutine()` otherwise.
239   */ 241   */
240   BOOST_CAPY_DECL 242   BOOST_CAPY_DECL
241   std::coroutine_handle<> 243   std::coroutine_handle<>
242   dispatch(continuation& c) const; 244   dispatch(continuation& c) const;
243   245  
244   /** Post a continuation to the thread pool. 246   /** Post a continuation to the thread pool.
245   247  
246   The continuation is resumed on one of the pool's 248   The continuation is resumed on one of the pool's
247   worker threads. The continuation must remain at a stable 249   worker threads. The continuation must remain at a stable
248   address until it is dequeued and resumed. 250   address until it is dequeued and resumed.
249   251  
250   @param c The continuation to execute. 252   @param c The continuation to execute.
251   */ 253   */
252   BOOST_CAPY_DECL 254   BOOST_CAPY_DECL
253   void 255   void
254   post(continuation& c) const; 256   post(continuation& c) const;
255   257  
256   /** Return true if two executors refer to the same thread pool. 258   /** Return true if two executors refer to the same thread pool.
257   259  
258   @param other The executor to compare against. 260   @param other The executor to compare against.
259   261  
260   @return `true` if both executors refer to the same pool. 262   @return `true` if both executors refer to the same pool.
261   */ 263   */
262   bool 264   bool
HITCBC 263   15 operator==(executor_type const& other) const noexcept 265   15 operator==(executor_type const& other) const noexcept
264   { 266   {
HITCBC 265   15 return pool_ == other.pool_; 267   15 return pool_ == other.pool_;
266   } 268   }
267   }; 269   };
268   270  
269   } // capy 271   } // capy
270   } // boost 272   } // boost
271   273  
272   #endif 274   #endif