1 // -*- C++ -*-
2
3 // Copyright (C) 2007-2023 Free Software Foundation, Inc.
4 //
5 // This file is part of the GNU ISO C++ Library. This library is free
6 // software; you can redistribute it and/or modify it under the terms
7 // of the GNU General Public License as published by the Free Software
8 // Foundation; either version 3, or (at your option) any later
9 // version.
10
11 // This library is distributed in the hope that it will be useful, but
12 // WITHOUT ANY WARRANTY; without even the implied warranty of
13 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 // General Public License for more details.
15
16 // Under Section 7 of GPL version 3, you are granted additional
17 // permissions described in the GCC Runtime Library Exception, version
18 // 3.1, as published by the Free Software Foundation.
19
20 // You should have received a copy of the GNU General Public License and
21 // a copy of the GCC Runtime Library Exception along with this program;
22 // see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
23 // <http://www.gnu.org/licenses/>.
24
25 /** @file parallel/workstealing.h
26 * @brief Parallelization of embarrassingly parallel execution by
27 * means of work-stealing.
28 *
29 * Work stealing is described in
30 *
31 * R. D. Blumofe and C. E. Leiserson.
32 * Scheduling multithreaded computations by work stealing.
33 * Journal of the ACM, 46(5):720-748, 1999.
34 *
35 * This file is a GNU parallel extension to the Standard C++ Library.
36 */
37
38 // Written by Felix Putze.
39
40 #ifndef _GLIBCXX_PARALLEL_WORKSTEALING_H
41 #define _GLIBCXX_PARALLEL_WORKSTEALING_H 1
42
43 #include <parallel/parallel.h>
44 #include <parallel/random_number.h>
45 #include <parallel/compatibility.h>
46
47 namespace __gnu_parallel
48 {
49
50 #define _GLIBCXX_JOB_VOLATILE volatile
51
52 /** @brief One __job for a certain thread. */
53 template<typename _DifferenceTp>
54 struct _Job
55 {
56 typedef _DifferenceTp _DifferenceType;
57
58 /** @brief First element.
59 *
60 * Changed by owning and stealing thread. By stealing thread,
61 * always incremented. */
62 _GLIBCXX_JOB_VOLATILE _DifferenceType _M_first;
63
64 /** @brief Last element.
65 *
66 * Changed by owning thread only. */
67 _GLIBCXX_JOB_VOLATILE _DifferenceType _M_last;
68
69 /** @brief Number of elements, i.e. @c _M_last-_M_first+1.
70 *
71 * Changed by owning thread only. */
72 _GLIBCXX_JOB_VOLATILE _DifferenceType _M_load;
73 };
74
75 /** @brief Work stealing algorithm for random access iterators.
76 *
77 * Uses O(1) additional memory. Synchronization at job lists is
78 * done with atomic operations.
79 * @param __begin Begin iterator of element sequence.
80 * @param __end End iterator of element sequence.
81 * @param __op User-supplied functor (comparator, predicate, adding
82 * functor, ...).
83 * @param __f Functor to @a process an element with __op (depends on
84 * desired functionality, e. g. for std::for_each(), ...).
85 * @param __r Functor to @a add a single __result to the already
86 * processed elements (depends on functionality).
87 * @param __base Base value for reduction.
88 * @param __output Pointer to position where final result is written to
89 * @param __bound Maximum number of elements processed (e. g. for
90 * std::count_n()).
91 * @return User-supplied functor (that may contain a part of the result).
92 */
93 template<typename _RAIter,
94 typename _Op,
95 typename _Fu,
96 typename _Red,
97 typename _Result>
98 _Op
99 __for_each_template_random_access_workstealing(_RAIter __begin,
100 _RAIter __end, _Op __op,
101 _Fu& __f, _Red __r,
102 _Result __base,
103 _Result& __output,
104 typename std::iterator_traits<_RAIter>::difference_type __bound)
105 {
106 _GLIBCXX_CALL(__end - __begin)
107
108 typedef std::iterator_traits<_RAIter> _TraitsType;
109 typedef typename _TraitsType::difference_type _DifferenceType;
110
111 const _Settings& __s = _Settings::get();
112
113 _DifferenceType __chunk_size =
114 static_cast<_DifferenceType>(__s.workstealing_chunk_size);
115
116 // How many jobs?
117 _DifferenceType __length = (__bound < 0) ? (__end - __begin) : __bound;
118
119 // To avoid false sharing in a cache line.
120 const int __stride = (__s.cache_line_size * 10
121 / sizeof(_Job<_DifferenceType>) + 1);
122
123 // Total number of threads currently working.
124 _ThreadIndex __busy = 0;
125
126 _Job<_DifferenceType> *__job;
127
128 omp_lock_t __output_lock;
129 omp_init_lock(&__output_lock);
130
131 // Write base value to output.
132 __output = __base;
133
134 // No more threads than jobs, at least one thread.
135 _ThreadIndex __num_threads = __gnu_parallel::max<_ThreadIndex>
136 (1, __gnu_parallel::min<_DifferenceType>(__length,
137 __get_max_threads()));
138
139 # pragma omp parallel shared(__busy) num_threads(__num_threads)
140 {
141 # pragma omp single
142 {
143 __num_threads = omp_get_num_threads();
144
145 // Create job description array.
146 __job = new _Job<_DifferenceType>[__num_threads * __stride];
147 }
148
149 // Initialization phase.
150
151 // Flags for every thread if it is doing productive work.
152 bool __iam_working = false;
153
154 // Thread id.
155 _ThreadIndex __iam = omp_get_thread_num();
156
157 // This job.
158 _Job<_DifferenceType>& __my_job = __job[__iam * __stride];
159
160 // Random number (for work stealing).
161 _ThreadIndex __victim;
162
163 // Local value for reduction.
164 _Result __result = _Result();
165
166 // Number of elements to steal in one attempt.
167 _DifferenceType __steal;
168
169 // Every thread has its own random number generator
170 // (modulo __num_threads).
171 _RandomNumber __rand_gen(__iam, __num_threads);
172
173 // This thread is currently working.
174 # pragma omp atomic
175 ++__busy;
176
177 __iam_working = true;
178
179 // How many jobs per thread? last thread gets the rest.
180 __my_job._M_first = static_cast<_DifferenceType>
181 (__iam * (__length / __num_threads));
182
183 __my_job._M_last = (__iam == (__num_threads - 1)
184 ? (__length - 1)
185 : ((__iam + 1) * (__length / __num_threads) - 1));
186 __my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
187
188 // Init result with _M_first value (to have a base value for reduction)
189 if (__my_job._M_first <= __my_job._M_last)
190 {
191 // Cannot use volatile variable directly.
192 _DifferenceType __my_first = __my_job._M_first;
193 __result = __f(__op, __begin + __my_first);
194 ++__my_job._M_first;
195 --__my_job._M_load;
196 }
197
198 _RAIter __current;
199
200 # pragma omp barrier
201
202 // Actual work phase
203 // Work on own or stolen current start
204 while (__busy > 0)
205 {
206 // Work until no productive thread left.
207 # pragma omp flush(__busy)
208
209 // Thread has own work to do
210 while (__my_job._M_first <= __my_job._M_last)
211 {
212 // fetch-and-add call
213 // Reserve current job block (size __chunk_size) in my queue.
214 _DifferenceType __current_job =
215 __fetch_and_add<_DifferenceType>(&(__my_job._M_first),
216 __chunk_size);
217
218 // Update _M_load, to make the three values consistent,
219 // _M_first might have been changed in the meantime
220 __my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
221 for (_DifferenceType __job_counter = 0;
222 __job_counter < __chunk_size
223 && __current_job <= __my_job._M_last;
224 ++__job_counter)
225 {
226 // Yes: process it!
227 __current = __begin + __current_job;
228 ++__current_job;
229
230 // Do actual work.
231 __result = __r(__result, __f(__op, __current));
232 }
233
234 # pragma omp flush(__busy)
235 }
236
237 // After reaching this point, a thread's __job list is empty.
238 if (__iam_working)
239 {
240 // This thread no longer has work.
241 # pragma omp atomic
242 --__busy;
243
244 __iam_working = false;
245 }
246
247 _DifferenceType __supposed_first, __supposed_last,
248 __supposed_load;
249 do
250 {
251 // Find random nonempty deque (not own), do consistency check.
252 __yield();
253 # pragma omp flush(__busy)
254 __victim = __rand_gen();
255 __supposed_first = __job[__victim * __stride]._M_first;
256 __supposed_last = __job[__victim * __stride]._M_last;
257 __supposed_load = __job[__victim * __stride]._M_load;
258 }
259 while (__busy > 0
260 && ((__supposed_load <= 0)
261 || ((__supposed_first + __supposed_load - 1)
262 != __supposed_last)));
263
264 if (__busy == 0)
265 break;
266
267 if (__supposed_load > 0)
268 {
269 // Has work and work to do.
270 // Number of elements to steal (at least one).
271 __steal = (__supposed_load < 2) ? 1 : __supposed_load / 2;
272
273 // Push __victim's current start forward.
274 _DifferenceType __stolen_first =
275 __fetch_and_add<_DifferenceType>
276 (&(__job[__victim * __stride]._M_first), __steal);
277 _DifferenceType __stolen_try = (__stolen_first + __steal
278 - _DifferenceType(1));
279
280 __my_job._M_first = __stolen_first;
281 __my_job._M_last = __gnu_parallel::min(__stolen_try,
282 __supposed_last);
283 __my_job._M_load = __my_job._M_last - __my_job._M_first + 1;
284
285 // Has potential work again.
286 # pragma omp atomic
287 ++__busy;
288 __iam_working = true;
289
290 # pragma omp flush(__busy)
291 }
292 # pragma omp flush(__busy)
293 } // end while __busy > 0
294 // Add accumulated result to output.
295 omp_set_lock(&__output_lock);
296 __output = __r(__output, __result);
297 omp_unset_lock(&__output_lock);
298 }
299
300 delete[] __job;
301
302 // Points to last element processed (needed as return value for
303 // some algorithms like transform)
304 __f._M_finish_iterator = __begin + __length;
305
306 omp_destroy_lock(&__output_lock);
307
308 return __op;
309 }
310 } // end namespace
311
312 #endif /* _GLIBCXX_PARALLEL_WORKSTEALING_H */