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/balanced_quicksort.h
26 * @brief Implementation of a dynamically load-balanced parallel quicksort.
27 *
28 * It works in-place and needs only logarithmic extra memory.
29 * The algorithm is similar to the one proposed in
30 *
31 * P. Tsigas and Y. Zhang.
32 * A simple, fast parallel implementation of quicksort and
33 * its performance evaluation on SUN enterprise 10000.
34 * In 11th Euromicro Conference on Parallel, Distributed and
35 * Network-Based Processing, page 372, 2003.
36 *
37 * This file is a GNU parallel extension to the Standard C++ Library.
38 */
39
40 // Written by Johannes Singler.
41
42 #ifndef _GLIBCXX_PARALLEL_BALANCED_QUICKSORT_H
43 #define _GLIBCXX_PARALLEL_BALANCED_QUICKSORT_H 1
44
45 #include <parallel/basic_iterator.h>
46 #include <bits/stl_algo.h>
47 #include <bits/stl_function.h>
48
49 #include <parallel/settings.h>
50 #include <parallel/partition.h>
51 #include <parallel/random_number.h>
52 #include <parallel/queue.h>
53
54 #if _GLIBCXX_PARALLEL_ASSERTIONS
55 #include <parallel/checkers.h>
56 #ifdef _GLIBCXX_HAVE_UNISTD_H
57 #include <unistd.h>
58 #endif
59 #endif
60
61 namespace __gnu_parallel
62 {
63 /** @brief Information local to one thread in the parallel quicksort run. */
64 template<typename _RAIter>
65 struct _QSBThreadLocal
66 {
67 typedef std::iterator_traits<_RAIter> _TraitsType;
68 typedef typename _TraitsType::difference_type _DifferenceType;
69
70 /** @brief Continuous part of the sequence, described by an
71 iterator pair. */
72 typedef std::pair<_RAIter, _RAIter> _Piece;
73
74 /** @brief Initial piece to work on. */
75 _Piece _M_initial;
76
77 /** @brief Work-stealing queue. */
78 _RestrictedBoundedConcurrentQueue<_Piece> _M_leftover_parts;
79
80 /** @brief Number of threads involved in this algorithm. */
81 _ThreadIndex _M_num_threads;
82
83 /** @brief Pointer to a counter of elements left over to sort. */
84 volatile _DifferenceType* _M_elements_leftover;
85
86 /** @brief The complete sequence to sort. */
87 _Piece _M_global;
88
89 /** @brief Constructor.
90 * @param __queue_size size of the work-stealing queue. */
91 _QSBThreadLocal(int __queue_size) : _M_leftover_parts(__queue_size) { }
92 };
93
94 /** @brief Balanced quicksort divide step.
95 * @param __begin Begin iterator of subsequence.
96 * @param __end End iterator of subsequence.
97 * @param __comp Comparator.
98 * @param __num_threads Number of threads that are allowed to work on
99 * this part.
100 * @pre @c (__end-__begin)>=1 */
101 template<typename _RAIter, typename _Compare>
102 typename std::iterator_traits<_RAIter>::difference_type
103 __qsb_divide(_RAIter __begin, _RAIter __end,
104 _Compare __comp, _ThreadIndex __num_threads)
105 {
106 _GLIBCXX_PARALLEL_ASSERT(__num_threads > 0);
107
108 typedef std::iterator_traits<_RAIter> _TraitsType;
109 typedef typename _TraitsType::value_type _ValueType;
110 typedef typename _TraitsType::difference_type _DifferenceType;
111
112 _RAIter __pivot_pos =
113 __median_of_three_iterators(__begin, __begin + (__end - __begin) / 2,
114 __end - 1, __comp);
115
116 #if defined(_GLIBCXX_PARALLEL_ASSERTIONS)
117 // Must be in between somewhere.
118 _DifferenceType __n = __end - __begin;
119
120 _GLIBCXX_PARALLEL_ASSERT((!__comp(*__pivot_pos, *__begin)
121 && !__comp(*(__begin + __n / 2),
122 *__pivot_pos))
123 || (!__comp(*__pivot_pos, *__begin)
124 && !__comp(*(__end - 1), *__pivot_pos))
125 || (!__comp(*__pivot_pos, *(__begin + __n / 2))
126 && !__comp(*__begin, *__pivot_pos))
127 || (!__comp(*__pivot_pos, *(__begin + __n / 2))
128 && !__comp(*(__end - 1), *__pivot_pos))
129 || (!__comp(*__pivot_pos, *(__end - 1))
130 && !__comp(*__begin, *__pivot_pos))
131 || (!__comp(*__pivot_pos, *(__end - 1))
132 && !__comp(*(__begin + __n / 2),
133 *__pivot_pos)));
134 #endif
135
136 // Swap pivot value to end.
137 if (__pivot_pos != (__end - 1))
138 std::iter_swap(__pivot_pos, __end - 1);
139 __pivot_pos = __end - 1;
140
141 __gnu_parallel::__binder2nd<_Compare, _ValueType, _ValueType, bool>
142 __pred(__comp, *__pivot_pos);
143
144 // Divide, returning __end - __begin - 1 in the worst case.
145 _DifferenceType __split_pos = __parallel_partition(__begin, __end - 1,
146 __pred,
147 __num_threads);
148
149 // Swap back pivot to middle.
150 std::iter_swap(__begin + __split_pos, __pivot_pos);
151 __pivot_pos = __begin + __split_pos;
152
153 #if _GLIBCXX_PARALLEL_ASSERTIONS
154 _RAIter __r;
155 for (__r = __begin; __r != __pivot_pos; ++__r)
156 _GLIBCXX_PARALLEL_ASSERT(__comp(*__r, *__pivot_pos));
157 for (; __r != __end; ++__r)
158 _GLIBCXX_PARALLEL_ASSERT(!__comp(*__r, *__pivot_pos));
159 #endif
160
161 return __split_pos;
162 }
163
164 /** @brief Quicksort conquer step.
165 * @param __tls Array of thread-local storages.
166 * @param __begin Begin iterator of subsequence.
167 * @param __end End iterator of subsequence.
168 * @param __comp Comparator.
169 * @param __iam Number of the thread processing this function.
170 * @param __num_threads
171 * Number of threads that are allowed to work on this part. */
172 template<typename _RAIter, typename _Compare>
173 void
174 __qsb_conquer(_QSBThreadLocal<_RAIter>** __tls,
175 _RAIter __begin, _RAIter __end,
176 _Compare __comp,
177 _ThreadIndex __iam, _ThreadIndex __num_threads,
178 bool __parent_wait)
179 {
180 typedef std::iterator_traits<_RAIter> _TraitsType;
181 typedef typename _TraitsType::value_type _ValueType;
182 typedef typename _TraitsType::difference_type _DifferenceType;
183
184 _DifferenceType __n = __end - __begin;
185
186 if (__num_threads <= 1 || __n <= 1)
187 {
188 __tls[__iam]->_M_initial.first = __begin;
189 __tls[__iam]->_M_initial.second = __end;
190
191 __qsb_local_sort_with_helping(__tls, __comp, __iam, __parent_wait);
192
193 return;
194 }
195
196 // Divide step.
197 _DifferenceType __split_pos =
198 __qsb_divide(__begin, __end, __comp, __num_threads);
199
200 #if _GLIBCXX_PARALLEL_ASSERTIONS
201 _GLIBCXX_PARALLEL_ASSERT(0 <= __split_pos &&
202 __split_pos < (__end - __begin));
203 #endif
204
205 _ThreadIndex
206 __num_threads_leftside = std::max<_ThreadIndex>
207 (1, std::min<_ThreadIndex>(__num_threads - 1, __split_pos
208 * __num_threads / __n));
209
210 # pragma omp atomic
211 *__tls[__iam]->_M_elements_leftover -= (_DifferenceType)1;
212
213 // Conquer step.
214 # pragma omp parallel num_threads(2)
215 {
216 bool __wait;
217 if(omp_get_num_threads() < 2)
218 __wait = false;
219 else
220 __wait = __parent_wait;
221
222 # pragma omp sections
223 {
224 # pragma omp section
225 {
226 __qsb_conquer(__tls, __begin, __begin + __split_pos, __comp,
227 __iam, __num_threads_leftside, __wait);
228 __wait = __parent_wait;
229 }
230 // The pivot_pos is left in place, to ensure termination.
231 # pragma omp section
232 {
233 __qsb_conquer(__tls, __begin + __split_pos + 1, __end, __comp,
234 __iam + __num_threads_leftside,
235 __num_threads - __num_threads_leftside, __wait);
236 __wait = __parent_wait;
237 }
238 }
239 }
240 }
241
242 /**
243 * @brief Quicksort step doing load-balanced local sort.
244 * @param __tls Array of thread-local storages.
245 * @param __comp Comparator.
246 * @param __iam Number of the thread processing this function.
247 */
248 template<typename _RAIter, typename _Compare>
249 void
250 __qsb_local_sort_with_helping(_QSBThreadLocal<_RAIter>** __tls,
251 _Compare& __comp, _ThreadIndex __iam,
252 bool __wait)
253 {
254 typedef std::iterator_traits<_RAIter> _TraitsType;
255 typedef typename _TraitsType::value_type _ValueType;
256 typedef typename _TraitsType::difference_type _DifferenceType;
257 typedef std::pair<_RAIter, _RAIter> _Piece;
258
259 _QSBThreadLocal<_RAIter>& __tl = *__tls[__iam];
260
261 _DifferenceType
262 __base_case_n = _Settings::get().sort_qsb_base_case_maximal_n;
263 if (__base_case_n < 2)
264 __base_case_n = 2;
265 _ThreadIndex __num_threads = __tl._M_num_threads;
266
267 // Every thread has its own random number generator.
268 _RandomNumber __rng(__iam + 1);
269
270 _Piece __current = __tl._M_initial;
271
272 _DifferenceType __elements_done = 0;
273 #if _GLIBCXX_PARALLEL_ASSERTIONS
274 _DifferenceType __total_elements_done = 0;
275 #endif
276
277 for (;;)
278 {
279 // Invariant: __current must be a valid (maybe empty) range.
280 _RAIter __begin = __current.first, __end = __current.second;
281 _DifferenceType __n = __end - __begin;
282
283 if (__n > __base_case_n)
284 {
285 // Divide.
286 _RAIter __pivot_pos = __begin + __rng(__n);
287
288 // Swap __pivot_pos value to end.
289 if (__pivot_pos != (__end - 1))
290 std::iter_swap(__pivot_pos, __end - 1);
291 __pivot_pos = __end - 1;
292
293 __gnu_parallel::__binder2nd
294 <_Compare, _ValueType, _ValueType, bool>
295 __pred(__comp, *__pivot_pos);
296
297 // Divide, leave pivot unchanged in last place.
298 _RAIter __split_pos1, __split_pos2;
299 __split_pos1 = __gnu_sequential::partition(__begin, __end - 1,
300 __pred);
301
302 // Left side: < __pivot_pos; __right side: >= __pivot_pos.
303 #if _GLIBCXX_PARALLEL_ASSERTIONS
304 _GLIBCXX_PARALLEL_ASSERT(__begin <= __split_pos1
305 && __split_pos1 < __end);
306 #endif
307 // Swap pivot back to middle.
308 if (__split_pos1 != __pivot_pos)
309 std::iter_swap(__split_pos1, __pivot_pos);
310 __pivot_pos = __split_pos1;
311
312 // In case all elements are equal, __split_pos1 == 0.
313 if ((__split_pos1 + 1 - __begin) < (__n >> 7)
314 || (__end - __split_pos1) < (__n >> 7))
315 {
316 // Very unequal split, one part smaller than one 128th
317 // elements not strictly larger than the pivot.
318 __gnu_parallel::__unary_negate<__gnu_parallel::__binder1st
319 <_Compare, _ValueType, _ValueType, bool>, _ValueType>
320 __pred(__gnu_parallel::__binder1st
321 <_Compare, _ValueType, _ValueType, bool>
322 (__comp, *__pivot_pos));
323
324 // Find other end of pivot-equal range.
325 __split_pos2 = __gnu_sequential::partition(__split_pos1 + 1,
326 __end, __pred);
327 }
328 else
329 // Only skip the pivot.
330 __split_pos2 = __split_pos1 + 1;
331
332 // Elements equal to pivot are done.
333 __elements_done += (__split_pos2 - __split_pos1);
334 #if _GLIBCXX_PARALLEL_ASSERTIONS
335 __total_elements_done += (__split_pos2 - __split_pos1);
336 #endif
337 // Always push larger part onto stack.
338 if (((__split_pos1 + 1) - __begin) < (__end - (__split_pos2)))
339 {
340 // Right side larger.
341 if ((__split_pos2) != __end)
342 __tl._M_leftover_parts.push_front
343 (std::make_pair(__split_pos2, __end));
344
345 //__current.first = __begin; //already set anyway
346 __current.second = __split_pos1;
347 continue;
348 }
349 else
350 {
351 // Left side larger.
352 if (__begin != __split_pos1)
353 __tl._M_leftover_parts.push_front(std::make_pair
354 (__begin, __split_pos1));
355
356 __current.first = __split_pos2;
357 //__current.second = __end; //already set anyway
358 continue;
359 }
360 }
361 else
362 {
363 __gnu_sequential::sort(__begin, __end, __comp);
364 __elements_done += __n;
365 #if _GLIBCXX_PARALLEL_ASSERTIONS
366 __total_elements_done += __n;
367 #endif
368
369 // Prefer own stack, small pieces.
370 if (__tl._M_leftover_parts.pop_front(__current))
371 continue;
372
373 # pragma omp atomic
374 *__tl._M_elements_leftover -= __elements_done;
375
376 __elements_done = 0;
377
378 #if _GLIBCXX_PARALLEL_ASSERTIONS
379 double __search_start = omp_get_wtime();
380 #endif
381
382 // Look for new work.
383 bool __successfully_stolen = false;
384 while (__wait && *__tl._M_elements_leftover > 0
385 && !__successfully_stolen
386 #if _GLIBCXX_PARALLEL_ASSERTIONS
387 // Possible dead-lock.
388 && (omp_get_wtime() < (__search_start + 1.0))
389 #endif
390 )
391 {
392 _ThreadIndex __victim;
393 __victim = __rng(__num_threads);
394
395 // Large pieces.
396 __successfully_stolen = (__victim != __iam)
397 && __tls[__victim]->_M_leftover_parts.pop_back(__current);
398 if (!__successfully_stolen)
399 __yield();
400 #if !defined(__ICC) && !defined(__ECC)
401 # pragma omp flush
402 #endif
403 }
404
405 #if _GLIBCXX_PARALLEL_ASSERTIONS
406 if (omp_get_wtime() >= (__search_start + 1.0))
407 {
408 sleep(1);
409 _GLIBCXX_PARALLEL_ASSERT(omp_get_wtime()
410 < (__search_start + 1.0));
411 }
412 #endif
413 if (!__successfully_stolen)
414 {
415 #if _GLIBCXX_PARALLEL_ASSERTIONS
416 _GLIBCXX_PARALLEL_ASSERT(*__tl._M_elements_leftover == 0);
417 #endif
418 return;
419 }
420 }
421 }
422 }
423
424 /** @brief Top-level quicksort routine.
425 * @param __begin Begin iterator of sequence.
426 * @param __end End iterator of sequence.
427 * @param __comp Comparator.
428 * @param __num_threads Number of threads that are allowed to work on
429 * this part.
430 */
431 template<typename _RAIter, typename _Compare>
432 void
433 __parallel_sort_qsb(_RAIter __begin, _RAIter __end,
434 _Compare __comp, _ThreadIndex __num_threads)
435 {
436 _GLIBCXX_CALL(__end - __begin)
437
438 typedef std::iterator_traits<_RAIter> _TraitsType;
439 typedef typename _TraitsType::value_type _ValueType;
440 typedef typename _TraitsType::difference_type _DifferenceType;
441 typedef std::pair<_RAIter, _RAIter> _Piece;
442
443 typedef _QSBThreadLocal<_RAIter> _TLSType;
444
445 _DifferenceType __n = __end - __begin;
446
447 if (__n <= 1)
448 return;
449
450 // At least one element per processor.
451 if (__num_threads > __n)
452 __num_threads = static_cast<_ThreadIndex>(__n);
453
454 // Initialize thread local storage
455 _TLSType** __tls = new _TLSType*[__num_threads];
456 _DifferenceType __queue_size = (__num_threads
457 * (_ThreadIndex)(__rd_log2(__n) + 1));
458 for (_ThreadIndex __t = 0; __t < __num_threads; ++__t)
459 __tls[__t] = new _QSBThreadLocal<_RAIter>(__queue_size);
460
461 // There can never be more than ceil(__rd_log2(__n)) ranges on the
462 // stack, because
463 // 1. Only one processor pushes onto the stack
464 // 2. The largest range has at most length __n
465 // 3. Each range is larger than half of the range remaining
466 volatile _DifferenceType __elements_leftover = __n;
467 for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
468 {
469 __tls[__i]->_M_elements_leftover = &__elements_leftover;
470 __tls[__i]->_M_num_threads = __num_threads;
471 __tls[__i]->_M_global = std::make_pair(__begin, __end);
472
473 // Just in case nothing is left to assign.
474 __tls[__i]->_M_initial = std::make_pair(__end, __end);
475 }
476
477 // Main recursion call.
478 __qsb_conquer(__tls, __begin, __begin + __n, __comp, 0,
479 __num_threads, true);
480
481 #if _GLIBCXX_PARALLEL_ASSERTIONS
482 // All stack must be empty.
483 _Piece __dummy;
484 for (_ThreadIndex __i = 1; __i < __num_threads; ++__i)
485 _GLIBCXX_PARALLEL_ASSERT(
486 !__tls[__i]->_M_leftover_parts.pop_back(__dummy));
487 #endif
488
489 for (_ThreadIndex __i = 0; __i < __num_threads; ++__i)
490 delete __tls[__i];
491 delete[] __tls;
492 }
493 } // namespace __gnu_parallel
494
495 #endif /* _GLIBCXX_PARALLEL_BALANCED_QUICKSORT_H */