libstdc++
balanced_quicksort.h
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1 // -*- C++ -*-
2 
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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 
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>
66  {
68  typedef typename _TraitsType::difference_type _DifferenceType;
69 
70  /** @brief Continuous part of the sequence, described by an
71  iterator pair. */
73 
74  /** @brief Initial piece to work on. */
76 
77  /** @brief Work-stealing queue. */
79 
80  /** @brief Number of threads involved in this algorithm. */
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. */
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>
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 
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
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 
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
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;
258 
259  _QSBThreadLocal<_RAIter>& __tl = *__tls[__iam];
260 
261  _DifferenceType
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 
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.
319  <_Compare, _ValueType, _ValueType, bool>, _ValueType>
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
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;
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 */
Includes the original header files concerned with iterators except for stream iterators....
Routines for checking the correctness of algorithm results. This file is a GNU parallel extension to ...
#define _GLIBCXX_PARALLEL_ASSERTIONS
Switch on many _GLIBCXX_PARALLEL_ASSERTions in parallel code. Should be switched on only locally.
#define _GLIBCXX_CALL(__n)
Macro to produce log message when entering a function.
Parallel implementation of std::partition(), std::nth_element(), and std::partial_sort()....
Lock-free double-ended queue. This file is a GNU parallel extension to the Standard C++ Library.
Random number generator based on the Mersenne twister. This file is a GNU parallel extension to the S...
Runtime settings and tuning parameters, heuristics to decide whether to use parallelized algorithms.
GNU parallel code for public use.
uint16_t _ThreadIndex
Unsigned integer to index a thread number. The maximum thread number (for each processor) must fit in...
Definition: types.h:123
void __parallel_sort_qsb(_RAIter __begin, _RAIter __end, _Compare __comp, _ThreadIndex __num_threads)
Top-level quicksort routine.
_RAIter __median_of_three_iterators(_RAIter __a, _RAIter __b, _RAIter __c, _Compare __comp)
Compute the median of three referenced elements, according to __comp.
Definition: base.h:398
void __qsb_local_sort_with_helping(_QSBThreadLocal< _RAIter > **__tls, _Compare &__comp, _ThreadIndex __iam, bool __wait)
Quicksort step doing load-balanced local sort.
void __qsb_conquer(_QSBThreadLocal< _RAIter > **__tls, _RAIter __begin, _RAIter __end, _Compare __comp, _ThreadIndex __iam, _ThreadIndex __num_threads, bool __parent_wait)
Quicksort conquer step.
std::iterator_traits< _RAIter >::difference_type __parallel_partition(_RAIter __begin, _RAIter __end, _Predicate __pred, _ThreadIndex __num_threads)
Parallel implementation of std::partition.
Definition: partition.h:56
void __yield()
Yield control to another thread, without waiting for the end of the time slice.
_Size __rd_log2(_Size __n)
Calculates the rounded-down logarithm of __n for base 2.
Definition: base.h:102
std::iterator_traits< _RAIter >::difference_type __qsb_divide(_RAIter __begin, _RAIter __end, _Compare __comp, _ThreadIndex __num_threads)
Balanced quicksort divide step.
Traits class for iterators.
Information local to one thread in the parallel quicksort run.
_Piece _M_initial
Initial piece to work on.
volatile _DifferenceType * _M_elements_leftover
Pointer to a counter of elements left over to sort.
_ThreadIndex _M_num_threads
Number of threads involved in this algorithm.
_QSBThreadLocal(int __queue_size)
Constructor.
std::pair< _RAIter, _RAIter > _Piece
Continuous part of the sequence, described by an iterator pair.
_RestrictedBoundedConcurrentQueue< _Piece > _M_leftover_parts
Work-stealing queue.
_Piece _M_global
The complete sequence to sort.
Similar to std::unary_negate, but giving the argument types explicitly.
Definition: base.h:175
Similar to std::binder1st, but giving the argument types explicitly.
Definition: base.h:194
Similar to std::binder2nd, but giving the argument types explicitly.
Definition: base.h:222
Subsequence description.
Double-ended queue of bounded size, allowing lock-free atomic access. push_front() and pop_front() mu...
Definition: queue.h:53
Random number generator, based on the Mersenne twister.
Definition: random_number.h:43
_SequenceIndex sort_qsb_base_case_maximal_n
Maximal subsequence __length to switch to unbalanced __base case. Applies to std::sort with dynamical...
Definition: settings.h:240
static const _Settings & get()
Get the global settings.