libstdc++-v3/include/parallel/multiway_mergesort.h

   1 // -*- C++ -*-
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   3 // Copyright (C) 2007 Free Software Foundation, Inc.
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  30
  31 /** @file parallel/multiway_mergesort.h
  32  *  @brief Parallel multiway merge sort.
  33  *  This file is a GNU parallel extension to the Standard C++ Library.
  34  */
  35
  36 // Written by Johannes Singler.
  37
  38 #ifndef _GLIBCXX_PARALLEL_MERGESORT_H
  39 #define _GLIBCXX_PARALLEL_MERGESORT_H 1
  40
  41 #include <vector>
  42
  43 #include <parallel/basic_iterator.h>
  44 #include <bits/stl_algo.h>
  45 #include <parallel/parallel.h>
  46 #include <parallel/multiway_merge.h>
  47 #include <parallel/timing.h>
  48
  49 namespace __gnu_parallel
  50 {
  51
  52   /** @brief Subsequence description. */
  53   template<typename _DifferenceTp>
  54   struct Piece
  55   {
  56     typedef _DifferenceTp difference_type;
  57
  58     /** @brief Begin of subsequence. */
  59     difference_type begin;
  60
  61     /** @brief End of subsequence. */
  62     difference_type end;
  63   };
  64
  65   /** @brief Data accessed by all threads.
  66    *
  67    *  PMWMS = parallel multiway mergesort */
  68   template<typename RandomAccessIterator>
  69   struct PMWMSSortingData
  70   {
  71     typedef std::iterator_traits<RandomAccessIterator> traits_type;
  72     typedef typename traits_type::value_type value_type;
  73     typedef typename traits_type::difference_type difference_type;
  74
  75     /** @brief Input begin. */
  76     RandomAccessIterator source;
  77
  78     /** @brief Start indices, per thread. */
  79     difference_type* starts;
  80
  81     /** @brief Temporary arrays for each thread.
  82      *
  83      *  Indirection Allows using the temporary storage in different
  84      *  ways, without code duplication.
  85      *  @see _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST */
  86     value_type** temporaries;
  87
  88 #if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
  89     /** @brief Storage in which to sort. */
  90     RandomAccessIterator* sorting_places;
  91
  92     /** @brief Storage into which to merge. */
  93     value_type** merging_places;
  94 #else
  95     /** @brief Storage in which to sort. */
  96     value_type** sorting_places;
  97
  98     /** @brief Storage into which to merge. */
  99     RandomAccessIterator* merging_places;
 100 #endif
 101     /** @brief Samples. */
 102     value_type* samples;
 103
 104     /** @brief Offsets to add to the found positions. */
 105     difference_type* offsets;
 106
 107     /** @brief Pieces of data to merge @c [thread][sequence] */
 108     std::vector<Piece<difference_type> >* pieces;
 109   };
 110
 111   /** @brief Thread local data for PMWMS. */
 112   template<typename RandomAccessIterator>
 113   struct PMWMSSorterPU
 114   {
 115     /** @brief Total number of thread involved. */
 116     thread_index_t num_threads;
 117     /** @brief Number of owning thread. */
 118     thread_index_t iam;
 119     /** @brief Stable sorting desired. */
 120     bool stable;
 121     /** @brief Pointer to global data. */
 122     PMWMSSortingData<RandomAccessIterator>* sd;
 123   };
 124
 125   /**
 126    *  @brief Select samples from a sequence.
 127    *  @param d Pointer to thread-local data. Result will be placed in
 128    *  @c d->ds->samples.
 129    *  @param num_samples Number of samples to select.
 130    */
 131   template<typename RandomAccessIterator, typename _DifferenceTp>
 132   inline void
 133   determine_samples(PMWMSSorterPU<RandomAccessIterator>* d,
 134                     _DifferenceTp& num_samples)
 135   {
 136     typedef _DifferenceTp difference_type;
 137
 138     PMWMSSortingData<RandomAccessIterator>* sd = d->sd;
 139
 140     num_samples = Settings::sort_mwms_oversampling * d->num_threads - 1;
 141
 142     difference_type es[num_samples + 2];
 143     equally_split(sd->starts[d->iam + 1] - sd->starts[d->iam], num_samples + 1, es);
 144
 145     for (difference_type i = 0; i < num_samples; i++)
 146       sd->samples[d->iam * num_samples + i] = sd->source[sd->starts[d->iam] + es[i + 1]];
 147   }
 148
 149   /** @brief PMWMS code executed by each thread.
 150    *  @param d Pointer to thread-local data.
 151    *  @param comp Comparator.
 152    */
 153   template<typename RandomAccessIterator, typename Comparator>
 154   inline void
 155   parallel_sort_mwms_pu(PMWMSSorterPU<RandomAccessIterator>* d,
 156                         Comparator& comp)
 157   {
 158     typedef std::iterator_traits<RandomAccessIterator> traits_type;
 159     typedef typename traits_type::value_type value_type;
 160     typedef typename traits_type::difference_type difference_type;
 161
 162     Timing<sequential_tag> t;
 163
 164     t.tic();
 165
 166     PMWMSSortingData<RandomAccessIterator>* sd = d->sd;
 167     thread_index_t iam = d->iam;
 168
 169     // Length of this thread's chunk, before merging.
 170     difference_type length_local = sd->starts[iam + 1] - sd->starts[iam];
 171
 172 #if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
 173     typedef RandomAccessIterator SortingPlacesIterator;
 174
 175     // Sort in input storage.
 176     sd->sorting_places[iam] = sd->source + sd->starts[iam];
 177 #else
 178     typedef value_type* SortingPlacesIterator;
 179
 180     // Sort in temporary storage, leave space for sentinel.
 181     sd->sorting_places[iam] = sd->temporaries[iam] = static_cast<value_type*>(::operator new(sizeof(value_type) *(length_local + 1)));
 182
 183     // Copy there.
 184     std::uninitialized_copy(sd->source + sd->starts[iam], sd->source + sd->starts[iam] + length_local, sd->sorting_places[iam]);
 185 #endif
 186
 187     // Sort locally.
 188     if (d->stable)
 189       __gnu_sequential::stable_sort(sd->sorting_places[iam], sd->sorting_places[iam] + length_local, comp);
 190     else
 191       __gnu_sequential::sort(sd->sorting_places[iam], sd->sorting_places[iam] + length_local, comp);
 192
 193 #if _GLIBCXX_ASSERTIONS
 194     _GLIBCXX_PARALLEL_ASSERT(is_sorted(sd->sorting_places[iam], sd->sorting_places[iam] + length_local, comp));
 195 #endif
 196
 197     // Invariant: locally sorted subsequence in sd->sorting_places[iam],
 198     // sd->sorting_places[iam] + length_local.
 199     t.tic("local sort");
 200
 201     if (Settings::sort_splitting == Settings::SAMPLING)
 202       {
 203         difference_type num_samples;
 204         determine_samples(d, num_samples);
 205
 206 #pragma omp barrier
 207
 208         t.tic("sample/wait");
 209
 210 #pragma omp single
 211         __gnu_sequential::sort(sd->samples, sd->samples + (num_samples * d->num_threads), comp);
 212
 213 #pragma omp barrier
 214
 215         for (int s = 0; s < d->num_threads; s++)
 216           {
 217             // For each sequence.
 218             if (num_samples * iam > 0)
 219               sd->pieces[iam][s].begin = std::lower_bound(sd->sorting_places[s],
 220                                  sd->sorting_places[s] + sd->starts[s + 1] - sd->starts[s],
 221                                  sd->samples[num_samples * iam],
 222                                  comp)
 223                 - sd->sorting_places[s];
 224             else
 225               // Absolute beginning.
 226               sd->pieces[iam][s].begin = 0;
 227
 228             if ((num_samples * (iam + 1)) < (num_samples * d->num_threads))
 229               sd->pieces[iam][s].end = std::lower_bound(sd->sorting_places[s],
 230                                                         sd->sorting_places[s] + sd->starts[s + 1] - sd->starts[s], sd->samples[num_samples * (iam + 1)], comp)
 231                 - sd->sorting_places[s];
 232             else
 233               // Absolute end.
 234               sd->pieces[iam][s].end = sd->starts[s + 1] - sd->starts[s];
 235           }
 236
 237       }
 238     else if (Settings::sort_splitting == Settings::EXACT)
 239       {
 240 #pragma omp barrier
 241
 242         t.tic("wait");
 243
 244         std::vector<std::pair<SortingPlacesIterator, SortingPlacesIterator> > seqs(d->num_threads);
 245         for (int s = 0; s < d->num_threads; s++)
 246           seqs[s] = std::make_pair(sd->sorting_places[s], sd->sorting_places[s] + sd->starts[s + 1] - sd->starts[s]);
 247
 248         std::vector<SortingPlacesIterator> offsets(d->num_threads);
 249
 250         // If not last thread.
 251         if (iam < d->num_threads - 1)
 252           multiseq_partition(seqs.begin(), seqs.end(), sd->starts[iam + 1], offsets.begin(), comp);
 253
 254         for (int seq = 0; seq < d->num_threads; seq++)
 255           {
 256             // For each sequence.
 257             if (iam < (d->num_threads - 1))
 258               sd->pieces[iam][seq].end = offsets[seq] - seqs[seq].first;
 259             else
 260               // Absolute end of this sequence.
 261               sd->pieces[iam][seq].end = sd->starts[seq + 1] - sd->starts[seq];
 262           }
 263
 264 #pragma omp barrier
 265
 266         for (int seq = 0; seq < d->num_threads; seq++)
 267           {
 268             // For each sequence.
 269             if (iam > 0)
 270               sd->pieces[iam][seq].begin = sd->pieces[iam - 1][seq].end;
 271             else
 272               // Absolute beginning.
 273               sd->pieces[iam][seq].begin = 0;
 274           }
 275       }
 276
 277     t.tic("split");
 278
 279     // Offset from target begin, length after merging.
 280     difference_type offset = 0, length_am = 0;
 281     for (int s = 0; s < d->num_threads; s++)
 282       {
 283         length_am += sd->pieces[iam][s].end - sd->pieces[iam][s].begin;
 284         offset += sd->pieces[iam][s].begin;
 285       }
 286
 287 #if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
 288     // Merge to temporary storage, uninitialized creation not possible
 289     // since there is no multiway_merge calling the placement new
 290     // instead of the assignment operator.
 291     sd->merging_places[iam] = sd->temporaries[iam] = new value_type[length_am];
 292 #else
 293     // Merge directly to target.
 294     sd->merging_places[iam] = sd->source + offset;
 295 #endif
 296     std::vector<std::pair<SortingPlacesIterator, SortingPlacesIterator> > seqs(d->num_threads);
 297
 298     for (int s = 0; s < d->num_threads; s++)
 299       {
 300         seqs[s] = std::make_pair(sd->sorting_places[s] + sd->pieces[iam][s].begin, sd->sorting_places[s] + sd->pieces[iam][s].end);
 301
 302 #if _GLIBCXX_ASSERTIONS
 303         _GLIBCXX_PARALLEL_ASSERT(is_sorted(seqs[s].first, seqs[s].second, comp));
 304 #endif
 305       }
 306
 307     multiway_merge(seqs.begin(), seqs.end(), sd->merging_places[iam], comp, length_am, d->stable, false, sequential_tag());
 308
 309     t.tic("merge");
 310
 311 #if _GLIBCXX_ASSERTIONS
 312     _GLIBCXX_PARALLEL_ASSERT(is_sorted(sd->merging_places[iam], sd->merging_places[iam] + length_am, comp));
 313 #endif
 314
 315 #       pragma omp barrier
 316
 317 #if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
 318     // Write back.
 319     std::copy(sd->merging_places[iam], sd->merging_places[iam] + length_am,
 320               sd->source + offset);
 321 #endif
 322
 323     delete[] sd->temporaries[iam];
 324
 325     t.tic("copy back");
 326
 327     t.print();
 328   }
 329
 330   /** @brief PMWMS main call.
 331    *  @param begin Begin iterator of sequence.
 332    *  @param end End iterator of sequence.
 333    *  @param comp Comparator.
 334    *  @param n Length of sequence.
 335    *  @param num_threads Number of threads to use.
 336    *  @param stable Stable sorting.
 337    */
 338   template<typename RandomAccessIterator, typename Comparator>
 339   inline void
 340   parallel_sort_mwms(RandomAccessIterator begin, RandomAccessIterator end, Comparator comp, typename std::iterator_traits<RandomAccessIterator>::difference_type n, int num_threads, bool stable)
 341   {
 342     _GLIBCXX_CALL(n)
 343
 344     typedef std::iterator_traits<RandomAccessIterator> traits_type;
 345     typedef typename traits_type::value_type value_type;
 346     typedef typename traits_type::difference_type difference_type;
 347
 348     if (n <= 1)
 349       return;
 350
 351     // At least one element per thread.
 352     if (num_threads > n)
 353       num_threads = static_cast<thread_index_t>(n);
 354
 355     PMWMSSortingData<RandomAccessIterator> sd;
 356
 357     sd.source = begin;
 358     sd.temporaries = new value_type*[num_threads];
 359
 360 #if _GLIBCXX_MULTIWAY_MERGESORT_COPY_LAST
 361     sd.sorting_places = new RandomAccessIterator[num_threads];
 362     sd.merging_places = new value_type*[num_threads];
 363 #else
 364     sd.sorting_places = new value_type*[num_threads];
 365     sd.merging_places = new RandomAccessIterator[num_threads];
 366 #endif
 367
 368     if (Settings::sort_splitting == Settings::SAMPLING)
 369       sd.samples = new value_type[num_threads * (Settings::sort_mwms_oversampling * num_threads - 1)];
 370     else
 371       sd.samples = NULL;
 372
 373     sd.offsets = new difference_type[num_threads - 1];
 374     sd.pieces = new std::vector<Piece<difference_type> >[num_threads];
 375     for (int s = 0; s < num_threads; s++)
 376       sd.pieces[s].resize(num_threads);
 377     PMWMSSorterPU<RandomAccessIterator>* pus = new PMWMSSorterPU<RandomAccessIterator>[num_threads];
 378     difference_type* starts = sd.starts = new difference_type[num_threads + 1];
 379
 380     difference_type chunk_length = n / num_threads, split = n % num_threads, start = 0;
 381     for (int i = 0; i < num_threads; i++)
 382       {
 383         starts[i] = start;
 384         start += (i < split) ? (chunk_length + 1) : chunk_length;
 385         pus[i].num_threads = num_threads;
 386         pus[i].iam = i;
 387         pus[i].sd = &sd;
 388         pus[i].stable = stable;
 389       }
 390     starts[num_threads] = start;
 391
 392     // Now sort in parallel.
 393 #pragma omp parallel num_threads(num_threads)
 394     parallel_sort_mwms_pu(&(pus[omp_get_thread_num()]), comp);
 395
 396     // XXX sd as RAII
 397     delete[] starts;
 398     delete[] sd.temporaries;
 399     delete[] sd.sorting_places;
 400     delete[] sd.merging_places;
 401
 402     if (Settings::sort_splitting == Settings::SAMPLING)
 403       delete[] sd.samples;
 404
 405     delete[] sd.offsets;
 406     delete[] sd.pieces;
 407
 408     delete[] pus;
 409   }
 410
 411 }
 412
 413 #endif