mingw/lib/gcc/mingw32/4.4.0/include/c++/parallel/workstealing.h

   1 // -*- C++ -*-
   2
   3 // Copyright (C) 2007, 2008, 2009 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 difference_type;
  57
  58     /** @brief First element.
  59      *
  60      *  Changed by owning and stealing thread. By stealing thread,
  61      *  always incremented. */
  62     _GLIBCXX_JOB_VOLATILE difference_type first;
  63
  64     /** @brief Last element.
  65      *
  66      *  Changed by owning thread only. */
  67     _GLIBCXX_JOB_VOLATILE difference_type last;
  68
  69     /** @brief Number of elements, i. e. @c last-first+1.
  70      *
  71      *  Changed by owning thread only. */
  72     _GLIBCXX_JOB_VOLATILE difference_type 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 "process" an element with op (depends on
  84   *  desired functionality, e. g. for std::for_each(), ...).
  85   *  @param r Functor to "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 RandomAccessIterator,
  94          typename Op,
  95          typename Fu,
  96          typename Red,
  97          typename Result>
  98   Op
  99   for_each_template_random_access_workstealing(RandomAccessIterator begin,
 100                                                RandomAccessIterator end,
 101                                                Op op, Fu& f, Red r,
 102                                                Result base, Result& output,
 103                                                typename std::iterator_traits
 104                                                <RandomAccessIterator>::
 105                                                difference_type bound)
 106   {
 107     _GLIBCXX_CALL(end - begin)
 108
 109     typedef std::iterator_traits<RandomAccessIterator> traits_type;
 110     typedef typename traits_type::difference_type difference_type;
 111
 112     const _Settings& __s = _Settings::get();
 113
 114     difference_type chunk_size = static_cast<difference_type>(__s.workstealing_chunk_size);
 115
 116     // How many jobs?
 117     difference_type 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 / sizeof(Job<difference_type>) + 1;
 121
 122     // Total number of threads currently working.
 123     thread_index_t busy = 0;
 124
 125     Job<difference_type> *job;
 126
 127     omp_lock_t output_lock;
 128     omp_init_lock(&output_lock);
 129
 130     // Write base value to output.
 131     output = base;
 132
 133     // No more threads than jobs, at least one thread.
 134     thread_index_t num_threads =
 135         __gnu_parallel::max<thread_index_t>(1,
 136             __gnu_parallel::min<difference_type>(length, get_max_threads()));
 137
 138 #   pragma omp parallel shared(busy) num_threads(num_threads)
 139       {
 140
 141 #       pragma omp single
 142           {
 143             num_threads = omp_get_num_threads();
 144
 145             // Create job description array.
 146             job = new Job<difference_type>[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         thread_index_t iam = omp_get_thread_num();
 156
 157         // This job.
 158         Job<difference_type>& my_job = job[iam * stride];
 159
 160         // Random number (for work stealing).
 161         thread_index_t victim;
 162
 163         // Local value for reduction.
 164         Result result = Result();
 165
 166         // Number of elements to steal in one attempt.
 167         difference_type steal;
 168
 169         // Every thread has its own random number generator
 170         // (modulo num_threads).
 171         random_number 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.first =
 181             static_cast<difference_type>(iam * (length / num_threads));
 182
 183         my_job.last = (iam == (num_threads - 1)) ?
 184             (length - 1) : ((iam + 1) * (length / num_threads) - 1);
 185         my_job.load = my_job.last - my_job.first + 1;
 186
 187         // Init result with first value (to have a base value for reduction).
 188         if (my_job.first <= my_job.last)
 189           {
 190             // Cannot use volatile variable directly.
 191             difference_type my_first = my_job.first;
 192             result = f(op, begin + my_first);
 193             ++my_job.first;
 194             --my_job.load;
 195           }
 196
 197         RandomAccessIterator current;
 198
 199 #       pragma omp barrier
 200
 201         // Actual work phase
 202         // Work on own or stolen start
 203         while (busy > 0)
 204           {
 205             // Work until no productive thread left.
 206 #           pragma omp flush(busy)
 207
 208             // Thread has own work to do
 209             while (my_job.first <= my_job.last)
 210               {
 211                 // fetch-and-add call
 212                 // Reserve current job block (size chunk_size) in my queue.
 213                 difference_type current_job =
 214                   fetch_and_add<difference_type>(&(my_job.first), chunk_size);
 215
 216                 // Update load, to make the three values consistent,
 217                 // first might have been changed in the meantime
 218                 my_job.load = my_job.last - my_job.first + 1;
 219                 for (difference_type job_counter = 0;
 220                      job_counter < chunk_size && current_job <= my_job.last;
 221                      ++job_counter)
 222                   {
 223                     // Yes: process it!
 224                     current = begin + current_job;
 225                     ++current_job;
 226
 227                     // Do actual work.
 228                     result = r(result, f(op, current));
 229                   }
 230
 231 #               pragma omp flush(busy)
 232               }
 233
 234             // After reaching this point, a thread's job list is empty.
 235             if (iam_working)
 236               {
 237                 // This thread no longer has work.
 238 #               pragma omp atomic
 239                 --busy;
 240
 241                 iam_working = false;
 242               }
 243
 244             difference_type supposed_first, supposed_last, supposed_load;
 245             do
 246               {
 247                 // Find random nonempty deque (not own), do consistency check.
 248                 yield();
 249 #               pragma omp flush(busy)
 250                 victim = rand_gen();
 251                 supposed_first = job[victim * stride].first;
 252                 supposed_last = job[victim * stride].last;
 253                 supposed_load = job[victim * stride].load;
 254               }
 255             while (busy > 0
 256               && ((supposed_load <= 0)
 257                 || ((supposed_first + supposed_load - 1) != supposed_last)));
 258
 259             if (busy == 0)
 260               break;
 261
 262             if (supposed_load > 0)
 263               {
 264                 // Has work and work to do.
 265                 // Number of elements to steal (at least one).
 266                 steal = (supposed_load < 2) ? 1 : supposed_load / 2;
 267
 268                 // Push victim's start forward.
 269                 difference_type stolen_first =
 270                     fetch_and_add<difference_type>(
 271                         &(job[victim * stride].first), steal);
 272                 difference_type stolen_try =
 273                     stolen_first + steal - difference_type(1);
 274
 275                 my_job.first = stolen_first;
 276                 my_job.last = __gnu_parallel::min(stolen_try, supposed_last);
 277                 my_job.load = my_job.last - my_job.first + 1;
 278
 279                 // Has potential work again.
 280 #               pragma omp atomic
 281                   ++busy;
 282                 iam_working = true;
 283
 284 #               pragma omp flush(busy)
 285               }
 286 #           pragma omp flush(busy)
 287           } // end while busy > 0
 288             // Add accumulated result to output.
 289         omp_set_lock(&output_lock);
 290         output = r(output, result);
 291         omp_unset_lock(&output_lock);
 292       }
 293
 294     delete[] job;
 295
 296     // Points to last element processed (needed as return value for
 297     // some algorithms like transform)
 298     f.finish_iterator = begin + length;
 299
 300     omp_destroy_lock(&output_lock);
 301
 302     return op;
 303   }
 304 } // end namespace
 305
 306 #endif /* _GLIBCXX_PARALLEL_WORKSTEALING_H */