libstdc++-v3/include/parallel/settings.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/settings.h
  26  *  @brief Runtime settings and tuning parameters, heuristics to decide
  27  *  whether to use parallelized algorithms.
  28  *  This file is a GNU parallel extension to the Standard C++ Library.
  29  *
  30  *  @section parallelization_decision
  31  *  The decision whether to run an algorithm in parallel.
  32  *
  33  *  There are several ways the user can switch on and off the parallel
  34  *  execution of an algorithm, both at compile- and run-time.
  35  *
  36  *  Only sequential execution can be forced at compile-time.  This
  37  *  reduces code size and protects code parts that have
  38  *  non-thread-safe side effects.
  39  *
  40  *  Ultimately, forcing parallel execution at compile-time makes
  41  *  sense.  Often, the sequential algorithm implementation is used as
  42  *  a subroutine, so no reduction in code size can be achieved.  Also,
  43  *  the machine the program is run on might have only one processor
  44  *  core, so to avoid overhead, the algorithm is executed
  45  *  sequentially.
  46  *
  47  *  To force sequential execution of an algorithm ultimately at
  48  *  compile-time, the user must add the tag
  49  *  __gnu_parallel::sequential_tag() to the end of the parameter list,
  50  *  e. g.
  51  *
  52  *  \code
  53  *  std::sort(v.begin(), v.end(), __gnu_parallel::sequential_tag());
  54  *  \endcode
  55  *
  56  *  This is compatible with all overloaded algorithm variants.  No
  57  *  additional code will be instantiated, at all.  The same holds for
  58  *  most algorithm calls with iterators not providing random access.
  59  *
  60  *  If the algorithm call is not forced to be executed sequentially
  61  *  at compile-time, the decision is made at run-time.
  62  *  The global variable __gnu_parallel::_Settings::algorithm_strategy
  63  *  is checked. It is a tristate variable corresponding to:
  64  *
  65  *  a. force_sequential, meaning the sequential algorithm is executed.
  66  *  b. force_parallel, meaning the parallel algorithm is executed.
  67  *  c. heuristic
  68  *
  69  *  For heuristic, the parallel algorithm implementation is called
  70  *  only if the input size is sufficiently large.  For most
  71  *  algorithms, the input size is the (combined) length of the input
  72  *  sequence(s).  The threshold can be set by the user, individually
  73  *  for each algorithm.  The according variables are called
  74  *  __gnu_parallel::_Settings::[algorithm]_minimal_n .
  75  *
  76  *  For some of the algorithms, there are even more tuning options,
  77  *  e. g. the ability to choose from multiple algorithm variants.  See
  78  *  below for details.
  79  */
  80
  81 // Written by Johannes Singler and Felix Putze.
  82
  83 #ifndef _GLIBCXX_PARALLEL_SETTINGS_H
  84 #define _GLIBCXX_PARALLEL_SETTINGS_H 1
  85
  86 #include <parallel/types.h>
  87
  88 /**
  89   * @brief Determine at compile(?)-time if the parallel variant of an
  90   * algorithm should be called.
  91   * @param c A condition that is convertible to bool that is overruled by
  92   * __gnu_parallel::_Settings::algorithm_strategy. Usually a decision
  93   * based on the input size.
  94   */
  95 #define _GLIBCXX_PARALLEL_CONDITION(c) (__gnu_parallel::_Settings::get().algorithm_strategy != __gnu_parallel::force_sequential && ((__gnu_parallel::get_max_threads() > 1 && (c)) || __gnu_parallel::_Settings::get().algorithm_strategy == __gnu_parallel::force_parallel))
  96
  97 /*
  98 inline bool
  99 parallel_condition(bool c)
 100 {
 101   bool ret = false;
 102   const _Settings& s = _Settings::get();
 103   if (s.algorithm_strategy != force_seqential)
 104     {
 105       if (s.algorithm_strategy == force_parallel)
 106         ret = true;
 107       else
 108         ret = get_max_threads() > 1 && c;
 109     }
 110   return ret;
 111 }
 112 */
 113
 114 namespace __gnu_parallel
 115 {
 116   /// class _Settings
 117   /// Run-time settings for the parallel mode, including all tunable parameters.
 118   struct _Settings
 119   {
 120     _AlgorithmStrategy          algorithm_strategy;
 121
 122     _SortAlgorithm              sort_algorithm;
 123     _PartialSumAlgorithm        partial_sum_algorithm;
 124     _MultiwayMergeAlgorithm     multiway_merge_algorithm;
 125     _FindAlgorithm              find_algorithm;
 126
 127     _SplittingAlgorithm         sort_splitting;
 128     _SplittingAlgorithm         merge_splitting;
 129     _SplittingAlgorithm         multiway_merge_splitting;
 130
 131     // Per-algorithm settings.
 132
 133     /// Minimal input size for accumulate.
 134     sequence_index_t            accumulate_minimal_n;
 135
 136     /// Minimal input size for adjacent_difference.
 137     unsigned int                adjacent_difference_minimal_n;
 138
 139     /// Minimal input size for count and count_if.
 140     sequence_index_t            count_minimal_n;
 141
 142     /// Minimal input size for fill.
 143     sequence_index_t            fill_minimal_n;
 144
 145     /// Block size increase factor for find.
 146     double                      find_increasing_factor;
 147
 148     /// Initial block size for find.
 149     sequence_index_t            find_initial_block_size;
 150
 151     /// Maximal block size for find.
 152     sequence_index_t            find_maximum_block_size;
 153
 154     /// Start with looking for this many elements sequentially, for find.
 155     sequence_index_t            find_sequential_search_size;
 156
 157     /// Minimal input size for for_each.
 158     sequence_index_t            for_each_minimal_n;
 159
 160     /// Minimal input size for generate.
 161     sequence_index_t            generate_minimal_n;
 162
 163     /// Minimal input size for max_element.
 164     sequence_index_t            max_element_minimal_n;
 165
 166     /// Minimal input size for merge.
 167     sequence_index_t            merge_minimal_n;
 168
 169     /// Oversampling factor for merge.
 170     unsigned int                merge_oversampling;
 171
 172     /// Minimal input size for min_element.
 173     sequence_index_t            min_element_minimal_n;
 174
 175     /// Minimal input size for multiway_merge.
 176     sequence_index_t            multiway_merge_minimal_n;
 177
 178     /// Oversampling factor for multiway_merge.
 179     int                         multiway_merge_minimal_k;
 180
 181     /// Oversampling factor for multiway_merge.
 182     unsigned int                multiway_merge_oversampling;
 183
 184     /// Minimal input size for nth_element.
 185     sequence_index_t            nth_element_minimal_n;
 186
 187     /// Chunk size for partition.
 188     sequence_index_t            partition_chunk_size;
 189
 190     /// Chunk size for partition, relative to input size.  If > 0.0,
 191     /// this value overrides partition_chunk_size.
 192     double                      partition_chunk_share;
 193
 194     /// Minimal input size for partition.
 195     sequence_index_t            partition_minimal_n;
 196
 197     /// Minimal input size for partial_sort.
 198     sequence_index_t            partial_sort_minimal_n;
 199
 200     /// Ratio for partial_sum. Assume "sum and write result" to be
 201     /// this factor slower than just "sum".
 202     float                       partial_sum_dilation;
 203
 204     /// Minimal input size for partial_sum.
 205     unsigned int                partial_sum_minimal_n;
 206
 207     /// Minimal input size for random_shuffle.
 208     unsigned int                random_shuffle_minimal_n;
 209
 210     /// Minimal input size for replace and replace_if.
 211     sequence_index_t            replace_minimal_n;
 212
 213     /// Minimal input size for set_difference.
 214     sequence_index_t            set_difference_minimal_n;
 215
 216     /// Minimal input size for set_intersection.
 217     sequence_index_t            set_intersection_minimal_n;
 218
 219     /// Minimal input size for set_symmetric_difference.
 220     sequence_index_t            set_symmetric_difference_minimal_n;
 221
 222     /// Minimal input size for set_union.
 223     sequence_index_t            set_union_minimal_n;
 224
 225     /// Minimal input size for parallel sorting.
 226     sequence_index_t            sort_minimal_n;
 227
 228     /// Oversampling factor for parallel std::sort (MWMS).
 229     unsigned int                sort_mwms_oversampling;
 230
 231     /// Such many samples to take to find a good pivot (quicksort).
 232     unsigned int                sort_qs_num_samples_preset;
 233
 234     /// Maximal subsequence length to switch to unbalanced base case.
 235     /// Applies to std::sort with dynamically load-balanced quicksort.
 236     sequence_index_t            sort_qsb_base_case_maximal_n;
 237
 238     /// Minimal input size for parallel std::transform.
 239     sequence_index_t            transform_minimal_n;
 240
 241     /// Minimal input size for unique_copy.
 242     sequence_index_t            unique_copy_minimal_n;
 243
 244     sequence_index_t            workstealing_chunk_size;
 245
 246     // Hardware dependent tuning parameters.
 247
 248     /// Size of the L1 cache in bytes (underestimation).
 249     unsigned long long          L1_cache_size;
 250
 251     /// Size of the L2 cache in bytes (underestimation).
 252     unsigned long long          L2_cache_size;
 253
 254     /// Size of the Translation Lookaside Buffer (underestimation).
 255     unsigned int                TLB_size;
 256
 257     /// Overestimation of cache line size.  Used to avoid false
 258     /// sharing, i. e. elements of different threads are at least this
 259     /// amount apart.
 260     unsigned int                cache_line_size;
 261
 262     // Statistics.
 263
 264     /// The number of stolen ranges in load-balanced quicksort.
 265     sequence_index_t            qsb_steals;
 266
 267     /// Get the global settings.
 268     static const _Settings&
 269     get() throw();
 270
 271     /// Set the global settings.
 272     static void
 273     set(_Settings&) throw();
 274
 275     explicit
 276     _Settings() : algorithm_strategy(heuristic), sort_algorithm(MWMS), partial_sum_algorithm(LINEAR), multiway_merge_algorithm(LOSER_TREE), find_algorithm(CONSTANT_SIZE_BLOCKS), sort_splitting(EXACT), merge_splitting(EXACT), multiway_merge_splitting(EXACT), accumulate_minimal_n(1000), adjacent_difference_minimal_n(1000), count_minimal_n(1000), fill_minimal_n(1000), find_increasing_factor(2.0), find_initial_block_size(256), find_maximum_block_size(8192), find_sequential_search_size(256), for_each_minimal_n(1000), generate_minimal_n(1000), max_element_minimal_n(1000), merge_minimal_n(1000), merge_oversampling(10), min_element_minimal_n(1000), multiway_merge_minimal_n(1000), multiway_merge_minimal_k(2), multiway_merge_oversampling(10), nth_element_minimal_n(1000), partition_chunk_size(1000), partition_chunk_share(0.0), partition_minimal_n(1000), partial_sort_minimal_n(1000), partial_sum_dilation(1.0f), partial_sum_minimal_n(1000), random_shuffle_minimal_n(1000), replace_minimal_n(1000), set_difference_minimal_n(1000), set_intersection_minimal_n(1000), set_symmetric_difference_minimal_n(1000), set_union_minimal_n(1000), sort_minimal_n(1000), sort_mwms_oversampling(10), sort_qs_num_samples_preset(100), sort_qsb_base_case_maximal_n(100), transform_minimal_n(1000), unique_copy_minimal_n(10000), workstealing_chunk_size(100), L1_cache_size(16 << 10), L2_cache_size(256 << 10), TLB_size(128), cache_line_size(64), qsb_steals(0)
 277     { }
 278   };
 279 }
 280
 281 #endif /* _GLIBCXX_PARALLEL_SETTINGS_H */