1 // -*- C++ -*- 2 3 // Copyright (C) 2007-2021 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 * 29 * This file is a GNU parallel extension to the Standard C++ Library. 30 * 31 * @section parallelization_decision Deciding 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 * - a. force_sequential, meaning the sequential algorithm is executed. 65 * - b. force_parallel, meaning the parallel algorithm is executed. 66 * - c. heuristic 67 * 68 * For heuristic, the parallel algorithm implementation is called 69 * only if the input size is sufficiently large. For most 70 * algorithms, the input size is the (combined) length of the input 71 * sequence(__s). The threshold can be set by the user, individually 72 * for each algorithm. The according variables are called 73 * gnu_parallel::_Settings::[algorithm]_minimal_n . 74 * 75 * For some of the algorithms, there are even more tuning options, 76 * e. g. the ability to choose from multiple algorithm variants. See 77 * below for details. 78 */ 79 80 // Written by Johannes Singler and Felix Putze. 81 82 #ifndef _GLIBCXX_PARALLEL_SETTINGS_H 83 #define _GLIBCXX_PARALLEL_SETTINGS_H 1 84 85 #include <parallel/types.h> 86 87 /** 88 * @brief Determine at compile(?)-time if the parallel variant of an 89 * algorithm should be called. 90 * @param __c A condition that is convertible to bool that is overruled by 91 * __gnu_parallel::_Settings::algorithm_strategy. Usually a decision 92 * based on the input size. 93 */ 94 #define _GLIBCXX_PARALLEL_CONDITION(__c) \ 95 (__gnu_parallel::_Settings::get().algorithm_strategy \ 96 != __gnu_parallel::force_sequential \ 97 && ((__gnu_parallel::__get_max_threads() > 1 && (__c)) \ 98 || __gnu_parallel::_Settings::get().algorithm_strategy \ 99 == __gnu_parallel::force_parallel)) 100 101 /* 102 inline bool 103 parallel_condition(bool __c) 104 { 105 bool ret = false; 106 const _Settings& __s = _Settings::get(); 107 if (__s.algorithm_strategy != force_seqential) 108 { 109 if (__s.algorithm_strategy == force_parallel) 110 ret = true; 111 else 112 ret = __get_max_threads() > 1 && __c; 113 } 114 return ret; 115 } 116 */ 117 118 namespace __gnu_parallel 119 { 120 /// class _Settings 121 /// Run-time settings for the parallel mode including all tunable parameters. 122 struct _Settings 123 { 124 _AlgorithmStrategy algorithm_strategy; 125 126 _SortAlgorithm sort_algorithm; 127 _PartialSumAlgorithm partial_sum_algorithm; 128 _MultiwayMergeAlgorithm multiway_merge_algorithm; 129 _FindAlgorithm find_algorithm; 130 131 _SplittingAlgorithm sort_splitting; 132 _SplittingAlgorithm merge_splitting; 133 _SplittingAlgorithm multiway_merge_splitting; 134 135 // Per-algorithm settings. 136 137 /// Minimal input size for accumulate. 138 _SequenceIndex accumulate_minimal_n; 139 140 /// Minimal input size for adjacent_difference. 141 unsigned int adjacent_difference_minimal_n; 142 143 /// Minimal input size for count and count_if. 144 _SequenceIndex count_minimal_n; 145 146 /// Minimal input size for fill. 147 _SequenceIndex fill_minimal_n; 148 149 /// Block size increase factor for find. 150 double find_increasing_factor; 151 152 /// Initial block size for find. 153 _SequenceIndex find_initial_block_size; 154 155 /// Maximal block size for find. 156 _SequenceIndex find_maximum_block_size; 157 158 /// Start with looking for this many elements sequentially, for find. 159 _SequenceIndex find_sequential_search_size; 160 161 /// Minimal input size for for_each. 162 _SequenceIndex for_each_minimal_n; 163 164 /// Minimal input size for generate. 165 _SequenceIndex generate_minimal_n; 166 167 /// Minimal input size for max_element. 168 _SequenceIndex max_element_minimal_n; 169 170 /// Minimal input size for merge. 171 _SequenceIndex merge_minimal_n; 172 173 /// Oversampling factor for merge. 174 unsigned int merge_oversampling; 175 176 /// Minimal input size for min_element. 177 _SequenceIndex min_element_minimal_n; 178 179 /// Minimal input size for multiway_merge. 180 _SequenceIndex multiway_merge_minimal_n; 181 182 /// Oversampling factor for multiway_merge. 183 int multiway_merge_minimal_k; 184 185 /// Oversampling factor for multiway_merge. 186 unsigned int multiway_merge_oversampling; 187 188 /// Minimal input size for nth_element. 189 _SequenceIndex nth_element_minimal_n; 190 191 /// Chunk size for partition. 192 _SequenceIndex partition_chunk_size; 193 194 /// Chunk size for partition, relative to input size. If > 0.0, 195 /// this value overrides partition_chunk_size. 196 double partition_chunk_share; 197 198 /// Minimal input size for partition. 199 _SequenceIndex partition_minimal_n; 200 201 /// Minimal input size for partial_sort. 202 _SequenceIndex partial_sort_minimal_n; 203 204 /// Ratio for partial_sum. Assume "sum and write result" to be 205 /// this factor slower than just "sum". 206 float partial_sum_dilation; 207 208 /// Minimal input size for partial_sum. 209 unsigned int partial_sum_minimal_n; 210 211 /// Minimal input size for random_shuffle. 212 unsigned int random_shuffle_minimal_n; 213 214 /// Minimal input size for replace and replace_if. 215 _SequenceIndex replace_minimal_n; 216 217 /// Minimal input size for set_difference. 218 _SequenceIndex set_difference_minimal_n; 219 220 /// Minimal input size for set_intersection. 221 _SequenceIndex set_intersection_minimal_n; 222 223 /// Minimal input size for set_symmetric_difference. 224 _SequenceIndex set_symmetric_difference_minimal_n; 225 226 /// Minimal input size for set_union. 227 _SequenceIndex set_union_minimal_n; 228 229 /// Minimal input size for parallel sorting. 230 _SequenceIndex sort_minimal_n; 231 232 /// Oversampling factor for parallel std::sort (MWMS). 233 unsigned int sort_mwms_oversampling; 234 235 /// Such many samples to take to find a good pivot (quicksort). 236 unsigned int sort_qs_num_samples_preset; 237 238 /// Maximal subsequence __length to switch to unbalanced __base case. 239 /// Applies to std::sort with dynamically load-balanced quicksort. 240 _SequenceIndex sort_qsb_base_case_maximal_n; 241 242 /// Minimal input size for parallel std::transform. 243 _SequenceIndex transform_minimal_n; 244 245 /// Minimal input size for unique_copy. 246 _SequenceIndex unique_copy_minimal_n; 247 248 _SequenceIndex workstealing_chunk_size; 249 250 // Hardware dependent tuning parameters. 251 252 /// size of the L1 cache in bytes (underestimation). 253 unsigned long long L1_cache_size; 254 255 /// size of the L2 cache in bytes (underestimation). 256 unsigned long long L2_cache_size; 257 258 /// size of the Translation Lookaside Buffer (underestimation). 259 unsigned int TLB_size; 260 261 /// Overestimation of cache line size. Used to avoid false 262 /// sharing, i.e. elements of different threads are at least this 263 /// amount apart. 264 unsigned int cache_line_size; 265 266 // Statistics. 267 268 /// The number of stolen ranges in load-balanced quicksort. 269 _SequenceIndex qsb_steals; 270 271 /// Minimal input size for search and search_n. 272 _SequenceIndex search_minimal_n; 273 274 /// Block size scale-down factor with respect to current position. 275 float find_scale_factor; 276 277 /// Get the global settings. 278 _GLIBCXX_CONST static const _Settings& 279 get() throw(); 280 281 /// Set the global settings. 282 static void 283 set(_Settings&) throw(); 284 285 explicit _Settings_Settings286 _Settings() : 287 algorithm_strategy(heuristic), 288 sort_algorithm(MWMS), 289 partial_sum_algorithm(LINEAR), 290 multiway_merge_algorithm(LOSER_TREE), 291 find_algorithm(CONSTANT_SIZE_BLOCKS), 292 sort_splitting(EXACT), 293 merge_splitting(EXACT), 294 multiway_merge_splitting(EXACT), 295 accumulate_minimal_n(1000), 296 adjacent_difference_minimal_n(1000), 297 count_minimal_n(1000), 298 fill_minimal_n(1000), 299 find_increasing_factor(2.0), 300 find_initial_block_size(256), 301 find_maximum_block_size(8192), 302 find_sequential_search_size(256), 303 for_each_minimal_n(1000), 304 generate_minimal_n(1000), 305 max_element_minimal_n(1000), 306 merge_minimal_n(1000), 307 merge_oversampling(10), 308 min_element_minimal_n(1000), 309 multiway_merge_minimal_n(1000), 310 multiway_merge_minimal_k(2), multiway_merge_oversampling(10), 311 nth_element_minimal_n(1000), 312 partition_chunk_size(1000), 313 partition_chunk_share(0.0), 314 partition_minimal_n(1000), 315 partial_sort_minimal_n(1000), 316 partial_sum_dilation(1.0f), 317 partial_sum_minimal_n(1000), 318 random_shuffle_minimal_n(1000), 319 replace_minimal_n(1000), 320 set_difference_minimal_n(1000), 321 set_intersection_minimal_n(1000), 322 set_symmetric_difference_minimal_n(1000), 323 set_union_minimal_n(1000), 324 sort_minimal_n(1000), 325 sort_mwms_oversampling(10), 326 sort_qs_num_samples_preset(100), 327 sort_qsb_base_case_maximal_n(100), 328 transform_minimal_n(1000), 329 unique_copy_minimal_n(10000), 330 workstealing_chunk_size(100), 331 L1_cache_size(16 << 10), 332 L2_cache_size(256 << 10), 333 TLB_size(128), 334 cache_line_size(64), 335 qsb_steals(0), 336 search_minimal_n(1000), 337 find_scale_factor(0.01f) 338 { } 339 }; 340 } 341 342 #endif /* _GLIBCXX_PARALLEL_SETTINGS_H */ 343