1 // -*- C++ -*-
2
3 // Copyright (C) 2007-2023 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
286 _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 */