libstdc++
settings.h
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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) \
96  (__gnu_parallel::_Settings::get().algorithm_strategy \
97  != __gnu_parallel::force_sequential \
98  && ((__gnu_parallel::__get_max_threads() > 1 && (__c)) \
99  || __gnu_parallel::_Settings::get().algorithm_strategy \
100  == __gnu_parallel::force_parallel))
101 
102 /*
103 inline bool
104 parallel_condition(bool __c)
105 {
106  bool ret = false;
107  const _Settings& __s = _Settings::get();
108  if (__s.algorithm_strategy != force_seqential)
109  {
110  if (__s.algorithm_strategy == force_parallel)
111  ret = true;
112  else
113  ret = __get_max_threads() > 1 && __c;
114  }
115  return ret;
116 }
117 */
118 
119 namespace __gnu_parallel
120 {
121  /// class _Settings
122  /// Run-time settings for the parallel mode including all tunable parameters.
123  struct _Settings
124  {
125  _AlgorithmStrategy algorithm_strategy;
126 
127  _SortAlgorithm sort_algorithm;
128  _PartialSumAlgorithm partial_sum_algorithm;
129  _MultiwayMergeAlgorithm multiway_merge_algorithm;
130  _FindAlgorithm find_algorithm;
131 
132  _SplittingAlgorithm sort_splitting;
133  _SplittingAlgorithm merge_splitting;
134  _SplittingAlgorithm multiway_merge_splitting;
135 
136  // Per-algorithm settings.
137 
138  /// Minimal input size for accumulate.
140 
141  /// Minimal input size for adjacent_difference.
143 
144  /// Minimal input size for count and count_if.
146 
147  /// Minimal input size for fill.
149 
150  /// Block size increase factor for find.
152 
153  /// Initial block size for find.
155 
156  /// Maximal block size for find.
158 
159  /// Start with looking for this many elements sequentially, for find.
161 
162  /// Minimal input size for for_each.
164 
165  /// Minimal input size for generate.
167 
168  /// Minimal input size for max_element.
170 
171  /// Minimal input size for merge.
173 
174  /// Oversampling factor for merge.
175  unsigned int merge_oversampling;
176 
177  /// Minimal input size for min_element.
179 
180  /// Minimal input size for multiway_merge.
182 
183  /// Oversampling factor for multiway_merge.
185 
186  /// Oversampling factor for multiway_merge.
188 
189  /// Minimal input size for nth_element.
191 
192  /// Chunk size for partition.
194 
195  /// Chunk size for partition, relative to input size. If > 0.0,
196  /// this value overrides partition_chunk_size.
198 
199  /// Minimal input size for partition.
201 
202  /// Minimal input size for partial_sort.
204 
205  /// Ratio for partial_sum. Assume "sum and write result" to be
206  /// this factor slower than just "sum".
208 
209  /// Minimal input size for partial_sum.
210  unsigned int partial_sum_minimal_n;
211 
212  /// Minimal input size for random_shuffle.
214 
215  /// Minimal input size for replace and replace_if.
217 
218  /// Minimal input size for set_difference.
220 
221  /// Minimal input size for set_intersection.
223 
224  /// Minimal input size for set_symmetric_difference.
226 
227  /// Minimal input size for set_union.
229 
230  /// Minimal input size for parallel sorting.
232 
233  /// Oversampling factor for parallel std::sort (MWMS).
235 
236  /// Such many samples to take to find a good pivot (quicksort).
238 
239  /// Maximal subsequence __length to switch to unbalanced __base case.
240  /// Applies to std::sort with dynamically load-balanced quicksort.
242 
243  /// Minimal input size for parallel std::transform.
245 
246  /// Minimal input size for unique_copy.
248 
249  _SequenceIndex workstealing_chunk_size;
250 
251  // Hardware dependent tuning parameters.
252 
253  /// size of the L1 cache in bytes (underestimation).
254  unsigned long long L1_cache_size;
255 
256  /// size of the L2 cache in bytes (underestimation).
257  unsigned long long L2_cache_size;
258 
259  /// size of the Translation Lookaside Buffer (underestimation).
260  unsigned int TLB_size;
261 
262  /// Overestimation of cache line size. Used to avoid false
263  /// sharing, i.e. elements of different threads are at least this
264  /// amount apart.
265  unsigned int cache_line_size;
266 
267  // Statistics.
268 
269  /// The number of stolen ranges in load-balanced quicksort.
271 
272  /// Minimal input size for search and search_n.
274 
275  /// Block size scale-down factor with respect to current position.
277 
278  /// Get the global settings.
279  _GLIBCXX_CONST static const _Settings&
280  get() throw();
281 
282  /// Set the global settings.
283  static void
284  set(_Settings&) throw();
285 
286  explicit
287  _Settings() :
288  algorithm_strategy(heuristic),
289  sort_algorithm(MWMS),
290  partial_sum_algorithm(LINEAR),
291  multiway_merge_algorithm(LOSER_TREE),
292  find_algorithm(CONSTANT_SIZE_BLOCKS),
293  sort_splitting(EXACT),
294  merge_splitting(EXACT),
295  multiway_merge_splitting(EXACT),
296  accumulate_minimal_n(1000),
298  count_minimal_n(1000),
299  fill_minimal_n(1000),
304  for_each_minimal_n(1000),
305  generate_minimal_n(1000),
306  max_element_minimal_n(1000),
307  merge_minimal_n(1000),
308  merge_oversampling(10),
309  min_element_minimal_n(1000),
312  nth_element_minimal_n(1000),
313  partition_chunk_size(1000),
315  partition_minimal_n(1000),
317  partial_sum_dilation(1.0f),
318  partial_sum_minimal_n(1000),
320  replace_minimal_n(1000),
324  set_union_minimal_n(1000),
325  sort_minimal_n(1000),
329  transform_minimal_n(1000),
330  unique_copy_minimal_n(10000),
331  workstealing_chunk_size(100),
332  L1_cache_size(16 << 10),
333  L2_cache_size(256 << 10),
334  TLB_size(128),
335  cache_line_size(64),
336  qsb_steals(0),
337  search_minimal_n(1000),
338  find_scale_factor(0.01f)
339  { }
340  };
341 }
342 
343 #endif /* _GLIBCXX_PARALLEL_SETTINGS_H */
unsigned long long L2_cache_size
size of the L2 cache in bytes (underestimation).
Definition: settings.h:257
_SequenceIndex find_sequential_search_size
Start with looking for this many elements sequentially, for find.
Definition: settings.h:160
_SequenceIndex transform_minimal_n
Minimal input size for parallel std::transform.
Definition: settings.h:244
_SequenceIndex find_initial_block_size
Initial block size for find.
Definition: settings.h:154
_SequenceIndex set_union_minimal_n
Minimal input size for set_union.
Definition: settings.h:228
class _Settings Run-time settings for the parallel mode including all tunable parameters.
Definition: settings.h:123
float partial_sum_dilation
Ratio for partial_sum. Assume "sum and write result" to be this factor slower than just "sum"...
Definition: settings.h:207
_SplittingAlgorithm
Sorting/merging algorithms: sampling, __exact.
Definition: types.h:98
_SequenceIndex generate_minimal_n
Minimal input size for generate.
Definition: settings.h:166
_SequenceIndex for_each_minimal_n
Minimal input size for for_each.
Definition: settings.h:163
_SequenceIndex set_difference_minimal_n
Minimal input size for set_difference.
Definition: settings.h:219
unsigned int sort_mwms_oversampling
Oversampling factor for parallel std::sort (MWMS).
Definition: settings.h:234
_SequenceIndex set_symmetric_difference_minimal_n
Minimal input size for set_symmetric_difference.
Definition: settings.h:225
_SequenceIndex merge_minimal_n
Minimal input size for merge.
Definition: settings.h:172
unsigned int random_shuffle_minimal_n
Minimal input size for random_shuffle.
Definition: settings.h:213
_SequenceIndex accumulate_minimal_n
Minimal input size for accumulate.
Definition: settings.h:139
unsigned int cache_line_size
Overestimation of cache line size. Used to avoid false sharing, i.e. elements of different threads ar...
Definition: settings.h:265
_SequenceIndex search_minimal_n
Minimal input size for search and search_n.
Definition: settings.h:273
unsigned int multiway_merge_oversampling
Oversampling factor for multiway_merge.
Definition: settings.h:187
static void set(_Settings &)
Set the global settings.
int multiway_merge_minimal_k
Oversampling factor for multiway_merge.
Definition: settings.h:184
_SequenceIndex set_intersection_minimal_n
Minimal input size for set_intersection.
Definition: settings.h:222
_SequenceIndex qsb_steals
The number of stolen ranges in load-balanced quicksort.
Definition: settings.h:270
Basic types and typedefs. This file is a GNU parallel extension to the Standard C++ Library...
_SequenceIndex find_maximum_block_size
Maximal block size for find.
Definition: settings.h:157
_SequenceIndex fill_minimal_n
Minimal input size for fill.
Definition: settings.h:148
_SequenceIndex count_minimal_n
Minimal input size for count and count_if.
Definition: settings.h:145
_SequenceIndex multiway_merge_minimal_n
Minimal input size for multiway_merge.
Definition: settings.h:181
_SequenceIndex partition_minimal_n
Minimal input size for partition.
Definition: settings.h:200
_SequenceIndex partial_sort_minimal_n
Minimal input size for partial_sort.
Definition: settings.h:203
_AlgorithmStrategy
Strategies for run-time algorithm selection:
Definition: types.h:67
_SequenceIndex unique_copy_minimal_n
Minimal input size for unique_copy.
Definition: settings.h:247
_SequenceIndex sort_qsb_base_case_maximal_n
Maximal subsequence __length to switch to unbalanced __base case. Applies to std::sort with dynamical...
Definition: settings.h:241
unsigned int adjacent_difference_minimal_n
Minimal input size for adjacent_difference.
Definition: settings.h:142
double find_increasing_factor
Block size increase factor for find.
Definition: settings.h:151
unsigned int TLB_size
size of the Translation Lookaside Buffer (underestimation).
Definition: settings.h:260
_MultiwayMergeAlgorithm
Merging algorithms:
Definition: types.h:85
_SequenceIndex min_element_minimal_n
Minimal input size for min_element.
Definition: settings.h:178
_SequenceIndex sort_minimal_n
Minimal input size for parallel sorting.
Definition: settings.h:231
_SequenceIndex nth_element_minimal_n
Minimal input size for nth_element.
Definition: settings.h:190
double partition_chunk_share
Chunk size for partition, relative to input size. If > 0.0, this value overrides partition_chunk_size...
Definition: settings.h:197
_SequenceIndex replace_minimal_n
Minimal input size for replace and replace_if.
Definition: settings.h:216
uint64_t _SequenceIndex
Unsigned integer to index __elements. The total number of elements for each algorithm must fit into t...
Definition: types.h:117
unsigned int partial_sum_minimal_n
Minimal input size for partial_sum.
Definition: settings.h:210
unsigned int sort_qs_num_samples_preset
Such many samples to take to find a good pivot (quicksort).
Definition: settings.h:237
_SequenceIndex max_element_minimal_n
Minimal input size for max_element.
Definition: settings.h:169
_FindAlgorithm
Find algorithms:
Definition: types.h:106
float find_scale_factor
Block size scale-down factor with respect to current position.
Definition: settings.h:276
unsigned long long L1_cache_size
size of the L1 cache in bytes (underestimation).
Definition: settings.h:254
unsigned int merge_oversampling
Oversampling factor for merge.
Definition: settings.h:175
_SequenceIndex partition_chunk_size
Chunk size for partition.
Definition: settings.h:193
_PartialSumAlgorithm
Partial sum algorithms: recursive, linear.
Definition: types.h:91
_SortAlgorithm
Sorting algorithms:
Definition: types.h:76