2 This file is a part of libcds - Concurrent Data Structures library
4 (C) Copyright Maxim Khizhinsky (libcds.dev@gmail.com) 2006-2016
6 Source code repo: http://github.com/khizmax/libcds/
7 Download: http://sourceforge.net/projects/libcds/files/
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31 #ifndef CDSLIB_CONTAINER_IMPL_FELDMAN_HASHSET_H
32 #define CDSLIB_CONTAINER_IMPL_FELDMAN_HASHSET_H
34 #include <cds/intrusive/impl/feldman_hashset.h>
35 #include <cds/container/details/feldman_hashset_base.h>
37 namespace cds { namespace container {
39 /// Hash set based on multi-level array
40 /** @ingroup cds_nonintrusive_set
41 @anchor cds_container_FeldmanHashSet_hp
44 - [2013] Steven Feldman, Pierre LaBorde, Damian Dechev "Concurrent Multi-level Arrays:
45 Wait-free Extensible Hash Maps"
47 [From the paper] The hardest problem encountered while developing a parallel hash map is how to perform
48 a global resize, the process of redistributing the elements in a hash map that occurs when adding new
49 buckets. The negative impact of blocking synchronization is multiplied during a global resize, because all
50 threads will be forced to wait on the thread that is performing the involved process of resizing the hash map
51 and redistributing the elements. \p %FeldmanHashSet implementation avoids global resizes through new array
52 allocation. By allowing concurrent expansion this structure is free from the overhead of an explicit resize,
53 which facilitates concurrent operations.
55 The presented design includes dynamic hashing, the use of sub-arrays within the hash map data structure;
56 which, in combination with <b>perfect hashing</b>, means that each element has a unique final, as well as current, position.
57 It is important to note that the perfect hash function required by our hash map is trivial to realize as
58 any hash function that permutes the bits of the key is suitable. This is possible because of our approach
59 to the hash function; we require that it produces hash values that are equal in size to that of the key.
60 We know that if we expand the hash map a fixed number of times there can be no collision as duplicate keys
61 are not provided for in the standard semantics of a hash map.
63 \p %FeldmanHashSet is a multi-level array which has an internal structure similar to a tree:
64 @image html feldman_hashset.png
65 The multi-level array differs from a tree in that each position on the tree could hold an array of nodes or a single node.
66 A position that holds a single node is a \p dataNode which holds the hash value of a key and the value that is associated
67 with that key; it is a simple struct holding two variables. A \p dataNode in the multi-level array could be marked.
68 A \p markedDataNode refers to a pointer to a \p dataNode that has been bitmarked at the least significant bit (LSB)
69 of the pointer to the node. This signifies that this \p dataNode is contended. An expansion must occur at this node;
70 any thread that sees this \p markedDataNode will try to replace it with an \p arrayNode; which is a position that holds
71 an array of nodes. The pointer to an \p arrayNode is differentiated from that of a pointer to a \p dataNode by a bitmark
72 on the second-least significant bit.
74 \p %FeldmanHashSet multi-level array is similar to a tree in that we keep a pointer to the root, which is a memory array
75 called \p head. The length of the \p head memory array is unique, whereas every other \p arrayNode has a uniform length;
76 a normal \p arrayNode has a fixed power-of-two length equal to the binary logarithm of a variable called \p arrayLength.
77 The maximum depth of the tree, \p maxDepth, is the maximum number of pointers that must be followed to reach any node.
78 We define \p currentDepth as the number of memory arrays that we need to traverse to reach the \p arrayNode on which
79 we need to operate; this is initially one, because of \p head.
81 That approach to the structure of the hash set uses an extensible hashing scheme; <b> the hash value is treated as a bit
82 string</b> and rehash incrementally.
84 @note Two important things you should keep in mind when you're using \p %FeldmanHashSet:
85 - all keys must be fixed-size. It means that you cannot use \p std::string as a key for \p %FeldmanHashSet.
86 Instead, for the strings you should use well-known hashing algorithms like <a href="https://en.wikipedia.org/wiki/Secure_Hash_Algorithm">SHA1, SHA2</a>,
87 <a href="https://en.wikipedia.org/wiki/MurmurHash">MurmurHash</a>, <a href="https://en.wikipedia.org/wiki/CityHash">CityHash</a>
88 or its successor <a href="https://code.google.com/p/farmhash/">FarmHash</a> and so on, which
89 converts variable-length strings to fixed-length bit-strings, and use that hash as a key in \p %FeldmanHashSet.
90 - \p %FeldmanHashSet uses a perfect hashing. It means that if two different keys, for example, of type \p std::string,
91 have identical hash then you cannot insert both that keys in the set. \p %FeldmanHashSet does not maintain the key,
92 it maintains its fixed-size hash value.
94 The set supports @ref cds_container_FeldmanHashSet_iterators "bidirectional thread-safe iterators".
97 - \p GC - safe memory reclamation schema. Can be \p gc::HP, \p gc::DHP or one of \ref cds_urcu_type "RCU type"
98 - \p T - a value type to be stored in the set
99 - \p Traits - type traits, the structure based on \p feldman_hashset::traits or result of \p feldman_hashset::make_traits metafunction.
100 \p Traits is the mandatory argument because it has one mandatory type - an @ref feldman_hashset::traits::hash_accessor "accessor"
101 to hash value of \p T. The set algorithm does not calculate that hash value.
103 There are several specializations of \p %FeldmanHashSet for each \p GC. You should include:
104 - <tt><cds/container/feldman_hashset_hp.h></tt> for \p gc::HP garbage collector
105 - <tt><cds/container/feldman_hashset_dhp.h></tt> for \p gc::DHP garbage collector
106 - <tt><cds/container/feldman_hashset_rcu.h></tt> for \ref cds_intrusive_FeldmanHashSet_rcu "RCU type". RCU specialization
107 has a slightly different interface.
112 #ifdef CDS_DOXYGEN_INVOKED
113 , class Traits = feldman_hashset::traits
119 #ifdef CDS_DOXYGEN_INVOKED
120 : protected cds::intrusive::FeldmanHashSet< GC, T, Traits >
122 : protected cds::container::details::make_feldman_hashset< GC, T, Traits >::type
126 typedef cds::container::details::make_feldman_hashset< GC, T, Traits > maker;
127 typedef typename maker::type base_class;
131 typedef GC gc; ///< Garbage collector
132 typedef T value_type; ///< type of value stored in the set
133 typedef Traits traits; ///< Traits template parameter, see \p feldman_hashset::traits
135 typedef typename base_class::hash_accessor hash_accessor; ///< Hash accessor functor
136 typedef typename base_class::hash_type hash_type; ///< Hash type deduced from \p hash_accessor return type
137 typedef typename base_class::hash_comparator hash_comparator; ///< hash compare functor based on \p opt::compare and \p opt::less option setter
139 typedef typename traits::item_counter item_counter; ///< Item counter type
140 typedef typename traits::allocator allocator; ///< Element allocator
141 typedef typename traits::node_allocator node_allocator; ///< Array node allocator
142 typedef typename traits::memory_model memory_model; ///< Memory model
143 typedef typename traits::back_off back_off; ///< Backoff strategy
144 typedef typename traits::stat stat; ///< Internal statistics type
146 typedef typename gc::template guarded_ptr< value_type > guarded_ptr; ///< Guarded pointer
148 /// Count of hazard pointers required
149 static CDS_CONSTEXPR size_t const c_nHazardPtrCount = base_class::c_nHazardPtrCount;
152 typedef feldman_hashset::level_statistics level_statistics;
156 typedef typename maker::cxx_node_allocator cxx_node_allocator;
157 typedef std::unique_ptr< value_type, typename maker::node_disposer > scoped_node_ptr;
161 ///@name Thread-safe iterators
163 /// Bidirectional iterator
164 /** @anchor cds_container_FeldmanHashSet_iterators
165 The set supports thread-safe iterators: you may iterate over the set in multi-threaded environment.
166 It is guaranteed that the iterators will remain valid even if another thread deletes the node the iterator points to:
167 Hazard Pointer embedded into the iterator object protects the node from physical reclamation.
169 @note Since the iterator object contains hazard pointer that is a thread-local resource,
170 the iterator should not be passed to another thread.
172 Each iterator object supports the following interface:
173 - dereference operators:
175 value_type [const] * operator ->() noexcept
176 value_type [const] & operator *() noexcept
178 - pre-increment and pre-decrement. Post-operators is not supported
179 - equality operators <tt>==</tt> and <tt>!=</tt>.
180 Iterators are equal iff they point to the same cell of the same array node.
181 Note that for two iterators \p it1 and \p it2, the conditon <tt> it1 == it2 </tt>
182 does not entail <tt> &(*it1) == &(*it2) </tt>
183 - helper member function \p release() that clears internal hazard pointer.
184 After \p release() the iterator points to \p nullptr but it still remain valid: further iterating is possible.
186 During iteration you may safely erase any item from the set;
187 @ref erase_at() function call doesn't invalidate any iterator.
188 If some iterator points to the item to be erased, that item is not deleted immediately
189 but only after that iterator will be advanced forward or backward.
191 @note It is possible the item can be iterated more that once, for example, if an iterator points to the item
192 in array node that is being splitted.
194 typedef typename base_class::iterator iterator;
195 typedef typename base_class::const_iterator const_iterator; ///< @ref cds_container_FeldmanHashSet_iterators "bidirectional const iterator" type
196 typedef typename base_class::reverse_iterator reverse_iterator; ///< @ref cds_container_FeldmanHashSet_iterators "bidirectional reverse iterator" type
197 typedef typename base_class::const_reverse_iterator const_reverse_iterator; ///< @ref cds_container_FeldmanHashSet_iterators "bidirectional reverse const iterator" type
199 /// Returns an iterator to the beginning of the set
202 return base_class::begin();
205 /// Returns an const iterator to the beginning of the set
206 const_iterator begin() const
208 return base_class::begin();
211 /// Returns an const iterator to the beginning of the set
212 const_iterator cbegin()
214 return base_class::cbegin();
217 /// Returns an iterator to the element following the last element of the set. This element acts as a placeholder; attempting to access it results in undefined behavior.
220 return base_class::end();
223 /// Returns a const iterator to the element following the last element of the set. This element acts as a placeholder; attempting to access it results in undefined behavior.
224 const_iterator end() const
226 return base_class::end();
229 /// Returns a const iterator to the element following the last element of the set. This element acts as a placeholder; attempting to access it results in undefined behavior.
230 const_iterator cend()
232 return base_class::cend();
235 /// Returns a reverse iterator to the first element of the reversed set
236 reverse_iterator rbegin()
238 return base_class::rbegin();
241 /// Returns a const reverse iterator to the first element of the reversed set
242 const_reverse_iterator rbegin() const
244 return base_class::rbegin();
247 /// Returns a const reverse iterator to the first element of the reversed set
248 const_reverse_iterator crbegin()
250 return base_class::crbegin();
253 /// Returns a reverse iterator to the element following the last element of the reversed set
255 It corresponds to the element preceding the first element of the non-reversed container.
256 This element acts as a placeholder, attempting to access it results in undefined behavior.
258 reverse_iterator rend()
260 return base_class::rend();
263 /// Returns a const reverse iterator to the element following the last element of the reversed set
265 It corresponds to the element preceding the first element of the non-reversed container.
266 This element acts as a placeholder, attempting to access it results in undefined behavior.
268 const_reverse_iterator rend() const
270 return base_class::rend();
273 /// Returns a const reverse iterator to the element following the last element of the reversed set
275 It corresponds to the element preceding the first element of the non-reversed container.
276 This element acts as a placeholder, attempting to access it results in undefined behavior.
278 const_reverse_iterator crend()
280 return base_class::crend();
285 /// Creates empty set
287 @param head_bits: 2<sup>head_bits</sup> specifies the size of head array, minimum is 4.
288 @param array_bits: 2<sup>array_bits</sup> specifies the size of array node, minimum is 2.
290 Equation for \p head_bits and \p array_bits:
292 sizeof(hash_type) * 8 == head_bits + N * array_bits
294 where \p N is multi-level array depth.
296 FeldmanHashSet( size_t head_bits = 8, size_t array_bits = 4 )
297 : base_class( head_bits, array_bits )
300 /// Destructs the set and frees all data
304 /// Inserts new element
306 The function creates an element with copy of \p val value and then inserts it into the set.
308 The type \p Q should contain as minimum the complete hash for the element.
309 The object of \ref value_type should be constructible from a value of type \p Q.
310 In trivial case, \p Q is equal to \ref value_type.
312 Returns \p true if \p val is inserted into the set, \p false otherwise.
314 template <typename Q>
315 bool insert( Q const& val )
317 scoped_node_ptr sp( cxx_node_allocator().New( val ));
318 if ( base_class::insert( *sp )) {
325 /// Inserts new element
327 The function allows to split creating of new item into two part:
328 - create item with key only
329 - insert new item into the set
330 - if inserting is success, calls \p f functor to initialize value-fields of \p val.
332 The functor signature is:
334 void func( value_type& val );
336 where \p val is the item inserted. User-defined functor \p f should guarantee that during changing
337 \p val no any other changes could be made on this set's item by concurrent threads.
338 The user-defined functor is called only if the inserting is success.
340 template <typename Q, typename Func>
341 bool insert( Q const& val, Func f )
343 scoped_node_ptr sp( cxx_node_allocator().New( val ));
344 if ( base_class::insert( *sp, f )) {
351 /// Updates the element
353 The operation performs inserting or replacing with lock-free manner.
355 If the \p val key not found in the set, then the new item created from \p val
356 will be inserted into the set iff \p bInsert is \p true.
357 Otherwise, if \p val is found, it is replaced with new item created from \p val
358 and previous item is disposed.
359 In both cases \p func functor is called.
361 The functor \p Func signature:
364 void operator()( value_type& cur, value_type * prev );
368 - \p cur - current element
369 - \p prev - pointer to previous element with such hash. \p prev is \p nullptr
370 if \p cur was just inserted.
372 The functor may change non-key fields of the \p item; however, \p func must guarantee
373 that during changing no any other modifications could be made on this item by concurrent threads.
375 Returns <tt> std::pair<bool, bool> </tt> where \p first is \p true if operation is successful,
376 i.e. the item has been inserted or updated,
377 \p second is \p true if the new item has been added or \p false if the item with key equal to \p val
380 template <typename Q, typename Func>
381 std::pair<bool, bool> update( Q const& val, Func func, bool bInsert = true )
383 scoped_node_ptr sp( cxx_node_allocator().New( val ));
384 std::pair<bool, bool> bRes = base_class::do_update( *sp, func, bInsert );
390 /// Inserts data of type \p value_type created in-place from <tt>std::forward<Args>(args)...</tt>
392 Returns \p true if inserting successful, \p false otherwise.
394 template <typename... Args>
395 bool emplace( Args&&... args )
397 scoped_node_ptr sp( cxx_node_allocator().MoveNew( std::forward<Args>(args)... ));
398 if ( base_class::insert( *sp )) {
405 /// Deletes the item from the set
407 The function searches \p hash in the set,
408 deletes the item found, and returns \p true.
409 If that item is not found the function returns \p false.
411 bool erase( hash_type const& hash )
413 return base_class::erase( hash );
416 /// Deletes the item from the set
418 The function searches \p hash in the set,
419 call \p f functor with item found, and deltes the element from the set.
421 The \p Func interface is
424 void operator()( value_type& item );
428 If \p hash is not found the function returns \p false.
430 template <typename Func>
431 bool erase( hash_type const& hash, Func f )
433 return base_class::erase( hash, f );
436 /// Deletes the item pointed by iterator \p iter
438 Returns \p true if the operation is successful, \p false otherwise.
440 The function does not invalidate the iterator, it remains valid and can be used for further traversing.
442 bool erase_at( iterator const& iter )
444 return base_class::erase_at( iter );
447 bool erase_at( reverse_iterator const& iter )
449 return base_class::erase_at( iter );
453 /// Extracts the item with specified \p hash
455 The function searches \p hash in the set,
456 unlinks it from the set, and returns a guarded pointer to the item extracted.
457 If \p hash is not found the function returns an empty guarded pointer.
459 The item returned is reclaimed by garbage collector \p GC
460 when returned \ref guarded_ptr object to be destroyed or released.
461 @note Each \p guarded_ptr object uses the GC's guard that can be limited resource.
465 typedef cds::container::FeldmanHashSet< your_template_args > my_set;
469 my_set::guarded_ptr gp( theSet.extract( 5 ));
474 // Destructor of gp releases internal HP guard
478 guarded_ptr extract( hash_type const& hash )
480 return base_class::extract( hash );
483 /// Finds an item by it's \p hash
485 The function searches the item by \p hash and calls the functor \p f for item found.
486 The interface of \p Func functor is:
489 void operator()( value_type& item );
492 where \p item is the item found.
494 The functor may change non-key fields of \p item. Note that the functor is only guarantee
495 that \p item cannot be disposed during the functor is executing.
496 The functor does not serialize simultaneous access to the set's \p item. If such access is
497 possible you must provide your own synchronization schema on item level to prevent unsafe item modifications.
499 The function returns \p true if \p hash is found, \p false otherwise.
501 template <typename Func>
502 bool find( hash_type const& hash, Func f )
504 return base_class::find( hash, f );
507 /// Checks whether the set contains \p hash
509 The function searches the item by its \p hash
510 and returns \p true if it is found, or \p false otherwise.
512 bool contains( hash_type const& hash )
514 return base_class::contains( hash );
517 /// Finds an item by it's \p hash and returns the item found
519 The function searches the item by its \p hash
520 and returns the guarded pointer to the item found.
521 If \p hash is not found the function returns an empty \p guarded_ptr.
523 @note Each \p guarded_ptr object uses one GC's guard which can be limited resource.
527 typedef cds::container::FeldmanHashSet< your_template_params > my_set;
531 my_set::guarded_ptr gp( theSet.get( 5 ));
532 if ( theSet.get( 5 )) {
536 // Destructor of guarded_ptr releases internal HP guard
540 guarded_ptr get( hash_type const& hash )
542 return base_class::get( hash );
545 /// Clears the set (non-atomic)
547 The function unlink all data node from the set.
548 The function is not atomic but is thread-safe.
549 After \p %clear() the set may not be empty because another threads may insert items.
556 /// Checks if the set is empty
558 Emptiness is checked by item counting: if item count is zero then the set is empty.
559 Thus, the correct item counting feature is an important part of the set implementation.
563 return base_class::empty();
566 /// Returns item count in the set
569 return base_class::size();
572 /// Returns const reference to internal statistics
573 stat const& statistics() const
575 return base_class::statistics();
578 /// Returns the size of head node
579 size_t head_size() const
581 return base_class::head_size();
584 /// Returns the size of the array node
585 size_t array_node_size() const
587 return base_class::array_node_size();
590 /// Collects tree level statistics into \p stat
592 The function traverses the set and collects statistics for each level of the tree
593 into \p feldman_hashset::level_statistics struct. The element of \p stat[i]
594 represents statistics for level \p i, level 0 is head array.
595 The function is thread-safe and may be called in multi-threaded environment.
597 Result can be useful for estimating efficiency of hash functor you use.
599 void get_level_statistics(std::vector< feldman_hashset::level_statistics>& stat) const
601 base_class::get_level_statistics(stat);
605 }} // namespace cds::container
607 #endif // #ifndef CDSLIB_CONTAINER_IMPL_FELDMAN_HASHSET_H