2 * Copyright 2016 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 * AtomicHashArray is the building block for AtomicHashMap. It provides the
19 * core lock-free functionality, but is limitted by the fact that it cannot
20 * grow past it's initialization size and is a little more awkward (no public
21 * constructor, for example). If you're confident that you won't run out of
22 * space, don't mind the awkardness, and really need bare-metal performance,
23 * feel free to use AHA directly.
25 * Check out AtomicHashMap.h for more thorough documentation on perf and
26 * general pros and cons relative to other hash maps.
28 * @author Spencer Ahrens <sahrens@fb.com>
29 * @author Jordan DeLong <delong.j@fb.com>
33 #define FOLLY_ATOMICHASHARRAY_H_
37 #include <boost/iterator/iterator_facade.hpp>
38 #include <boost/noncopyable.hpp>
40 #include <folly/Hash.h>
41 #include <folly/ThreadCachedInt.h>
45 struct AtomicHashArrayLinearProbeFcn
47 inline size_t operator()(size_t idx,
48 size_t /* numProbes */,
49 size_t capacity) const {
50 idx += 1; // linear probing
52 // Avoid modulus because it's slow
53 return LIKELY(idx < capacity) ? idx : (idx - capacity);
57 struct AtomicHashArrayQuadraticProbeFcn
59 inline size_t operator()(size_t idx, size_t numProbes, size_t capacity) const{
60 idx += numProbes; // quadratic probing
62 // Avoid modulus because it's slow
63 return LIKELY(idx < capacity) ? idx : (idx - capacity);
67 // Enables specializing checkLegalKey without specializing its class.
69 // Local copy of folly::gen::Identity, to avoid heavy dependencies.
73 auto operator()(Value&& value) const ->
74 decltype(std::forward<Value>(value)) {
75 return std::forward<Value>(value);
79 template <typename NotKeyT, typename KeyT>
80 inline void checkLegalKeyIfKeyTImpl(NotKeyT /* ignored */,
83 KeyT /* erasedKey */) {}
85 template <typename KeyT>
86 inline void checkLegalKeyIfKeyTImpl(KeyT key_in, KeyT emptyKey,
87 KeyT lockedKey, KeyT erasedKey) {
88 DCHECK_NE(key_in, emptyKey);
89 DCHECK_NE(key_in, lockedKey);
90 DCHECK_NE(key_in, erasedKey);
94 template <class KeyT, class ValueT,
95 class HashFcn = std::hash<KeyT>,
96 class EqualFcn = std::equal_to<KeyT>,
97 class Allocator = std::allocator<char>,
98 class ProbeFcn = AtomicHashArrayLinearProbeFcn,
99 class KeyConvertFcn = detail::AHAIdentity>
102 template <class KeyT, class ValueT,
103 class HashFcn = std::hash<KeyT>,
104 class EqualFcn = std::equal_to<KeyT>,
105 class Allocator = std::allocator<char>,
106 class ProbeFcn = AtomicHashArrayLinearProbeFcn,
107 class KeyConvertFcn = detail::AHAIdentity>
108 class AtomicHashArray : boost::noncopyable {
109 static_assert((std::is_convertible<KeyT,int32_t>::value ||
110 std::is_convertible<KeyT,int64_t>::value ||
111 std::is_convertible<KeyT,const void*>::value),
112 "You are trying to use AtomicHashArray with disallowed key "
113 "types. You must use atomically compare-and-swappable integer "
114 "keys, or a different container class.");
116 typedef KeyT key_type;
117 typedef ValueT mapped_type;
118 typedef HashFcn hasher;
119 typedef EqualFcn key_equal;
120 typedef KeyConvertFcn key_convert;
121 typedef std::pair<const KeyT, ValueT> value_type;
122 typedef std::size_t size_type;
123 typedef std::ptrdiff_t difference_type;
124 typedef value_type& reference;
125 typedef const value_type& const_reference;
126 typedef value_type* pointer;
127 typedef const value_type* const_pointer;
129 const size_t capacity_;
130 const size_t maxEntries_;
131 const KeyT kEmptyKey_;
132 const KeyT kLockedKey_;
133 const KeyT kErasedKey_;
135 template<class ContT, class IterVal>
138 typedef aha_iterator<const AtomicHashArray,const value_type> const_iterator;
139 typedef aha_iterator<AtomicHashArray,value_type> iterator;
141 // You really shouldn't need this if you use the SmartPtr provided by create,
142 // but if you really want to do something crazy like stick the released
143 // pointer into a DescriminatedPtr or something, you'll need this to clean up
145 static void destroy(AtomicHashArray*);
148 const size_t kAnchorMask_;
151 void operator()(AtomicHashArray* ptr) {
152 AtomicHashArray::destroy(ptr);
157 typedef std::unique_ptr<AtomicHashArray, Deleter> SmartPtr;
162 * Creates AtomicHashArray objects. Use instead of constructor/destructor.
164 * We do things this way in order to avoid the perf penalty of a second
165 * pointer indirection when composing these into AtomicHashMap, which needs
166 * to store an array of pointers so that it can perform atomic operations on
169 * Instead of a mess of arguments, we take a max size and a Config struct to
170 * simulate named ctor parameters. The Config struct has sensible defaults
171 * for everything, but is overloaded - if you specify a positive capacity,
172 * that will be used directly instead of computing it based on
175 * Create returns an AHA::SmartPtr which is a unique_ptr with a custom
176 * deleter to make sure everything is cleaned up properly.
182 double maxLoadFactor;
184 int entryCountThreadCacheSize;
185 size_t capacity; // if positive, overrides maxLoadFactor
188 // Cannot have constexpr ctor because some compilers rightly complain.
189 Config() : emptyKey((KeyT)-1),
194 entryCountThreadCacheSize(1000),
198 // Cannot have pre-instantiated const Config instance because of SIOF.
199 static SmartPtr create(size_t maxSize, const Config& c = Config());
205 * Returns the iterator to the element if found, otherwise end().
207 * As an optional feature, the type of the key to look up (LookupKeyT) is
208 * allowed to be different from the type of keys actually stored (KeyT).
210 * This enables use cases where materializing the key is costly and usually
211 * redudant, e.g., canonicalizing/interning a set of strings and being able
212 * to look up by StringPiece. To use this feature, LookupHashFcn must take
213 * a LookupKeyT, and LookupEqualFcn must take KeyT and LookupKeyT as first
214 * and second parameter, respectively.
216 * See folly/test/ArrayHashArrayTest.cpp for sample usage.
218 template <typename LookupKeyT = key_type,
219 typename LookupHashFcn = hasher,
220 typename LookupEqualFcn = key_equal>
221 iterator find(LookupKeyT k) {
222 return iterator(this,
223 findInternal<LookupKeyT, LookupHashFcn, LookupEqualFcn>(k).idx);
226 template <typename LookupKeyT = key_type,
227 typename LookupHashFcn = hasher,
228 typename LookupEqualFcn = key_equal>
229 const_iterator find(LookupKeyT k) const {
230 return const_cast<AtomicHashArray*>(this)->
231 find<LookupKeyT, LookupHashFcn, LookupEqualFcn>(k);
237 * Returns a pair with iterator to the element at r.first and bool success.
238 * Retrieve the index with ret.first.getIndex().
240 * Fails on key collision (does not overwrite) or if map becomes
241 * full, at which point no element is inserted, iterator is set to end(),
242 * and success is set false. On collisions, success is set false, but the
243 * iterator is set to the existing entry.
245 std::pair<iterator,bool> insert(const value_type& r) {
246 return emplace(r.first, r.second);
248 std::pair<iterator,bool> insert(value_type&& r) {
249 return emplace(r.first, std::move(r.second));
255 * Same contract as insert(), but performs in-place construction
256 * of the value type using the specified arguments.
258 * Also, like find(), this method optionally allows 'key_in' to have a type
259 * different from that stored in the table; see find(). If and only if no
260 * equal key is already present, this method converts 'key_in' to a key of
261 * type KeyT using the provided LookupKeyToKeyFcn.
263 template <typename LookupKeyT = key_type,
264 typename LookupHashFcn = hasher,
265 typename LookupEqualFcn = key_equal,
266 typename LookupKeyToKeyFcn = key_convert,
268 std::pair<iterator,bool> emplace(LookupKeyT key_in, ArgTs&&... vCtorArgs) {
269 SimpleRetT ret = insertInternal<LookupKeyT,
274 std::forward<ArgTs>(vCtorArgs)...);
275 return std::make_pair(iterator(this, ret.idx), ret.success);
278 // returns the number of elements erased - should never exceed 1
279 size_t erase(KeyT k);
281 // clears all keys and values in the map and resets all counters. Not thread
285 // Exact number of elements in the map - note that readFull() acquires a
286 // mutex. See folly/ThreadCachedInt.h for more details.
287 size_t size() const {
288 return numEntries_.readFull() -
289 numErases_.load(std::memory_order_relaxed);
292 bool empty() const { return size() == 0; }
295 iterator it(this, 0);
296 it.advancePastEmpty();
299 const_iterator begin() const {
300 const_iterator it(this, 0);
301 it.advancePastEmpty();
305 iterator end() { return iterator(this, capacity_); }
306 const_iterator end() const { return const_iterator(this, capacity_); }
308 // See AtomicHashMap::findAt - access elements directly
309 // WARNING: The following 2 functions will fail silently for hashtable
310 // with capacity > 2^32
311 iterator findAt(uint32_t idx) {
312 DCHECK_LT(idx, capacity_);
313 return iterator(this, idx);
315 const_iterator findAt(uint32_t idx) const {
316 return const_cast<AtomicHashArray*>(this)->findAt(idx);
319 iterator makeIter(size_t idx) { return iterator(this, idx); }
320 const_iterator makeIter(size_t idx) const {
321 return const_iterator(this, idx);
324 // The max load factor allowed for this map
325 double maxLoadFactor() const { return ((double) maxEntries_) / capacity_; }
327 void setEntryCountThreadCacheSize(uint32_t newSize) {
328 numEntries_.setCacheSize(newSize);
329 numPendingEntries_.setCacheSize(newSize);
332 int getEntryCountThreadCacheSize() const {
333 return numEntries_.getCacheSize();
336 /* Private data and helper functions... */
339 friend class AtomicHashMap<KeyT,
346 struct SimpleRetT { size_t idx; bool success;
347 SimpleRetT(size_t i, bool s) : idx(i), success(s) {}
348 SimpleRetT() = default;
353 template <typename LookupKeyT = key_type,
354 typename LookupHashFcn = hasher,
355 typename LookupEqualFcn = key_equal,
356 typename LookupKeyToKeyFcn = detail::AHAIdentity,
358 SimpleRetT insertInternal(LookupKeyT key, ArgTs&&... vCtorArgs);
360 template <typename LookupKeyT = key_type,
361 typename LookupHashFcn = hasher,
362 typename LookupEqualFcn = key_equal>
363 SimpleRetT findInternal(const LookupKeyT key);
365 template <typename MaybeKeyT>
366 void checkLegalKeyIfKey(MaybeKeyT key) {
367 detail::checkLegalKeyIfKeyTImpl(key, kEmptyKey_, kLockedKey_, kErasedKey_);
370 static std::atomic<KeyT>* cellKeyPtr(const value_type& r) {
371 // We need some illegal casting here in order to actually store
372 // our value_type as a std::pair<const,>. But a little bit of
373 // undefined behavior never hurt anyone ...
374 static_assert(sizeof(std::atomic<KeyT>) == sizeof(KeyT),
375 "std::atomic is implemented in an unexpected way for AHM");
377 const_cast<std::atomic<KeyT>*>(
378 reinterpret_cast<std::atomic<KeyT> const*>(&r.first));
381 static KeyT relaxedLoadKey(const value_type& r) {
382 return cellKeyPtr(r)->load(std::memory_order_relaxed);
385 static KeyT acquireLoadKey(const value_type& r) {
386 return cellKeyPtr(r)->load(std::memory_order_acquire);
389 // Fun with thread local storage - atomic increment is expensive
390 // (relatively), so we accumulate in the thread cache and periodically
391 // flush to the actual variable, and walk through the unflushed counts when
392 // reading the value, so be careful of calling size() too frequently. This
393 // increases insertion throughput several times over while keeping the count
395 ThreadCachedInt<uint64_t> numEntries_; // Successful key inserts
396 ThreadCachedInt<uint64_t> numPendingEntries_; // Used by insertInternal
397 std::atomic<int64_t> isFull_; // Used by insertInternal
398 std::atomic<int64_t> numErases_; // Successful key erases
400 value_type cells_[0]; // This must be the last field of this class
402 // Force constructor/destructor private since create/destroy should be
403 // used externally instead
404 AtomicHashArray(size_t capacity, KeyT emptyKey, KeyT lockedKey,
405 KeyT erasedKey, double maxLoadFactor, size_t cacheSize);
407 ~AtomicHashArray() = default;
409 inline void unlockCell(value_type* const cell, KeyT newKey) {
410 cellKeyPtr(*cell)->store(newKey, std::memory_order_release);
413 inline bool tryLockCell(value_type* const cell) {
414 KeyT expect = kEmptyKey_;
415 return cellKeyPtr(*cell)->compare_exchange_strong(expect, kLockedKey_,
416 std::memory_order_acq_rel);
419 template <class LookupKeyT = key_type, class LookupHashFcn = hasher>
420 inline size_t keyToAnchorIdx(const LookupKeyT k) const {
421 const size_t hashVal = LookupHashFcn()(k);
422 const size_t probe = hashVal & kAnchorMask_;
423 return LIKELY(probe < capacity_) ? probe : hashVal % capacity_;
427 }; // AtomicHashArray
431 #include <folly/AtomicHashArray-inl.h>