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/*
* Copyright 2012 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#ifndef FOLLY_ATOMICHASHARRAY_H_
#error "This should only be included by AtomicHashArray.h"
#endif
#include "folly/Bits.h"
#include "folly/detail/AtomicHashUtils.h"
namespace folly {
// AtomicHashArray private constructor --
template <class KeyT, class ValueT, class HashFcn>
AtomicHashArray<KeyT, ValueT, HashFcn>::
AtomicHashArray(size_t capacity, KeyT emptyKey, KeyT lockedKey,
KeyT erasedKey, double maxLoadFactor, size_t cacheSize)
: capacity_(capacity), maxEntries_(size_t(maxLoadFactor * capacity_ + 0.5)),
kEmptyKey_(emptyKey), kLockedKey_(lockedKey), kErasedKey_(erasedKey),
kAnchorMask_(nextPowTwo(capacity_) - 1), numEntries_(0, cacheSize),
numPendingEntries_(0, cacheSize), isFull_(0), numErases_(0) {
}
/*
* findInternal --
*
* Sets ret.second to value found and ret.index to index
* of key and returns true, or if key does not exist returns false and
* ret.index is set to capacity_.
*/
template <class KeyT, class ValueT, class HashFcn>
typename AtomicHashArray<KeyT, ValueT, HashFcn>::SimpleRetT
AtomicHashArray<KeyT, ValueT, HashFcn>::
findInternal(const KeyT key_in) {
DCHECK_NE(key_in, kEmptyKey_);
DCHECK_NE(key_in, kLockedKey_);
DCHECK_NE(key_in, kErasedKey_);
for (size_t idx = keyToAnchorIdx(key_in), numProbes = 0;
;
idx = probeNext(idx, numProbes)) {
const KeyT key = acquireLoadKey(cells_[idx]);
if (LIKELY(key == key_in)) {
return SimpleRetT(idx, true);
}
if (UNLIKELY(key == kEmptyKey_)) {
// if we hit an empty element, this key does not exist
return SimpleRetT(capacity_, false);
}
++numProbes;
if (UNLIKELY(numProbes >= capacity_)) {
// probed every cell...fail
return SimpleRetT(capacity_, false);
}
}
}
/*
* insertInternal --
*
* Returns false on failure due to key collision or full.
* Also sets ret.index to the index of the key. If the map is full, sets
* ret.index = capacity_. Also sets ret.second to cell value, thus if insert
* successful this will be what we just inserted, if there is a key collision
* this will be the previously inserted value, and if the map is full it is
* default.
*/
template <class KeyT, class ValueT, class HashFcn>
template <class T>
typename AtomicHashArray<KeyT, ValueT, HashFcn>::SimpleRetT
AtomicHashArray<KeyT, ValueT, HashFcn>::
insertInternal(KeyT key_in, T&& value) {
const short NO_NEW_INSERTS = 1;
const short NO_PENDING_INSERTS = 2;
CHECK_NE(key_in, kEmptyKey_);
CHECK_NE(key_in, kLockedKey_);
CHECK_NE(key_in, kErasedKey_);
size_t idx = keyToAnchorIdx(key_in);
size_t numProbes = 0;
for (;;) {
DCHECK_LT(idx, capacity_);
value_type* cell = &cells_[idx];
if (relaxedLoadKey(*cell) == kEmptyKey_) {
// NOTE: isFull_ is set based on numEntries_.readFast(), so it's
// possible to insert more than maxEntries_ entries. However, it's not
// possible to insert past capacity_.
++numPendingEntries_;
if (isFull_.load(std::memory_order_acquire)) {
--numPendingEntries_;
// Before deciding whether this insert succeeded, this thread needs to
// wait until no other thread can add a new entry.
// Correctness assumes isFull_ is true at this point. If
// another thread now does ++numPendingEntries_, we expect it
// to pass the isFull_.load() test above. (It shouldn't insert
// a new entry.)
FOLLY_SPIN_WAIT(
isFull_.load(std::memory_order_acquire) != NO_PENDING_INSERTS
&& numPendingEntries_.readFull() != 0
);
isFull_.store(NO_PENDING_INSERTS, std::memory_order_release);
if (relaxedLoadKey(*cell) == kEmptyKey_) {
// Don't insert past max load factor
return SimpleRetT(capacity_, false);
}
} else {
// An unallocated cell. Try once to lock it. If we succeed, insert here.
// If we fail, fall through to comparison below; maybe the insert that
// just beat us was for this very key....
if (tryLockCell(cell)) {
// Write the value - done before unlocking
try {
DCHECK(relaxedLoadKey(*cell) == kLockedKey_);
/*
* This happens using the copy constructor because we won't have
* constructed a lhs to use an assignment operator on when
* values are being set.
*/
new (&cell->second) ValueT(std::forward<T>(value));
unlockCell(cell, key_in); // Sets the new key
} catch (...) {
// Transition back to empty key---requires handling
// locked->empty below.
unlockCell(cell, kEmptyKey_);
--numPendingEntries_;
throw;
}
DCHECK(relaxedLoadKey(*cell) == key_in);
--numPendingEntries_;
++numEntries_; // This is a thread cached atomic increment :)
if (numEntries_.readFast() >= maxEntries_) {
isFull_.store(NO_NEW_INSERTS, std::memory_order_relaxed);
}
return SimpleRetT(idx, true);
}
--numPendingEntries_;
}
}
DCHECK(relaxedLoadKey(*cell) != kEmptyKey_);
if (kLockedKey_ == acquireLoadKey(*cell)) {
FOLLY_SPIN_WAIT(
kLockedKey_ == acquireLoadKey(*cell)
);
}
const KeyT thisKey = acquireLoadKey(*cell);
if (thisKey == key_in) {
// Found an existing entry for our key, but we don't overwrite the
// previous value.
return SimpleRetT(idx, false);
} else if (thisKey == kEmptyKey_ || thisKey == kLockedKey_) {
// We need to try again (i.e., don't increment numProbes or
// advance idx): this case can happen if the constructor for
// ValueT threw for this very cell (the rethrow block above).
continue;
}
++numProbes;
if (UNLIKELY(numProbes >= capacity_)) {
// probed every cell...fail
return SimpleRetT(capacity_, false);
}
idx = probeNext(idx, numProbes);
}
}
/*
* erase --
*
* This will attempt to erase the given key key_in if the key is found. It
* returns 1 iff the key was located and marked as erased, and 0 otherwise.
*
* Memory is not freed or reclaimed by erase, i.e. the cell containing the
* erased key will never be reused. If there's an associated value, we won't
* touch it either.
*/
template <class KeyT, class ValueT, class HashFcn>
size_t AtomicHashArray<KeyT, ValueT, HashFcn>::
erase(KeyT key_in) {
CHECK_NE(key_in, kEmptyKey_);
CHECK_NE(key_in, kLockedKey_);
CHECK_NE(key_in, kErasedKey_);
for (size_t idx = keyToAnchorIdx(key_in), numProbes = 0;
;
idx = probeNext(idx, numProbes)) {
DCHECK_LT(idx, capacity_);
value_type* cell = &cells_[idx];
KeyT currentKey = acquireLoadKey(*cell);
if (currentKey == kEmptyKey_ || currentKey == kLockedKey_) {
// If we hit an empty (or locked) element, this key does not exist. This
// is similar to how it's handled in find().
return 0;
}
if (key_in == currentKey) {
// Found an existing entry for our key, attempt to mark it erased.
// Some other thread may have erased our key, but this is ok.
KeyT expect = key_in;
if (cellKeyPtr(*cell)->compare_exchange_strong(expect, kErasedKey_)) {
numErases_.fetch_add(1, std::memory_order_relaxed);
// Even if there's a value in the cell, we won't delete (or even
// default construct) it because some other thread may be accessing it.
// Locking it meanwhile won't work either since another thread may be
// holding a pointer to it.
// We found the key and successfully erased it.
return 1;
}
// If another thread succeeds in erasing our key, we'll stop our search.
return 0;
}
++numProbes;
if (UNLIKELY(numProbes >= capacity_)) {
// probed every cell...fail
return 0;
}
}
}
template <class KeyT, class ValueT, class HashFcn>
const typename AtomicHashArray<KeyT, ValueT, HashFcn>::Config
AtomicHashArray<KeyT, ValueT, HashFcn>::defaultConfig;
template <class KeyT, class ValueT, class HashFcn>
typename AtomicHashArray<KeyT, ValueT, HashFcn>::SmartPtr
AtomicHashArray<KeyT, ValueT, HashFcn>::
create(size_t maxSize, const Config& c) {
CHECK_LE(c.maxLoadFactor, 1.0);
CHECK_GT(c.maxLoadFactor, 0.0);
CHECK_NE(c.emptyKey, c.lockedKey);
size_t capacity = size_t(maxSize / c.maxLoadFactor);
size_t sz = sizeof(AtomicHashArray) + sizeof(value_type) * capacity;
std::unique_ptr<void, void(*)(void*)> mem(malloc(sz), free);
new(mem.get()) AtomicHashArray(capacity, c.emptyKey, c.lockedKey, c.erasedKey,
c.maxLoadFactor, c.entryCountThreadCacheSize);
SmartPtr map(static_cast<AtomicHashArray*>(mem.release()));
/*
* Mark all cells as empty.
*
* Note: we're bending the rules a little here accessing the key
* element in our cells even though the cell object has not been
* constructed, and casting them to atomic objects (see cellKeyPtr).
* (Also, in fact we never actually invoke the value_type
* constructor.) This is in order to avoid needing to default
* construct a bunch of value_type when we first start up: if you
* have an expensive default constructor for the value type this can
* noticeably speed construction time for an AHA.
*/
FOR_EACH_RANGE(i, 0, map->capacity_) {
cellKeyPtr(map->cells_[i])->store(map->kEmptyKey_,
std::memory_order_relaxed);
}
return map;
}
template <class KeyT, class ValueT, class HashFcn>
void AtomicHashArray<KeyT, ValueT, HashFcn>::
destroy(AtomicHashArray* p) {
assert(p);
FOR_EACH_RANGE(i, 0, p->capacity_) {
if (p->cells_[i].first != p->kEmptyKey_) {
p->cells_[i].~value_type();
}
}
p->~AtomicHashArray();
free(p);
}
// clear -- clears all keys and values in the map and resets all counters
template <class KeyT, class ValueT, class HashFcn>
void AtomicHashArray<KeyT, ValueT, HashFcn>::
clear() {
FOR_EACH_RANGE(i, 0, capacity_) {
if (cells_[i].first != kEmptyKey_) {
cells_[i].~value_type();
*const_cast<KeyT*>(&cells_[i].first) = kEmptyKey_;
}
CHECK(cells_[i].first == kEmptyKey_);
}
numEntries_.set(0);
numPendingEntries_.set(0);
isFull_.store(0, std::memory_order_relaxed);
numErases_.store(0, std::memory_order_relaxed);
}
// Iterator implementation
template <class KeyT, class ValueT, class HashFcn>
template <class ContT, class IterVal>
struct AtomicHashArray<KeyT, ValueT, HashFcn>::aha_iterator
: boost::iterator_facade<aha_iterator<ContT,IterVal>,
IterVal,
boost::forward_traversal_tag>
{
explicit aha_iterator() : aha_(0) {}
// Conversion ctor for interoperability between const_iterator and
// iterator. The enable_if<> magic keeps us well-behaved for
// is_convertible<> (v. the iterator_facade documentation).
template<class OtherContT, class OtherVal>
aha_iterator(const aha_iterator<OtherContT,OtherVal>& o,
typename std::enable_if<
std::is_convertible<OtherVal*,IterVal*>::value >::type* = 0)
: aha_(o.aha_)
, offset_(o.offset_)
{}
explicit aha_iterator(ContT* array, size_t offset)
: aha_(array)
, offset_(offset)
{
advancePastEmpty();
}
// Returns unique index that can be used with findAt().
// WARNING: The following function will fail silently for hashtable
// with capacity > 2^32
uint32_t getIndex() const { return offset_; }
private:
friend class AtomicHashArray;
friend class boost::iterator_core_access;
void increment() {
++offset_;
advancePastEmpty();
}
bool equal(const aha_iterator& o) const {
return aha_ == o.aha_ && offset_ == o.offset_;
}
IterVal& dereference() const {
return aha_->cells_[offset_];
}
void advancePastEmpty() {
while (offset_ < aha_->capacity_ && !isValid()) {
++offset_;
}
}
bool isValid() const {
KeyT key = acquireLoadKey(aha_->cells_[offset_]);
return key != aha_->kEmptyKey_ &&
key != aha_->kLockedKey_ &&
key != aha_->kErasedKey_;
}
private:
ContT* aha_;
size_t offset_;
}; // aha_iterator
} // namespace folly
#undef FOLLY_SPIN_WAIT
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