Summary:
= Major Changes =
* Add a new mem-table representation, HashCuckooRep, which is based cuckoo hash.
Cuckoo hash uses multiple hash functions. This allows each key to have multiple
possible locations in the mem-table.
- Put: When insert a key, it will try to find whether one of its possible
locations is vacant and store the key. If none of its possible
locations are available, then it will kick out a victim key and
store at that location. The kicked-out victim key will then be
stored at a vacant space of its possible locations or kick-out
another victim. In this diff, the kick-out path (known as
cuckoo-path) is found using BFS, which guarantees to be the shortest.
- Get: Simply tries all possible locations of a key --- this guarantees
worst-case constant time complexity.
- Time complexity: O(1) for Get, and average O(1) for Put if the
fullness of the mem-table is below 80%.
- Default using two hash functions, the number of hash functions used
by the cuckoo-hash may dynamically increase if it fails to find a
short-enough kick-out path.
- Currently, HashCuckooRep does not support iteration and snapshots,
as our current main purpose of this is to optimize point access.
= Minor Changes =
* Add IsSnapshotSupported() to DB to indicate whether the current DB
supports snapshots. If it returns false, then DB::GetSnapshot() will
always return nullptr.
Test Plan:
Run existing tests. Will develop a test specifically for cuckoo hash in
the next diff.
Reviewers: sdong, haobo
Reviewed By: sdong
CC: leveldb, dhruba, igor
Differential Revision: https://reviews.facebook.net/D16155
main
Yueh-Hsuan Chiang
11 years ago
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// Copyright (c) 2014, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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//
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#ifndef ROCKSDB_LITE |
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#include "util/hash_cuckoo_rep.h" |
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#include <algorithm> |
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#include <atomic> |
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#include <limits> |
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#include <queue> |
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#include <string> |
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#include <memory> |
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#include <vector> |
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#include "rocksdb/memtablerep.h" |
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#include "util/murmurhash.h" |
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#include "db/memtable.h" |
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#include "db/skiplist.h" |
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#include "util/stl_wrappers.h" |
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namespace rocksdb { |
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namespace { |
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// the default maximum size of the cuckoo path searching queue
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static const int kCuckooPathMaxSearchSteps = 100; |
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struct CuckooStep { |
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static const int kNullStep = -1; |
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// the bucket id in the cuckoo array.
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int bucket_id_; |
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// index of cuckoo-step array that points to its previous step,
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// -1 if it the beginning step.
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int prev_step_id_; |
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// the depth of the current step.
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unsigned int depth_; |
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CuckooStep() : bucket_id_(-1), prev_step_id_(kNullStep), depth_(1) {} |
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CuckooStep(CuckooStep&&) = default; |
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CuckooStep& operator=(CuckooStep&&) = default; |
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CuckooStep(const CuckooStep&) = delete; |
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CuckooStep& operator=(const CuckooStep&) = delete; |
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CuckooStep(int bucket_id, int prev_step_id, int depth) |
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: bucket_id_(bucket_id), prev_step_id_(prev_step_id), depth_(depth) {} |
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}; |
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class HashCuckooRep : public MemTableRep { |
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public: |
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explicit HashCuckooRep(const MemTableRep::KeyComparator& compare, |
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Arena* arena, const size_t bucket_count, |
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const unsigned int hash_func_count) |
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: MemTableRep(arena), |
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compare_(compare), |
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arena_(arena), |
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bucket_count_(bucket_count), |
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cuckoo_path_max_depth_(kDefaultCuckooPathMaxDepth), |
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occupied_count_(0), |
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hash_function_count_(hash_func_count), |
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backup_table_(nullptr) { |
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char* mem = reinterpret_cast<char*>( |
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arena_->Allocate(sizeof(std::atomic<const char*>) * bucket_count_)); |
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cuckoo_array_ = new (mem) std::atomic<const char*>[bucket_count_]; |
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for (unsigned int bid = 0; bid < bucket_count_; ++bid) { |
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cuckoo_array_[bid].store(nullptr, std::memory_order_relaxed); |
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} |
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cuckoo_path_ = reinterpret_cast<int*>( |
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arena_->Allocate(sizeof(int*) * (cuckoo_path_max_depth_ + 1))); |
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is_nearly_full_ = false; |
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} |
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// return false, indicating HashCuckooRep does not support merge operator.
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virtual bool IsMergeOperatorSupported() const override { return false; } |
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// return false, indicating HashCuckooRep does not support snapshot.
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virtual bool IsSnapshotSupported() const override { return false; } |
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// Returns true iff an entry that compares equal to key is in the collection.
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virtual bool Contains(const char* internal_key) const override; |
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virtual ~HashCuckooRep() override {} |
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// Insert the specified key (internal_key) into the mem-table. Assertion
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// fails if
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// the current mem-table already contains the specified key.
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virtual void Insert(KeyHandle handle) override; |
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// This function returns std::numeric_limits<size_t>::max() in the following
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// three cases to disallow further write operations:
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// 1. when the fullness reaches kMaxFullnes.
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// 2. when the backup_table_ is used.
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//
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// otherwise, this function will always return 0.
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virtual size_t ApproximateMemoryUsage() override { |
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if (is_nearly_full_) { |
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return std::numeric_limits<size_t>::max(); |
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} |
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return 0; |
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} |
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virtual void Get(const LookupKey& k, void* callback_args, |
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bool (*callback_func)(void* arg, |
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const char* entry)) override; |
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class Iterator : public MemTableRep::Iterator { |
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std::shared_ptr<std::vector<const char*>> bucket_; |
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typename std::vector<const char*>::const_iterator mutable cit_; |
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const KeyComparator& compare_; |
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std::string tmp_; // For passing to EncodeKey
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bool mutable sorted_; |
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void DoSort() const; |
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public: |
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explicit Iterator(std::shared_ptr<std::vector<const char*>> bucket, |
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const KeyComparator& compare); |
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// Initialize an iterator over the specified collection.
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// The returned iterator is not valid.
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// explicit Iterator(const MemTableRep* collection);
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virtual ~Iterator() override{}; |
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// Returns true iff the iterator is positioned at a valid node.
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virtual bool Valid() const override; |
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// Returns the key at the current position.
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// REQUIRES: Valid()
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virtual const char* key() const override; |
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// Advances to the next position.
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// REQUIRES: Valid()
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virtual void Next() override; |
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// Advances to the previous position.
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// REQUIRES: Valid()
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virtual void Prev() override; |
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// Advance to the first entry with a key >= target
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virtual void Seek(const Slice& user_key, const char* memtable_key) override; |
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// Position at the first entry in collection.
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// Final state of iterator is Valid() iff collection is not empty.
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virtual void SeekToFirst() override; |
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// Position at the last entry in collection.
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// Final state of iterator is Valid() iff collection is not empty.
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virtual void SeekToLast() override; |
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}; |
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struct CuckooStepBuffer { |
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CuckooStepBuffer() : write_index_(0), read_index_(0) {} |
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~CuckooStepBuffer() {} |
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int write_index_; |
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int read_index_; |
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CuckooStep steps_[kCuckooPathMaxSearchSteps]; |
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CuckooStep& NextWriteBuffer() { return steps_[write_index_++]; } |
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inline const CuckooStep& ReadNext() { return steps_[read_index_++]; } |
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inline bool HasNewWrite() { return write_index_ > read_index_; } |
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inline void reset() { |
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write_index_ = 0; |
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read_index_ = 0; |
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} |
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inline bool IsFull() { return write_index_ >= kCuckooPathMaxSearchSteps; } |
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// returns the number of steps that has been read
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inline int ReadCount() { return read_index_; } |
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// returns the number of steps that has been written to the buffer.
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inline int WriteCount() { return write_index_; } |
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}; |
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private: |
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const MemTableRep::KeyComparator& compare_; |
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// the pointer to Arena to allocate memory, immutable after construction.
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Arena* const arena_; |
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// the number of hash bucket in the hash table.
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const size_t bucket_count_; |
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// the maxinum depth of the cuckoo path.
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const unsigned int cuckoo_path_max_depth_; |
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// the current number of entries in cuckoo_array_ which has been occupied.
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size_t occupied_count_; |
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// the current number of hash functions used in the cuckoo hash.
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unsigned int hash_function_count_; |
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// the backup MemTableRep to handle the case where cuckoo hash cannot find
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// a vacant bucket for inserting the key of a put request.
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std::shared_ptr<MemTableRep> backup_table_; |
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// the array to store pointers, pointing to the actual data.
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std::atomic<const char*>* cuckoo_array_; |
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// a buffer to store cuckoo path
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int* cuckoo_path_; |
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// a boolean flag indicating whether the fullness of bucket array
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// reaches the point to make the current memtable immutable.
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bool is_nearly_full_; |
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// the default maximum depth of the cuckoo path.
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static const unsigned int kDefaultCuckooPathMaxDepth = 10; |
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CuckooStepBuffer step_buffer_; |
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// returns the bucket id assogied to the input slice based on the
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unsigned int GetHash(const Slice& slice, const int hash_func_id) const { |
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// the seeds used in the Murmur hash to produce different hash functions.
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static const int kMurmurHashSeeds[HashCuckooRepFactory::kMaxHashCount] = { |
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545609244, 1769731426, 763324157, 13099088, 592422103, |
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1899789565, 248369300, 1984183468, 1613664382, 1491157517}; |
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return MurmurHash(slice.data(), slice.size(), |
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kMurmurHashSeeds[hash_func_id]) % |
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bucket_count_; |
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} |
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// A cuckoo path is a sequence of bucket ids, where each id points to a
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// location of cuckoo_array_. This path describes the displacement sequence
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// of entries in order to store the desired data specified by the input user
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// key. The path starts from one of the locations associated with the
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// specified user key and ends at a vacant space in the cuckoo array. This
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// function will update the cuckoo_path.
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//
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// @return true if it found a cuckoo path.
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bool FindCuckooPath(const char* internal_key, const Slice& user_key, |
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int* cuckoo_path, size_t* cuckoo_path_length, |
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int initial_hash_id = 0); |
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// Perform quick insert by checking whether there is a vacant bucket in one
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// of the possible locations of the input key. If so, then the function will
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// return true and the key will be stored in that vacant bucket.
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//
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// This function is a helper function of FindCuckooPath that discovers the
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// first possible steps of a cuckoo path. It begins by first computing
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// the possible locations of the input keys (and stores them in bucket_ids.)
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// Then, if one of its possible locations is vacant, then the input key will
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// be stored in that vacant space and the function will return true.
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// Otherwise, the function will return false indicating a complete search
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// of cuckoo-path is needed.
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bool QuickInsert(const char* internal_key, const Slice& user_key, |
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int bucket_ids[], const int initial_hash_id); |
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// Unhide default implementations of GetIterator
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using MemTableRep::GetIterator; |
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// Returns the pointer to the internal iterator to the buckets where buckets
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// are sorted according to the user specified KeyComparator. Note that
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// any insert after this function call may affect the sorted nature of
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// the returned iterator.
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virtual MemTableRep::Iterator* GetIterator() override { |
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std::vector<const char*> compact_buckets; |
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for (unsigned int bid = 0; bid < bucket_count_; ++bid) { |
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const char* bucket = cuckoo_array_[bid].load(std::memory_order_relaxed); |
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if (bucket != nullptr) { |
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compact_buckets.push_back(bucket); |
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} |
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} |
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MemTableRep* backup_table = backup_table_.get(); |
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if (backup_table != nullptr) { |
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std::unique_ptr<MemTableRep::Iterator> iter(backup_table->GetIterator()); |
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for (iter->SeekToFirst(); iter->Valid(); iter->Next()) { |
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compact_buckets.push_back(iter->key()); |
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} |
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} |
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return new Iterator( |
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std::shared_ptr<std::vector<const char*>>( |
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new std::vector<const char*>(std::move(compact_buckets))), |
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compare_); |
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} |
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}; |
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void HashCuckooRep::Get(const LookupKey& key, void* callback_args, |
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bool (*callback_func)(void* arg, const char* entry)) { |
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Slice user_key = key.user_key(); |
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for (unsigned int hid = 0; hid < hash_function_count_; ++hid) { |
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const char* bucket = |
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cuckoo_array_[GetHash(user_key, hid)].load(std::memory_order_acquire); |
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if (bucket != nullptr) { |
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auto bucket_user_key = UserKey(bucket); |
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if (user_key.compare(bucket_user_key) == 0) { |
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callback_func(callback_args, bucket); |
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break; |
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} |
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} else { |
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// as Put() always stores at the vacant bucket located by the
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// hash function with the smallest possible id, when we first
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// find a vacant bucket in Get(), that means a miss.
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break; |
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} |
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} |
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MemTableRep* backup_table = backup_table_.get(); |
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if (backup_table != nullptr) { |
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backup_table->Get(key, callback_args, callback_func); |
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} |
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} |
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void HashCuckooRep::Insert(KeyHandle handle) { |
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static const float kMaxFullness = 0.90; |
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auto* key = static_cast<char*>(handle); |
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int initial_hash_id = 0; |
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size_t cuckoo_path_length = 0; |
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auto user_key = UserKey(key); |
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// find cuckoo path
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if (FindCuckooPath(key, user_key, cuckoo_path_, &cuckoo_path_length, |
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initial_hash_id) == false) { |
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// if true, then we can't find a vacant bucket for this key even we
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// have used up all the hash functions. Then use a backup memtable to
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// store such key, which will further make this mem-table become
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// immutable.
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if (backup_table_.get() == nullptr) { |
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VectorRepFactory factory(10); |
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backup_table_.reset(factory.CreateMemTableRep(compare_, arena_, nullptr)); |
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is_nearly_full_ = true; |
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} |
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backup_table_->Insert(key); |
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return; |
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} |
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// when reaching this point, means the insert can be done successfully.
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occupied_count_++; |
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if (occupied_count_ >= bucket_count_ * kMaxFullness) { |
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is_nearly_full_ = true; |
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} |
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// perform kickout process if the length of cuckoo path > 1.
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if (cuckoo_path_length == 0) return; |
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// the cuckoo path stores the kickout path in reverse order.
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// so the kickout or displacement is actually performed
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// in reverse order, which avoids false-negatives on read
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// by moving each key involved in the cuckoo path to the new
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// location before replacing it.
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for (size_t i = 1; i < cuckoo_path_length; ++i) { |
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int kicked_out_bid = cuckoo_path_[i - 1]; |
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int current_bid = cuckoo_path_[i]; |
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// since we only allow one writer at a time, it is safe to do relaxed read.
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cuckoo_array_[kicked_out_bid] |
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.store(cuckoo_array_[current_bid].load(std::memory_order_relaxed), |
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std::memory_order_release); |
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} |
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int insert_key_bid = cuckoo_path_[cuckoo_path_length - 1]; |
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cuckoo_array_[insert_key_bid].store(key, std::memory_order_release); |
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} |
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bool HashCuckooRep::Contains(const char* internal_key) const { |
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auto user_key = UserKey(internal_key); |
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for (unsigned int hid = 0; hid < hash_function_count_; ++hid) { |
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const char* stored_key = |
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cuckoo_array_[GetHash(user_key, hid)].load(std::memory_order_acquire); |
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if (stored_key != nullptr) { |
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if (compare_(internal_key, stored_key) == 0) { |
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return true; |
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} |
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} |
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} |
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return false; |
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} |
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bool HashCuckooRep::QuickInsert(const char* internal_key, const Slice& user_key, |
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int bucket_ids[], const int initial_hash_id) { |
||||||
|
int cuckoo_bucket_id = -1; |
||||||
|
|
||||||
|
// Below does the followings:
|
||||||
|
// 0. Calculate all possible locations of the input key.
|
||||||
|
// 1. Check if there is a bucket having same user_key as the input does.
|
||||||
|
// 2. If there exists such bucket, then replace this bucket by the newly
|
||||||
|
// insert data and return. This step also performs duplication check.
|
||||||
|
// 3. If no such bucket exists but exists a vacant bucket, then insert the
|
||||||
|
// input data into it.
|
||||||
|
// 4. If step 1 to 3 all fail, then return false.
|
||||||
|
for (unsigned int hid = initial_hash_id; hid < hash_function_count_; ++hid) { |
||||||
|
bucket_ids[hid] = GetHash(user_key, hid); |
||||||
|
// since only one PUT is allowed at a time, and this is part of the PUT
|
||||||
|
// operation, so we can safely perform relaxed load.
|
||||||
|
const char* stored_key = |
||||||
|
cuckoo_array_[bucket_ids[hid]].load(std::memory_order_relaxed); |
||||||
|
if (stored_key == nullptr) { |
||||||
|
if (cuckoo_bucket_id == -1) { |
||||||
|
cuckoo_bucket_id = bucket_ids[hid]; |
||||||
|
} |
||||||
|
} else { |
||||||
|
const auto bucket_user_key = UserKey(stored_key); |
||||||
|
if (bucket_user_key.compare(user_key) == 0) { |
||||||
|
cuckoo_bucket_id = bucket_ids[hid]; |
||||||
|
break; |
||||||
|
} |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
if (cuckoo_bucket_id != -1) { |
||||||
|
cuckoo_array_[cuckoo_bucket_id] |
||||||
|
.store(internal_key, std::memory_order_release); |
||||||
|
return true; |
||||||
|
} |
||||||
|
|
||||||
|
return false; |
||||||
|
} |
||||||
|
|
||||||
|
// Perform pre-check and find the shortest cuckoo path. A cuckoo path
|
||||||
|
// is a displacement sequence for inserting the specified input key.
|
||||||
|
//
|
||||||
|
// @return true if it successfully found a vacant space or cuckoo-path.
|
||||||
|
// If the return value is true but the length of cuckoo_path is zero,
|
||||||
|
// then it indicates that a vacant bucket or an bucket with matched user
|
||||||
|
// key with the input is found, and a quick insertion is done.
|
||||||
|
bool HashCuckooRep::FindCuckooPath(const char* internal_key, |
||||||
|
const Slice& user_key, int* cuckoo_path, |
||||||
|
size_t* cuckoo_path_length, |
||||||
|
const int initial_hash_id) { |
||||||
|
int bucket_ids[HashCuckooRepFactory::kMaxHashCount]; |
||||||
|
*cuckoo_path_length = 0; |
||||||
|
|
||||||
|
if (QuickInsert(internal_key, user_key, bucket_ids, initial_hash_id)) { |
||||||
|
return true; |
||||||
|
} |
||||||
|
// If this step is reached, then it means:
|
||||||
|
// 1. no vacant bucket in any of the possible locations of the input key.
|
||||||
|
// 2. none of the possible locations of the input key has the same user
|
||||||
|
// key as the input `internal_key`.
|
||||||
|
|
||||||
|
// the front and back indices for the step_queue_
|
||||||
|
step_buffer_.reset(); |
||||||
|
|
||||||
|
for (unsigned int hid = initial_hash_id; hid < hash_function_count_; ++hid) { |
||||||
|
/// CuckooStep& current_step = step_queue_[front_pos++];
|
||||||
|
CuckooStep& current_step = step_buffer_.NextWriteBuffer(); |
||||||
|
current_step.bucket_id_ = bucket_ids[hid]; |
||||||
|
current_step.prev_step_id_ = CuckooStep::kNullStep; |
||||||
|
current_step.depth_ = 1; |
||||||
|
} |
||||||
|
|
||||||
|
while (step_buffer_.HasNewWrite()) { |
||||||
|
int step_id = step_buffer_.read_index_; |
||||||
|
const CuckooStep& step = step_buffer_.ReadNext(); |
||||||
|
// Since it's a BFS process, then the first step with its depth deeper
|
||||||
|
// than the maximum allowed depth indicates all the remaining steps
|
||||||
|
// in the step buffer queue will all exceed the maximum depth.
|
||||||
|
// Return false immediately indicating we can't find a vacant bucket
|
||||||
|
// for the input key before the maximum allowed depth.
|
||||||
|
if (step.depth_ >= cuckoo_path_max_depth_) { |
||||||
|
return false; |
||||||
|
} |
||||||
|
// again, we can perform no barrier load safely here as the current
|
||||||
|
// thread is the only writer.
|
||||||
|
auto bucket_user_key = |
||||||
|
UserKey(cuckoo_array_[step.bucket_id_].load(std::memory_order_relaxed)); |
||||||
|
if (step.prev_step_id_ != CuckooStep::kNullStep) { |
||||||
|
if (bucket_user_key.compare(user_key) == 0) { |
||||||
|
// then there is a loop in the current path, stop discovering this path.
|
||||||
|
continue; |
||||||
|
} |
||||||
|
} |
||||||
|
// if the current bucket stores at its nth location, then we only consider
|
||||||
|
// its mth location where m > n. This property makes sure that all reads
|
||||||
|
// will not miss if we do have data associated to the query key.
|
||||||
|
//
|
||||||
|
// The n and m in the above statement is the start_hid and hid in the code.
|
||||||
|
unsigned int start_hid = hash_function_count_; |
||||||
|
for (unsigned int hid = 0; hid < hash_function_count_; ++hid) { |
||||||
|
bucket_ids[hid] = GetHash(bucket_user_key, hid); |
||||||
|
if (step.bucket_id_ == bucket_ids[hid]) { |
||||||
|
start_hid = hid; |
||||||
|
} |
||||||
|
} |
||||||
|
// must found a bucket which is its current "home".
|
||||||
|
assert(start_hid != hash_function_count_); |
||||||
|
|
||||||
|
// explore all possible next steps from the current step.
|
||||||
|
for (unsigned int hid = start_hid + 1; hid < hash_function_count_; ++hid) { |
||||||
|
CuckooStep& next_step = step_buffer_.NextWriteBuffer(); |
||||||
|
next_step.bucket_id_ = bucket_ids[hid]; |
||||||
|
next_step.prev_step_id_ = step_id; |
||||||
|
next_step.depth_ = step.depth_ + 1; |
||||||
|
// once a vacant bucket is found, trace back all its previous steps
|
||||||
|
// to generate a cuckoo path.
|
||||||
|
if (cuckoo_array_[next_step.bucket_id_].load(std::memory_order_relaxed) == |
||||||
|
nullptr) { |
||||||
|
// store the last step in the cuckoo path. Note that cuckoo_path
|
||||||
|
// stores steps in reverse order. This allows us to move keys along
|
||||||
|
// the cuckoo path by storing each key to the new place first before
|
||||||
|
// removing it from the old place. This property ensures reads will
|
||||||
|
// not missed due to moving keys along the cuckoo path.
|
||||||
|
cuckoo_path[(*cuckoo_path_length)++] = next_step.bucket_id_; |
||||||
|
int depth; |
||||||
|
for (depth = step.depth_; depth > 0 && step_id != CuckooStep::kNullStep; |
||||||
|
depth--) { |
||||||
|
const CuckooStep& prev_step = step_buffer_.steps_[step_id]; |
||||||
|
cuckoo_path[(*cuckoo_path_length)++] = prev_step.bucket_id_; |
||||||
|
step_id = prev_step.prev_step_id_; |
||||||
|
} |
||||||
|
assert(depth == 0 && step_id == CuckooStep::kNullStep); |
||||||
|
return true; |
||||||
|
} |
||||||
|
if (step_buffer_.IsFull()) { |
||||||
|
// if true, then it reaches maxinum number of cuckoo search steps.
|
||||||
|
return false; |
||||||
|
} |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
// tried all possible paths but still not unable to find a cuckoo path
|
||||||
|
// which path leads to a vacant bucket.
|
||||||
|
return false; |
||||||
|
} |
||||||
|
|
||||||
|
HashCuckooRep::Iterator::Iterator( |
||||||
|
std::shared_ptr<std::vector<const char*>> bucket, |
||||||
|
const KeyComparator& compare) |
||||||
|
: bucket_(bucket), |
||||||
|
cit_(bucket_->end()), |
||||||
|
compare_(compare), |
||||||
|
sorted_(false) {} |
||||||
|
|
||||||
|
void HashCuckooRep::Iterator::DoSort() const { |
||||||
|
if (!sorted_) { |
||||||
|
std::sort(bucket_->begin(), bucket_->end(), |
||||||
|
stl_wrappers::Compare(compare_)); |
||||||
|
cit_ = bucket_->begin(); |
||||||
|
sorted_ = true; |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
// Returns true iff the iterator is positioned at a valid node.
|
||||||
|
bool HashCuckooRep::Iterator::Valid() const { |
||||||
|
DoSort(); |
||||||
|
return cit_ != bucket_->end(); |
||||||
|
} |
||||||
|
|
||||||
|
// Returns the key at the current position.
|
||||||
|
// REQUIRES: Valid()
|
||||||
|
const char* HashCuckooRep::Iterator::key() const { |
||||||
|
assert(Valid()); |
||||||
|
return *cit_; |
||||||
|
} |
||||||
|
|
||||||
|
// Advances to the next position.
|
||||||
|
// REQUIRES: Valid()
|
||||||
|
void HashCuckooRep::Iterator::Next() { |
||||||
|
assert(Valid()); |
||||||
|
if (cit_ == bucket_->end()) { |
||||||
|
return; |
||||||
|
} |
||||||
|
++cit_; |
||||||
|
} |
||||||
|
|
||||||
|
// Advances to the previous position.
|
||||||
|
// REQUIRES: Valid()
|
||||||
|
void HashCuckooRep::Iterator::Prev() { |
||||||
|
assert(Valid()); |
||||||
|
if (cit_ == bucket_->begin()) { |
||||||
|
// If you try to go back from the first element, the iterator should be
|
||||||
|
// invalidated. So we set it to past-the-end. This means that you can
|
||||||
|
// treat the container circularly.
|
||||||
|
cit_ = bucket_->end(); |
||||||
|
} else { |
||||||
|
--cit_; |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
// Advance to the first entry with a key >= target
|
||||||
|
void HashCuckooRep::Iterator::Seek(const Slice& user_key, |
||||||
|
const char* memtable_key) { |
||||||
|
DoSort(); |
||||||
|
// Do binary search to find first value not less than the target
|
||||||
|
const char* encoded_key = |
||||||
|
(memtable_key != nullptr) ? memtable_key : EncodeKey(&tmp_, user_key); |
||||||
|
cit_ = std::equal_range(bucket_->begin(), bucket_->end(), encoded_key, |
||||||
|
[this](const char* a, const char* b) { |
||||||
|
return compare_(a, b) < 0; |
||||||
|
}).first; |
||||||
|
} |
||||||
|
|
||||||
|
// Position at the first entry in collection.
|
||||||
|
// Final state of iterator is Valid() iff collection is not empty.
|
||||||
|
void HashCuckooRep::Iterator::SeekToFirst() { |
||||||
|
DoSort(); |
||||||
|
cit_ = bucket_->begin(); |
||||||
|
} |
||||||
|
|
||||||
|
// Position at the last entry in collection.
|
||||||
|
// Final state of iterator is Valid() iff collection is not empty.
|
||||||
|
void HashCuckooRep::Iterator::SeekToLast() { |
||||||
|
DoSort(); |
||||||
|
cit_ = bucket_->end(); |
||||||
|
if (bucket_->size() != 0) { |
||||||
|
--cit_; |
||||||
|
} |
||||||
|
} |
||||||
|
|
||||||
|
} // anom namespace
|
||||||
|
|
||||||
|
MemTableRep* HashCuckooRepFactory::CreateMemTableRep( |
||||||
|
const MemTableRep::KeyComparator& compare, Arena* arena, |
||||||
|
const SliceTransform* transform) { |
||||||
|
// The estimated average fullness. The write performance of any close hash
|
||||||
|
// degrades as the fullness of the mem-table increases. Setting kFullness
|
||||||
|
// to a value around 0.7 can better avoid write performance degradation while
|
||||||
|
// keeping efficient memory usage.
|
||||||
|
static const float kFullness = 0.7; |
||||||
|
size_t pointer_size = sizeof(std::atomic<const char*>); |
||||||
|
assert(write_buffer_size_ >= (average_data_size_ + pointer_size)); |
||||||
|
size_t bucket_count = |
||||||
|
(write_buffer_size_ / (average_data_size_ + pointer_size)) / kFullness + |
||||||
|
1; |
||||||
|
unsigned int hash_function_count = hash_function_count_; |
||||||
|
if (hash_function_count < 2) { |
||||||
|
hash_function_count = 2; |
||||||
|
} |
||||||
|
if (hash_function_count > kMaxHashCount) { |
||||||
|
hash_function_count = kMaxHashCount; |
||||||
|
} |
||||||
|
return new HashCuckooRep(compare, arena, bucket_count, hash_function_count); |
||||||
|
} |
||||||
|
|
||||||
|
MemTableRepFactory* NewHashCuckooRepFactory(size_t write_buffer_size, |
||||||
|
size_t average_data_size, |
||||||
|
unsigned int hash_function_count) { |
||||||
|
return new HashCuckooRepFactory(write_buffer_size, average_data_size, |
||||||
|
hash_function_count); |
||||||
|
} |
||||||
|
|
||||||
|
} // namespace rocksdb
|
||||||
|
#endif // ROCKSDB_LITE
|
||||||
@ -0,0 +1,42 @@ |
|||||||
|
// Copyright (c) 2013, Facebook, Inc. All rights reserved.
|
||||||
|
// This source code is licensed under the BSD-style license found in the
|
||||||
|
// LICENSE file in the root directory of this source tree. An additional grant
|
||||||
|
// of patent rights can be found in the PATENTS file in the same directory.
|
||||||
|
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
|
||||||
|
// Use of this source code is governed by a BSD-style license that can be
|
||||||
|
// found in the LICENSE file. See the AUTHORS file for names of contributors.
|
||||||
|
|
||||||
|
#ifndef ROCKSDB_LITE |
||||||
|
#pragma once |
||||||
|
#include "rocksdb/slice_transform.h" |
||||||
|
#include "rocksdb/memtablerep.h" |
||||||
|
|
||||||
|
namespace rocksdb { |
||||||
|
|
||||||
|
class HashCuckooRepFactory : public MemTableRepFactory { |
||||||
|
public: |
||||||
|
// maxinum number of hash functions used in the cuckoo hash.
|
||||||
|
static const int kMaxHashCount = 10; |
||||||
|
|
||||||
|
explicit HashCuckooRepFactory(size_t write_buffer_size, |
||||||
|
size_t average_data_size, |
||||||
|
unsigned int hash_function_count) |
||||||
|
: write_buffer_size_(write_buffer_size), |
||||||
|
average_data_size_(average_data_size), |
||||||
|
hash_function_count_(hash_function_count) {} |
||||||
|
|
||||||
|
virtual ~HashCuckooRepFactory() {} |
||||||
|
|
||||||
|
virtual MemTableRep* CreateMemTableRep( |
||||||
|
const MemTableRep::KeyComparator& compare, Arena* arena, |
||||||
|
const SliceTransform* transform) override; |
||||||
|
|
||||||
|
virtual const char* Name() const override { return "HashCuckooRepFactory"; } |
||||||
|
|
||||||
|
private: |
||||||
|
size_t write_buffer_size_; |
||||||
|
size_t average_data_size_; |
||||||
|
const unsigned int hash_function_count_; |
||||||
|
}; |
||||||
|
} // namespace rocksdb
|
||||||
|
#endif // ROCKSDB_LITE
|
||||||
Loading…
Reference in new issue