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
parent
f1c9aa6ebe
commit
9d9d2965cb
@ -0,0 +1,627 @@ |
||||
|
||||
// Copyright (c) 2014, 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.
|
||||
//
|
||||
|
||||
#ifndef ROCKSDB_LITE |
||||
#include "util/hash_cuckoo_rep.h" |
||||
|
||||
#include <algorithm> |
||||
#include <atomic> |
||||
#include <limits> |
||||
#include <queue> |
||||
#include <string> |
||||
#include <memory> |
||||
#include <vector> |
||||
|
||||
#include "rocksdb/memtablerep.h" |
||||
#include "util/murmurhash.h" |
||||
#include "db/memtable.h" |
||||
#include "db/skiplist.h" |
||||
#include "util/stl_wrappers.h" |
||||
|
||||
namespace rocksdb { |
||||
namespace { |
||||
|
||||
// the default maximum size of the cuckoo path searching queue
|
||||
static const int kCuckooPathMaxSearchSteps = 100; |
||||
|
||||
struct CuckooStep { |
||||
static const int kNullStep = -1; |
||||
// the bucket id in the cuckoo array.
|
||||
int bucket_id_; |
||||
// index of cuckoo-step array that points to its previous step,
|
||||
// -1 if it the beginning step.
|
||||
int prev_step_id_; |
||||
// the depth of the current step.
|
||||
unsigned int depth_; |
||||
|
||||
CuckooStep() : bucket_id_(-1), prev_step_id_(kNullStep), depth_(1) {} |
||||
|
||||
CuckooStep(CuckooStep&&) = default; |
||||
CuckooStep& operator=(CuckooStep&&) = default; |
||||
|
||||
CuckooStep(const CuckooStep&) = delete; |
||||
CuckooStep& operator=(const CuckooStep&) = delete; |
||||
|
||||
CuckooStep(int bucket_id, int prev_step_id, int depth) |
||||
: bucket_id_(bucket_id), prev_step_id_(prev_step_id), depth_(depth) {} |
||||
}; |
||||
|
||||
class HashCuckooRep : public MemTableRep { |
||||
public: |
||||
explicit HashCuckooRep(const MemTableRep::KeyComparator& compare, |
||||
Arena* arena, const size_t bucket_count, |
||||
const unsigned int hash_func_count) |
||||
: MemTableRep(arena), |
||||
compare_(compare), |
||||
arena_(arena), |
||||
bucket_count_(bucket_count), |
||||
cuckoo_path_max_depth_(kDefaultCuckooPathMaxDepth), |
||||
occupied_count_(0), |
||||
hash_function_count_(hash_func_count), |
||||
backup_table_(nullptr) { |
||||
char* mem = reinterpret_cast<char*>( |
||||
arena_->Allocate(sizeof(std::atomic<const char*>) * bucket_count_)); |
||||
cuckoo_array_ = new (mem) std::atomic<const char*>[bucket_count_]; |
||||
for (unsigned int bid = 0; bid < bucket_count_; ++bid) { |
||||
cuckoo_array_[bid].store(nullptr, std::memory_order_relaxed); |
||||
} |
||||
|
||||
cuckoo_path_ = reinterpret_cast<int*>( |
||||
arena_->Allocate(sizeof(int*) * (cuckoo_path_max_depth_ + 1))); |
||||
is_nearly_full_ = false; |
||||
} |
||||
|
||||
// return false, indicating HashCuckooRep does not support merge operator.
|
||||
virtual bool IsMergeOperatorSupported() const override { return false; } |
||||
|
||||
// return false, indicating HashCuckooRep does not support snapshot.
|
||||
virtual bool IsSnapshotSupported() const override { return false; } |
||||
|
||||
// Returns true iff an entry that compares equal to key is in the collection.
|
||||
virtual bool Contains(const char* internal_key) const override; |
||||
|
||||
virtual ~HashCuckooRep() override {} |
||||
|
||||
// Insert the specified key (internal_key) into the mem-table. Assertion
|
||||
// fails if
|
||||
// the current mem-table already contains the specified key.
|
||||
virtual void Insert(KeyHandle handle) override; |
||||
|
||||
// This function returns std::numeric_limits<size_t>::max() in the following
|
||||
// three cases to disallow further write operations:
|
||||
// 1. when the fullness reaches kMaxFullnes.
|
||||
// 2. when the backup_table_ is used.
|
||||
//
|
||||
// otherwise, this function will always return 0.
|
||||
virtual size_t ApproximateMemoryUsage() override { |
||||
if (is_nearly_full_) { |
||||
return std::numeric_limits<size_t>::max(); |
||||
} |
||||
return 0; |
||||
} |
||||
|
||||
virtual void Get(const LookupKey& k, void* callback_args, |
||||
bool (*callback_func)(void* arg, |
||||
const char* entry)) override; |
||||
|
||||
class Iterator : public MemTableRep::Iterator { |
||||
std::shared_ptr<std::vector<const char*>> bucket_; |
||||
typename std::vector<const char*>::const_iterator mutable cit_; |
||||
const KeyComparator& compare_; |
||||
std::string tmp_; // For passing to EncodeKey
|
||||
bool mutable sorted_; |
||||
void DoSort() const; |
||||
|
||||
public: |
||||
explicit Iterator(std::shared_ptr<std::vector<const char*>> bucket, |
||||
const KeyComparator& compare); |
||||
|
||||
// Initialize an iterator over the specified collection.
|
||||
// The returned iterator is not valid.
|
||||
// explicit Iterator(const MemTableRep* collection);
|
||||
virtual ~Iterator() override{}; |
||||
|
||||
// Returns true iff the iterator is positioned at a valid node.
|
||||
virtual bool Valid() const override; |
||||
|
||||
// Returns the key at the current position.
|
||||
// REQUIRES: Valid()
|
||||
virtual const char* key() const override; |
||||
|
||||
// Advances to the next position.
|
||||
// REQUIRES: Valid()
|
||||
virtual void Next() override; |
||||
|
||||
// Advances to the previous position.
|
||||
// REQUIRES: Valid()
|
||||
virtual void Prev() override; |
||||
|
||||
// Advance to the first entry with a key >= target
|
||||
virtual void Seek(const Slice& user_key, const char* memtable_key) override; |
||||
|
||||
// Position at the first entry in collection.
|
||||
// Final state of iterator is Valid() iff collection is not empty.
|
||||
virtual void SeekToFirst() override; |
||||
|
||||
// Position at the last entry in collection.
|
||||
// Final state of iterator is Valid() iff collection is not empty.
|
||||
virtual void SeekToLast() override; |
||||
}; |
||||
|
||||
struct CuckooStepBuffer { |
||||
CuckooStepBuffer() : write_index_(0), read_index_(0) {} |
||||
~CuckooStepBuffer() {} |
||||
|
||||
int write_index_; |
||||
int read_index_; |
||||
CuckooStep steps_[kCuckooPathMaxSearchSteps]; |
||||
|
||||
CuckooStep& NextWriteBuffer() { return steps_[write_index_++]; } |
||||
|
||||
inline const CuckooStep& ReadNext() { return steps_[read_index_++]; } |
||||
|
||||
inline bool HasNewWrite() { return write_index_ > read_index_; } |
||||
|
||||
inline void reset() { |
||||
write_index_ = 0; |
||||
read_index_ = 0; |
||||
} |
||||
|
||||
inline bool IsFull() { return write_index_ >= kCuckooPathMaxSearchSteps; } |
||||
|
||||
// returns the number of steps that has been read
|
||||
inline int ReadCount() { return read_index_; } |
||||
|
||||
// returns the number of steps that has been written to the buffer.
|
||||
inline int WriteCount() { return write_index_; } |
||||
}; |
||||
|
||||
private: |
||||
const MemTableRep::KeyComparator& compare_; |
||||
// the pointer to Arena to allocate memory, immutable after construction.
|
||||
Arena* const arena_; |
||||
// the number of hash bucket in the hash table.
|
||||
const size_t bucket_count_; |
||||
// the maxinum depth of the cuckoo path.
|
||||
const unsigned int cuckoo_path_max_depth_; |
||||
// the current number of entries in cuckoo_array_ which has been occupied.
|
||||
size_t occupied_count_; |
||||
// the current number of hash functions used in the cuckoo hash.
|
||||
unsigned int hash_function_count_; |
||||
// the backup MemTableRep to handle the case where cuckoo hash cannot find
|
||||
// a vacant bucket for inserting the key of a put request.
|
||||
std::shared_ptr<MemTableRep> backup_table_; |
||||
// the array to store pointers, pointing to the actual data.
|
||||
std::atomic<const char*>* cuckoo_array_; |
||||
// a buffer to store cuckoo path
|
||||
int* cuckoo_path_; |
||||
// a boolean flag indicating whether the fullness of bucket array
|
||||
// reaches the point to make the current memtable immutable.
|
||||
bool is_nearly_full_; |
||||
|
||||
// the default maximum depth of the cuckoo path.
|
||||
static const unsigned int kDefaultCuckooPathMaxDepth = 10; |
||||
|
||||
CuckooStepBuffer step_buffer_; |
||||
|
||||
// returns the bucket id assogied to the input slice based on the
|
||||
unsigned int GetHash(const Slice& slice, const int hash_func_id) const { |
||||
// the seeds used in the Murmur hash to produce different hash functions.
|
||||
static const int kMurmurHashSeeds[HashCuckooRepFactory::kMaxHashCount] = { |
||||
545609244, 1769731426, 763324157, 13099088, 592422103, |
||||
1899789565, 248369300, 1984183468, 1613664382, 1491157517}; |
||||
return MurmurHash(slice.data(), slice.size(), |
||||
kMurmurHashSeeds[hash_func_id]) % |
||||
bucket_count_; |
||||
} |
||||
|
||||
// A cuckoo path is a sequence of bucket ids, where each id points to a
|
||||
// location of cuckoo_array_. This path describes the displacement sequence
|
||||
// of entries in order to store the desired data specified by the input user
|
||||
// key. The path starts from one of the locations associated with the
|
||||
// specified user key and ends at a vacant space in the cuckoo array. This
|
||||
// function will update the cuckoo_path.
|
||||
//
|
||||
// @return true if it found a cuckoo path.
|
||||
bool FindCuckooPath(const char* internal_key, const Slice& user_key, |
||||
int* cuckoo_path, size_t* cuckoo_path_length, |
||||
int initial_hash_id = 0); |
||||
|
||||
// Perform quick insert by checking whether there is a vacant bucket in one
|
||||
// of the possible locations of the input key. If so, then the function will
|
||||
// return true and the key will be stored in that vacant bucket.
|
||||
//
|
||||
// This function is a helper function of FindCuckooPath that discovers the
|
||||
// first possible steps of a cuckoo path. It begins by first computing
|
||||
// the possible locations of the input keys (and stores them in bucket_ids.)
|
||||
// Then, if one of its possible locations is vacant, then the input key will
|
||||
// be stored in that vacant space and the function will return true.
|
||||
// Otherwise, the function will return false indicating a complete search
|
||||
// of cuckoo-path is needed.
|
||||
bool QuickInsert(const char* internal_key, const Slice& user_key, |
||||
int bucket_ids[], const int initial_hash_id); |
||||
|
||||
// Unhide default implementations of GetIterator
|
||||
using MemTableRep::GetIterator; |
||||
// Returns the pointer to the internal iterator to the buckets where buckets
|
||||
// are sorted according to the user specified KeyComparator. Note that
|
||||
// any insert after this function call may affect the sorted nature of
|
||||
// the returned iterator.
|
||||
virtual MemTableRep::Iterator* GetIterator() override { |
||||
std::vector<const char*> compact_buckets; |
||||
for (unsigned int bid = 0; bid < bucket_count_; ++bid) { |
||||
const char* bucket = cuckoo_array_[bid].load(std::memory_order_relaxed); |
||||
if (bucket != nullptr) { |
||||
compact_buckets.push_back(bucket); |
||||
} |
||||
} |
||||
MemTableRep* backup_table = backup_table_.get(); |
||||
if (backup_table != nullptr) { |
||||
std::unique_ptr<MemTableRep::Iterator> iter(backup_table->GetIterator()); |
||||
for (iter->SeekToFirst(); iter->Valid(); iter->Next()) { |
||||
compact_buckets.push_back(iter->key()); |
||||
} |
||||
} |
||||
return new Iterator( |
||||
std::shared_ptr<std::vector<const char*>>( |
||||
new std::vector<const char*>(std::move(compact_buckets))), |
||||
compare_); |
||||
} |
||||
}; |
||||
|
||||
void HashCuckooRep::Get(const LookupKey& key, void* callback_args, |
||||
bool (*callback_func)(void* arg, const char* entry)) { |
||||
Slice user_key = key.user_key(); |
||||
for (unsigned int hid = 0; hid < hash_function_count_; ++hid) { |
||||
const char* bucket = |
||||
cuckoo_array_[GetHash(user_key, hid)].load(std::memory_order_acquire); |
||||
if (bucket != nullptr) { |
||||
auto bucket_user_key = UserKey(bucket); |
||||
if (user_key.compare(bucket_user_key) == 0) { |
||||
callback_func(callback_args, bucket); |
||||
break; |
||||
} |
||||
} else { |
||||
// as Put() always stores at the vacant bucket located by the
|
||||
// hash function with the smallest possible id, when we first
|
||||
// find a vacant bucket in Get(), that means a miss.
|
||||
break; |
||||
} |
||||
} |
||||
MemTableRep* backup_table = backup_table_.get(); |
||||
if (backup_table != nullptr) { |
||||
backup_table->Get(key, callback_args, callback_func); |
||||
} |
||||
} |
||||
|
||||
void HashCuckooRep::Insert(KeyHandle handle) { |
||||
static const float kMaxFullness = 0.90; |
||||
|
||||
auto* key = static_cast<char*>(handle); |
||||
int initial_hash_id = 0; |
||||
size_t cuckoo_path_length = 0; |
||||
auto user_key = UserKey(key); |
||||
// find cuckoo path
|
||||
if (FindCuckooPath(key, user_key, cuckoo_path_, &cuckoo_path_length, |
||||
initial_hash_id) == false) { |
||||
// if true, then we can't find a vacant bucket for this key even we
|
||||
// have used up all the hash functions. Then use a backup memtable to
|
||||
// store such key, which will further make this mem-table become
|
||||
// immutable.
|
||||
if (backup_table_.get() == nullptr) { |
||||
VectorRepFactory factory(10); |
||||
backup_table_.reset(factory.CreateMemTableRep(compare_, arena_, nullptr)); |
||||
is_nearly_full_ = true; |
||||
} |
||||
backup_table_->Insert(key); |
||||
return; |
||||
} |
||||
// when reaching this point, means the insert can be done successfully.
|
||||
occupied_count_++; |
||||
if (occupied_count_ >= bucket_count_ * kMaxFullness) { |
||||
is_nearly_full_ = true; |
||||
} |
||||
|
||||
// perform kickout process if the length of cuckoo path > 1.
|
||||
if (cuckoo_path_length == 0) return; |
||||
|
||||
// the cuckoo path stores the kickout path in reverse order.
|
||||
// so the kickout or displacement is actually performed
|
||||
// in reverse order, which avoids false-negatives on read
|
||||
// by moving each key involved in the cuckoo path to the new
|
||||
// location before replacing it.
|
||||
for (size_t i = 1; i < cuckoo_path_length; ++i) { |
||||
int kicked_out_bid = cuckoo_path_[i - 1]; |
||||
int current_bid = cuckoo_path_[i]; |
||||
// since we only allow one writer at a time, it is safe to do relaxed read.
|
||||
cuckoo_array_[kicked_out_bid] |
||||
.store(cuckoo_array_[current_bid].load(std::memory_order_relaxed), |
||||
std::memory_order_release); |
||||
} |
||||
int insert_key_bid = cuckoo_path_[cuckoo_path_length - 1]; |
||||
cuckoo_array_[insert_key_bid].store(key, std::memory_order_release); |
||||
} |
||||
|
||||
bool HashCuckooRep::Contains(const char* internal_key) const { |
||||
auto user_key = UserKey(internal_key); |
||||
for (unsigned int hid = 0; hid < hash_function_count_; ++hid) { |
||||
const char* stored_key = |
||||
cuckoo_array_[GetHash(user_key, hid)].load(std::memory_order_acquire); |
||||
if (stored_key != nullptr) { |
||||
if (compare_(internal_key, stored_key) == 0) { |
||||
return true; |
||||
} |
||||
} |
||||
} |
||||
return false; |
||||
} |
||||
|
||||
bool HashCuckooRep::QuickInsert(const char* internal_key, const Slice& user_key, |
||||
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