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rocksdb/db/memtable.cc

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22 KiB

// 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.
#include "db/memtable.h"
#include <memory>
#include <algorithm>
Add a new mem-table representation based on cuckoo hash. 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
11 years ago
#include <limits>
#include "db/dbformat.h"
#include "db/merge_context.h"
#include "rocksdb/comparator.h"
#include "rocksdb/env.h"
#include "rocksdb/iterator.h"
#include "rocksdb/merge_operator.h"
#include "rocksdb/slice_transform.h"
#include "table/merger.h"
#include "util/arena.h"
#include "util/coding.h"
#include "util/murmurhash.h"
#include "util/mutexlock.h"
#include "util/perf_context_imp.h"
#include "util/statistics.h"
#include "util/stop_watch.h"
namespace rocksdb {
MemTable::MemTable(const InternalKeyComparator& cmp, const Options& options)
: comparator_(cmp),
refs_(0),
kArenaBlockSize(OptimizeBlockSize(options.arena_block_size)),
kWriteBufferSize(options.write_buffer_size),
arena_(options.arena_block_size),
table_(options.memtable_factory->CreateMemTableRep(
comparator_, &arena_, options.prefix_extractor.get(),
options.info_log.get())),
num_entries_(0),
flush_in_progress_(false),
flush_completed_(false),
file_number_(0),
first_seqno_(0),
mem_next_logfile_number_(0),
locks_(options.inplace_update_support ? options.inplace_update_num_locks
: 0),
prefix_extractor_(options.prefix_extractor.get()),
should_flush_(ShouldFlushNow()) {
// if should_flush_ == true without an entry inserted, something must have
// gone wrong already.
assert(!should_flush_);
if (prefix_extractor_ && options.memtable_prefix_bloom_bits > 0) {
prefix_bloom_.reset(new DynamicBloom(
options.memtable_prefix_bloom_bits, options.bloom_locality,
options.memtable_prefix_bloom_probes, nullptr,
options.memtable_prefix_bloom_huge_page_tlb_size,
options.info_log.get()));
}
}
MemTable::~MemTable() {
assert(refs_ == 0);
}
size_t MemTable::ApproximateMemoryUsage() {
Add a new mem-table representation based on cuckoo hash. 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
11 years ago
size_t arena_usage = arena_.ApproximateMemoryUsage();
size_t table_usage = table_->ApproximateMemoryUsage();
// let MAX_USAGE = std::numeric_limits<size_t>::max()
// then if arena_usage + total_usage >= MAX_USAGE, return MAX_USAGE.
// the following variation is to avoid numeric overflow.
if (arena_usage >= std::numeric_limits<size_t>::max() - table_usage) {
return std::numeric_limits<size_t>::max();
}
// otherwise, return the actual usage
return arena_usage + table_usage;
}
bool MemTable::ShouldFlushNow() const {
// In a lot of times, we cannot allocate arena blocks that exactly matches the
// buffer size. Thus we have to decide if we should over-allocate or
// under-allocate.
// This constant avariable can be interpreted as: if we still have more than
// "kAllowOverAllocationRatio * kArenaBlockSize" space left, we'd try to over
// allocate one more block.
const double kAllowOverAllocationRatio = 0.6;
// If arena still have room for new block allocation, we can safely say it
// shouldn't flush.
auto allocated_memory =
table_->ApproximateMemoryUsage() + arena_.MemoryAllocatedBytes();
// if we can still allocate one more block without exceeding the
// over-allocation ratio, then we should not flush.
if (allocated_memory + kArenaBlockSize <
kWriteBufferSize + kArenaBlockSize * kAllowOverAllocationRatio) {
return false;
}
// if user keeps adding entries that exceeds kWriteBufferSize, we need to
// flush earlier even though we still have much available memory left.
if (allocated_memory >
kWriteBufferSize + kArenaBlockSize * kAllowOverAllocationRatio) {
return true;
}
// In this code path, Arena has already allocated its "last block", which
// means the total allocatedmemory size is either:
// (1) "moderately" over allocated the memory (no more than `0.6 * arena
// block size`. Or,
// (2) the allocated memory is less than write buffer size, but we'll stop
// here since if we allocate a new arena block, we'll over allocate too much
// more (half of the arena block size) memory.
//
// In either case, to avoid over-allocate, the last block will stop allocation
// when its usage reaches a certain ratio, which we carefully choose "0.75
// full" as the stop condition because it addresses the following issue with
// great simplicity: What if the next inserted entry's size is
// bigger than AllocatedAndUnused()?
//
// The answer is: if the entry size is also bigger than 0.25 *
// kArenaBlockSize, a dedicated block will be allocated for it; otherwise
// arena will anyway skip the AllocatedAndUnused() and allocate a new, empty
// and regular block. In either case, we *overly* over-allocated.
//
// Therefore, setting the last block to be at most "0.75 full" avoids both
// cases.
//
// NOTE: the average percentage of waste space of this approach can be counted
// as: "arena block size * 0.25 / write buffer size". User who specify a small
// write buffer size and/or big arena block size may suffer.
return arena_.AllocatedAndUnused() < kArenaBlockSize / 4;
}
int MemTable::KeyComparator::operator()(const char* prefix_len_key1,
const char* prefix_len_key2) const {
// Internal keys are encoded as length-prefixed strings.
Slice k1 = GetLengthPrefixedSlice(prefix_len_key1);
Slice k2 = GetLengthPrefixedSlice(prefix_len_key2);
return comparator.Compare(k1, k2);
}
int MemTable::KeyComparator::operator()(const char* prefix_len_key,
const Slice& key)
const {
// Internal keys are encoded as length-prefixed strings.
Slice a = GetLengthPrefixedSlice(prefix_len_key);
return comparator.Compare(a, key);
}
Slice MemTableRep::UserKey(const char* key) const {
Slice slice = GetLengthPrefixedSlice(key);
return Slice(slice.data(), slice.size() - 8);
}
KeyHandle MemTableRep::Allocate(const size_t len, char** buf) {
*buf = arena_->Allocate(len);
return static_cast<KeyHandle>(*buf);
}
// Encode a suitable internal key target for "target" and return it.
// Uses *scratch as scratch space, and the returned pointer will point
// into this scratch space.
const char* EncodeKey(std::string* scratch, const Slice& target) {
scratch->clear();
PutVarint32(scratch, target.size());
scratch->append(target.data(), target.size());
return scratch->data();
}
class MemTableIterator: public Iterator {
public:
MemTableIterator(const MemTable& mem, const ReadOptions& options,
bool enforce_total_order, Arena* arena)
: bloom_(nullptr),
prefix_extractor_(mem.prefix_extractor_),
valid_(false),
arena_mode_(arena != nullptr) {
if (prefix_extractor_ != nullptr && !enforce_total_order) {
bloom_ = mem.prefix_bloom_.get();
iter_ = mem.table_->GetDynamicPrefixIterator(arena);
} else {
iter_ = mem.table_->GetIterator(arena);
}
}
~MemTableIterator() {
if (arena_mode_) {
iter_->~Iterator();
} else {
delete iter_;
}
}
virtual bool Valid() const { return valid_; }
virtual void Seek(const Slice& k) {
if (bloom_ != nullptr &&
!bloom_->MayContain(prefix_extractor_->Transform(ExtractUserKey(k)))) {
valid_ = false;
return;
}
iter_->Seek(k, nullptr);
valid_ = iter_->Valid();
}
virtual void SeekToFirst() {
iter_->SeekToFirst();
valid_ = iter_->Valid();
}
virtual void SeekToLast() {
iter_->SeekToLast();
valid_ = iter_->Valid();
}
virtual void Next() {
assert(Valid());
iter_->Next();
valid_ = iter_->Valid();
}
virtual void Prev() {
assert(Valid());
iter_->Prev();
valid_ = iter_->Valid();
}
virtual Slice key() const {
assert(Valid());
return GetLengthPrefixedSlice(iter_->key());
}
virtual Slice value() const {
assert(Valid());
Slice key_slice = GetLengthPrefixedSlice(iter_->key());
return GetLengthPrefixedSlice(key_slice.data() + key_slice.size());
}
virtual Status status() const { return Status::OK(); }
private:
DynamicBloom* bloom_;
const SliceTransform* const prefix_extractor_;
MemTableRep::Iterator* iter_;
bool valid_;
bool arena_mode_;
// No copying allowed
MemTableIterator(const MemTableIterator&);
void operator=(const MemTableIterator&);
};
Iterator* MemTable::NewIterator(const ReadOptions& options,
bool enforce_total_order, Arena* arena) {
if (arena == nullptr) {
return new MemTableIterator(*this, options, enforce_total_order, nullptr);
} else {
auto mem = arena->AllocateAligned(sizeof(MemTableIterator));
return new (mem)
MemTableIterator(*this, options, enforce_total_order, arena);
}
}
port::RWMutex* MemTable::GetLock(const Slice& key) {
static murmur_hash hash;
return &locks_[hash(key) % locks_.size()];
}
void MemTable::Add(SequenceNumber s, ValueType type,
const Slice& key, /* user key */
const Slice& value) {
// Format of an entry is concatenation of:
// key_size : varint32 of internal_key.size()
// key bytes : char[internal_key.size()]
// value_size : varint32 of value.size()
// value bytes : char[value.size()]
size_t key_size = key.size();
size_t val_size = value.size();
size_t internal_key_size = key_size + 8;
const size_t encoded_len =
VarintLength(internal_key_size) + internal_key_size +
VarintLength(val_size) + val_size;
char* buf = nullptr;
KeyHandle handle = table_->Allocate(encoded_len, &buf);
assert(buf != nullptr);
char* p = EncodeVarint32(buf, internal_key_size);
memcpy(p, key.data(), key_size);
p += key_size;
EncodeFixed64(p, (s << 8) | type);
p += 8;
p = EncodeVarint32(p, val_size);
memcpy(p, value.data(), val_size);
assert((unsigned)(p + val_size - buf) == (unsigned)encoded_len);
table_->Insert(handle);
num_entries_++;
if (prefix_bloom_) {
assert(prefix_extractor_);
prefix_bloom_->Add(prefix_extractor_->Transform(key));
}
// The first sequence number inserted into the memtable
assert(first_seqno_ == 0 || s > first_seqno_);
if (first_seqno_ == 0) {
first_seqno_ = s;
}
should_flush_ = ShouldFlushNow();
}
// Callback from MemTable::Get()
namespace {
struct Saver {
Status* status;
const LookupKey* key;
bool* found_final_value; // Is value set correctly? Used by KeyMayExist
bool* merge_in_progress;
std::string* value;
const MergeOperator* merge_operator;
// the merge operations encountered;
MergeContext* merge_context;
MemTable* mem;
Logger* logger;
Statistics* statistics;
bool inplace_update_support;
};
} // namespace
static bool SaveValue(void* arg, const char* entry) {
Saver* s = reinterpret_cast<Saver*>(arg);
MergeContext* merge_context = s->merge_context;
const MergeOperator* merge_operator = s->merge_operator;
assert(s != nullptr && merge_context != nullptr);
// entry format is:
// klength varint32
// userkey char[klength-8]
// tag uint64
// vlength varint32
// value char[vlength]
// Check that it belongs to same user key. We do not check the
// sequence number since the Seek() call above should have skipped
// all entries with overly large sequence numbers.
uint32_t key_length;
const char* key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
if (s->mem->GetInternalKeyComparator().user_comparator()->Compare(
Slice(key_ptr, key_length - 8), s->key->user_key()) == 0) {
// Correct user key
const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
switch (static_cast<ValueType>(tag & 0xff)) {
case kTypeValue: {
if (s->inplace_update_support) {
s->mem->GetLock(s->key->user_key())->ReadLock();
}
Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
*(s->status) = Status::OK();
if (*(s->merge_in_progress)) {
assert(merge_operator);
if (!merge_operator->FullMerge(s->key->user_key(), &v,
merge_context->GetOperands(), s->value,
s->logger)) {
RecordTick(s->statistics, NUMBER_MERGE_FAILURES);
*(s->status) =
Status::Corruption("Error: Could not perform merge.");
}
} else {
s->value->assign(v.data(), v.size());
}
if (s->inplace_update_support) {
s->mem->GetLock(s->key->user_key())->Unlock();
}
*(s->found_final_value) = true;
return false;
}
case kTypeDeletion: {
if (*(s->merge_in_progress)) {
assert(merge_operator);
*(s->status) = Status::OK();
if (!merge_operator->FullMerge(s->key->user_key(), nullptr,
merge_context->GetOperands(), s->value,
s->logger)) {
RecordTick(s->statistics, NUMBER_MERGE_FAILURES);
*(s->status) =
Status::Corruption("Error: Could not perform merge.");
}
} else {
*(s->status) = Status::NotFound();
}
*(s->found_final_value) = true;
return false;
}
case kTypeMerge: {
std::string merge_result; // temporary area for merge results later
Slice v = GetLengthPrefixedSlice(key_ptr + key_length);
*(s->merge_in_progress) = true;
merge_context->PushOperand(v);
return true;
}
default:
assert(false);
return true;
}
}
// s->state could be Corrupt, merge or notfound
return false;
}
bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
MergeContext& merge_context, const Options& options) {
PERF_TIMER_AUTO(get_from_memtable_time);
Slice user_key = key.user_key();
bool found_final_value = false;
bool merge_in_progress = s->IsMergeInProgress();
if (prefix_bloom_ &&
!prefix_bloom_->MayContain(prefix_extractor_->Transform(user_key))) {
// iter is null if prefix bloom says the key does not exist
} else {
Saver saver;
saver.status = s;
saver.found_final_value = &found_final_value;
saver.merge_in_progress = &merge_in_progress;
saver.key = &key;
saver.value = value;
saver.status = s;
saver.mem = this;
saver.merge_context = &merge_context;
saver.merge_operator = options.merge_operator.get();
saver.logger = options.info_log.get();
saver.inplace_update_support = options.inplace_update_support;
saver.statistics = options.statistics.get();
table_->Get(key, &saver, SaveValue);
}
[RocksDB] [MergeOperator] The new Merge Interface! Uses merge sequences. Summary: Here are the major changes to the Merge Interface. It has been expanded to handle cases where the MergeOperator is not associative. It does so by stacking up merge operations while scanning through the key history (i.e.: during Get() or Compaction), until a valid Put/Delete/end-of-history is encountered; it then applies all of the merge operations in the correct sequence starting with the base/sentinel value. I have also introduced an "AssociativeMerge" function which allows the user to take advantage of associative merge operations (such as in the case of counters). The implementation will always attempt to merge the operations/operands themselves together when they are encountered, and will resort to the "stacking" method if and only if the "associative-merge" fails. This implementation is conjectured to allow MergeOperator to handle the general case, while still providing the user with the ability to take advantage of certain efficiencies in their own merge-operator / data-structure. NOTE: This is a preliminary diff. This must still go through a lot of review, revision, and testing. Feedback welcome! Test Plan: -This is a preliminary diff. I have only just begun testing/debugging it. -I will be testing this with the existing MergeOperator use-cases and unit-tests (counters, string-append, and redis-lists) -I will be "desk-checking" and walking through the code with the help gdb. -I will find a way of stress-testing the new interface / implementation using db_bench, db_test, merge_test, and/or db_stress. -I will ensure that my tests cover all cases: Get-Memtable, Get-Immutable-Memtable, Get-from-Disk, Iterator-Range-Scan, Flush-Memtable-to-L0, Compaction-L0-L1, Compaction-Ln-L(n+1), Put/Delete found, Put/Delete not-found, end-of-history, end-of-file, etc. -A lot of feedback from the reviewers. Reviewers: haobo, dhruba, zshao, emayanke Reviewed By: haobo CC: leveldb Differential Revision: https://reviews.facebook.net/D11499
12 years ago
// No change to value, since we have not yet found a Put/Delete
if (!found_final_value && merge_in_progress) {
*s = Status::MergeInProgress("");
}
PERF_TIMER_STOP(get_from_memtable_time);
PERF_COUNTER_ADD(get_from_memtable_count, 1);
return found_final_value;
}
void MemTable::Update(SequenceNumber seq,
const Slice& key,
const Slice& value) {
LookupKey lkey(key, seq);
Slice mem_key = lkey.memtable_key();
std::unique_ptr<MemTableRep::Iterator> iter(
table_->GetIterator(lkey.user_key()));
iter->Seek(lkey.internal_key(), mem_key.data());
if (iter->Valid()) {
// entry format is:
// key_length varint32
// userkey char[klength-8]
// tag uint64
// vlength varint32
// value char[vlength]
// Check that it belongs to same user key. We do not check the
// sequence number since the Seek() call above should have skipped
// all entries with overly large sequence numbers.
const char* entry = iter->key();
uint32_t key_length = 0;
const char* key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
if (comparator_.comparator.user_comparator()->Compare(
Slice(key_ptr, key_length - 8), lkey.user_key()) == 0) {
// Correct user key
const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
switch (static_cast<ValueType>(tag & 0xff)) {
case kTypeValue: {
Slice prev_value = GetLengthPrefixedSlice(key_ptr + key_length);
uint32_t prev_size = prev_value.size();
uint32_t new_size = value.size();
// Update value, if new value size <= previous value size
if (new_size <= prev_size ) {
char* p = EncodeVarint32(const_cast<char*>(key_ptr) + key_length,
new_size);
WriteLock wl(GetLock(lkey.user_key()));
memcpy(p, value.data(), value.size());
assert((unsigned)((p + value.size()) - entry) ==
(unsigned)(VarintLength(key_length) + key_length +
VarintLength(value.size()) + value.size()));
return;
}
}
default:
// If the latest value is kTypeDeletion, kTypeMerge or kTypeLogData
// we don't have enough space for update inplace
Add(seq, kTypeValue, key, value);
return;
}
}
}
// key doesn't exist
Add(seq, kTypeValue, key, value);
}
bool MemTable::UpdateCallback(SequenceNumber seq,
const Slice& key,
const Slice& delta,
const Options& options) {
LookupKey lkey(key, seq);
Slice memkey = lkey.memtable_key();
std::unique_ptr<MemTableRep::Iterator> iter(
table_->GetIterator(lkey.user_key()));
iter->Seek(lkey.internal_key(), memkey.data());
if (iter->Valid()) {
// entry format is:
// key_length varint32
// userkey char[klength-8]
// tag uint64
// vlength varint32
// value char[vlength]
// Check that it belongs to same user key. We do not check the
// sequence number since the Seek() call above should have skipped
// all entries with overly large sequence numbers.
const char* entry = iter->key();
uint32_t key_length = 0;
const char* key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
if (comparator_.comparator.user_comparator()->Compare(
Slice(key_ptr, key_length - 8), lkey.user_key()) == 0) {
// Correct user key
const uint64_t tag = DecodeFixed64(key_ptr + key_length - 8);
switch (static_cast<ValueType>(tag & 0xff)) {
case kTypeValue: {
Slice prev_value = GetLengthPrefixedSlice(key_ptr + key_length);
uint32_t prev_size = prev_value.size();
char* prev_buffer = const_cast<char*>(prev_value.data());
uint32_t new_prev_size = prev_size;
std::string str_value;
WriteLock wl(GetLock(lkey.user_key()));
auto status = options.inplace_callback(prev_buffer, &new_prev_size,
delta, &str_value);
if (status == UpdateStatus::UPDATED_INPLACE) {
// Value already updated by callback.
assert(new_prev_size <= prev_size);
if (new_prev_size < prev_size) {
// overwrite the new prev_size
char* p = EncodeVarint32(const_cast<char*>(key_ptr) + key_length,
new_prev_size);
if (VarintLength(new_prev_size) < VarintLength(prev_size)) {
// shift the value buffer as well.
memcpy(p, prev_buffer, new_prev_size);
}
}
RecordTick(options.statistics.get(), NUMBER_KEYS_UPDATED);
should_flush_ = ShouldFlushNow();
return true;
} else if (status == UpdateStatus::UPDATED) {
Add(seq, kTypeValue, key, Slice(str_value));
RecordTick(options.statistics.get(), NUMBER_KEYS_WRITTEN);
should_flush_ = ShouldFlushNow();
return true;
} else if (status == UpdateStatus::UPDATE_FAILED) {
// No action required. Return.
should_flush_ = ShouldFlushNow();
return true;
}
}
default:
break;
}
}
}
// If the latest value is not kTypeValue
// or key doesn't exist
return false;
}
size_t MemTable::CountSuccessiveMergeEntries(const LookupKey& key) {
Slice memkey = key.memtable_key();
// A total ordered iterator is costly for some memtablerep (prefix aware
// reps). By passing in the user key, we allow efficient iterator creation.
// The iterator only needs to be ordered within the same user key.
std::unique_ptr<MemTableRep::Iterator> iter(
table_->GetIterator(key.user_key()));
iter->Seek(key.internal_key(), memkey.data());
size_t num_successive_merges = 0;
for (; iter->Valid(); iter->Next()) {
const char* entry = iter->key();
uint32_t key_length = 0;
const char* iter_key_ptr = GetVarint32Ptr(entry, entry + 5, &key_length);
if (comparator_.comparator.user_comparator()->Compare(
Slice(iter_key_ptr, key_length - 8), key.user_key()) != 0) {
break;
}
const uint64_t tag = DecodeFixed64(iter_key_ptr + key_length - 8);
if (static_cast<ValueType>(tag & 0xff) != kTypeMerge) {
break;
}
++num_successive_merges;
}
return num_successive_merges;
}
void MemTableRep::Get(const LookupKey& k, void* callback_args,
bool (*callback_func)(void* arg, const char* entry)) {
auto iter = GetIterator(k.user_key());
for (iter->Seek(k.internal_key(), k.memtable_key().data());
iter->Valid() && callback_func(callback_args, iter->key());
iter->Next()) {
}
}
} // namespace rocksdb