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rocksdb/db/memtable.cc
Kai Liu 11da8bc5df A heuristic way to check if a memtable is full 
Summary:
This is is based on https://reviews.facebook.net/D15027. It's not finished but I would like to give a prototype to avoid arena over-allocation while making better use of the already allocated memory blocks.

Instead of check approximate memtable size, we will take a deeper look at the arena, which incorporate essential idea that @sdong suggests: flush when arena has allocated its last and the last is "almost full"

Test Plan: N/A

Reviewers: haobo, sdong

Reviewed By: sdong

CC: leveldb, sdong

Differential Revision: https://reviews.facebook.net/D15051
2014-03-12 16:40:14 -07:00

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// 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>
#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 "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())),
flush_in_progress_(false),
flush_completed_(false),
file_number_(0),
first_seqno_(0),
mem_next_logfile_number_(0),
mem_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.memtable_prefix_bloom_probes));
}
}
MemTable::~MemTable() {
assert(refs_ == 0);
}
size_t MemTable::ApproximateMemoryUsage() {
return arena_.ApproximateMemoryUsage() + table_->ApproximateMemoryUsage();
}
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 (allocated_memory + kArenaBlockSize * kAllowOverAllocationRatio <
kWriteBufferSize) {
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 * (1 + 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 `0.4 * 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);
}
// 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)
: mem_(mem), iter_(), dynamic_prefix_seek_(false), valid_(false) {
if (options.prefix) {
iter_.reset(mem_.table_->GetPrefixIterator(*options.prefix));
} else if (options.prefix_seek) {
dynamic_prefix_seek_ = true;
iter_.reset(mem_.table_->GetDynamicPrefixIterator());
} else {
iter_.reset(mem_.table_->GetIterator());
}
}
virtual bool Valid() const { return valid_; }
virtual void Seek(const Slice& k) {
if (dynamic_prefix_seek_ && mem_.prefix_bloom_ &&
!mem_.prefix_bloom_->MayContain(
mem_.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:
const MemTable& mem_;
std::shared_ptr<MemTableRep::Iterator> iter_;
bool dynamic_prefix_seek_;
bool valid_;
// No copying allowed
MemTableIterator(const MemTableIterator&);
void operator=(const MemTableIterator&);
};
Iterator* MemTable::NewIterator(const ReadOptions& options) {
return new MemTableIterator(*this, options);
}
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 = arena_.Allocate(encoded_len);
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(buf);
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);
while (merge_context->GetNumOperands() >= 2) {
// Attempt to associative merge. (Returns true if successful)
if (merge_operator->PartialMerge(
s->key->user_key(), merge_context->GetOperand(0),
merge_context->GetOperand(1), &merge_result, s->logger)) {
merge_context->PushPartialMergeResult(merge_result);
} else {
// Stack them because user can't associative merge
break;
}
}
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) {
StopWatchNano memtable_get_timer(options.env, false);
StartPerfTimer(&memtable_get_timer);
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);
}
// No change to value, since we have not yet found a Put/Delete
if (!found_final_value && merge_in_progress) {
*s = Status::MergeInProgress("");
}
BumpPerfTime(&perf_context.get_from_memtable_time, &memtable_get_timer);
BumpPerfCount(&perf_context.get_from_memtable_count);
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::shared_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
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