rocksdb/db/memtable.h
Zhongyi Xie 2f41ecfe75 Refactor trimming logic for immutable memtables (#5022)
Summary:
MyRocks currently sets `max_write_buffer_number_to_maintain` in order to maintain enough history for transaction conflict checking. The effectiveness of this approach depends on the size of memtables. When memtables are small, it may not keep enough history; when memtables are large, this may consume too much memory.
We are proposing a new way to configure memtable list history: by limiting the memory usage of immutable memtables. The new option is `max_write_buffer_size_to_maintain` and it will take precedence over the old `max_write_buffer_number_to_maintain` if they are both set to non-zero values. The new option accounts for the total memory usage of flushed immutable memtables and mutable memtable. When the total usage exceeds the limit, RocksDB may start dropping immutable memtables (which is also called trimming history), starting from the oldest one.
The semantics of the old option actually works both as an upper bound and lower bound. History trimming will start if number of immutable memtables exceeds the limit, but it will never go below (limit-1) due to history trimming.
In order the mimic the behavior with the new option, history trimming will stop if dropping the next immutable memtable causes the total memory usage go below the size limit. For example, assuming the size limit is set to 64MB, and there are 3 immutable memtables with sizes of 20, 30, 30. Although the total memory usage is 80MB > 64MB, dropping the oldest memtable will reduce the memory usage to 60MB < 64MB, so in this case no memtable will be dropped.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5022

Differential Revision: D14394062

Pulled By: miasantreble

fbshipit-source-id: 60457a509c6af89d0993f988c9b5c2aa9e45f5c5
2019-08-23 13:55:34 -07:00

515 lines
20 KiB
C++

// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root 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.
#pragma once
#include <atomic>
#include <deque>
#include <functional>
#include <memory>
#include <string>
#include <unordered_map>
#include <vector>
#include "db/dbformat.h"
#include "db/range_tombstone_fragmenter.h"
#include "db/read_callback.h"
#include "db/version_edit.h"
#include "memory/allocator.h"
#include "memory/concurrent_arena.h"
#include "monitoring/instrumented_mutex.h"
#include "options/cf_options.h"
#include "rocksdb/db.h"
#include "rocksdb/env.h"
#include "rocksdb/memtablerep.h"
#include "util/dynamic_bloom.h"
#include "util/hash.h"
namespace rocksdb {
class Mutex;
class MemTableIterator;
class MergeContext;
struct ImmutableMemTableOptions {
explicit ImmutableMemTableOptions(const ImmutableCFOptions& ioptions,
const MutableCFOptions& mutable_cf_options);
size_t arena_block_size;
uint32_t memtable_prefix_bloom_bits;
size_t memtable_huge_page_size;
bool memtable_whole_key_filtering;
bool inplace_update_support;
size_t inplace_update_num_locks;
UpdateStatus (*inplace_callback)(char* existing_value,
uint32_t* existing_value_size,
Slice delta_value,
std::string* merged_value);
size_t max_successive_merges;
Statistics* statistics;
MergeOperator* merge_operator;
Logger* info_log;
};
// Batched counters to updated when inserting keys in one write batch.
// In post process of the write batch, these can be updated together.
// Only used in concurrent memtable insert case.
struct MemTablePostProcessInfo {
uint64_t data_size = 0;
uint64_t num_entries = 0;
uint64_t num_deletes = 0;
};
// Note: Many of the methods in this class have comments indicating that
// external synchronization is required as these methods are not thread-safe.
// It is up to higher layers of code to decide how to prevent concurrent
// invokation of these methods. This is usually done by acquiring either
// the db mutex or the single writer thread.
//
// Some of these methods are documented to only require external
// synchronization if this memtable is immutable. Calling MarkImmutable() is
// not sufficient to guarantee immutability. It is up to higher layers of
// code to determine if this MemTable can still be modified by other threads.
// Eg: The Superversion stores a pointer to the current MemTable (that can
// be modified) and a separate list of the MemTables that can no longer be
// written to (aka the 'immutable memtables').
class MemTable {
public:
struct KeyComparator : public MemTableRep::KeyComparator {
const InternalKeyComparator comparator;
explicit KeyComparator(const InternalKeyComparator& c) : comparator(c) { }
virtual int operator()(const char* prefix_len_key1,
const char* prefix_len_key2) const override;
virtual int operator()(const char* prefix_len_key,
const DecodedType& key) const override;
};
// MemTables are reference counted. The initial reference count
// is zero and the caller must call Ref() at least once.
//
// earliest_seq should be the current SequenceNumber in the db such that any
// key inserted into this memtable will have an equal or larger seq number.
// (When a db is first created, the earliest sequence number will be 0).
// If the earliest sequence number is not known, kMaxSequenceNumber may be
// used, but this may prevent some transactions from succeeding until the
// first key is inserted into the memtable.
explicit MemTable(const InternalKeyComparator& comparator,
const ImmutableCFOptions& ioptions,
const MutableCFOptions& mutable_cf_options,
WriteBufferManager* write_buffer_manager,
SequenceNumber earliest_seq, uint32_t column_family_id);
// Do not delete this MemTable unless Unref() indicates it not in use.
~MemTable();
// Increase reference count.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
void Ref() { ++refs_; }
// Drop reference count.
// If the refcount goes to zero return this memtable, otherwise return null.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
MemTable* Unref() {
--refs_;
assert(refs_ >= 0);
if (refs_ <= 0) {
return this;
}
return nullptr;
}
// Returns an estimate of the number of bytes of data in use by this
// data structure.
//
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
size_t ApproximateMemoryUsage();
// As a cheap version of `ApproximateMemoryUsage()`, this function doens't
// require external synchronization. The value may be less accurate though
size_t ApproximateMemoryUsageFast() {
return approximate_memory_usage_.load(std::memory_order_relaxed);
}
// This method heuristically determines if the memtable should continue to
// host more data.
bool ShouldScheduleFlush() const {
return flush_state_.load(std::memory_order_relaxed) == FLUSH_REQUESTED;
}
// Returns true if a flush should be scheduled and the caller should
// be the one to schedule it
bool MarkFlushScheduled() {
auto before = FLUSH_REQUESTED;
return flush_state_.compare_exchange_strong(before, FLUSH_SCHEDULED,
std::memory_order_relaxed,
std::memory_order_relaxed);
}
// Return an iterator that yields the contents of the memtable.
//
// The caller must ensure that the underlying MemTable remains live
// while the returned iterator is live. The keys returned by this
// iterator are internal keys encoded by AppendInternalKey in the
// db/dbformat.{h,cc} module.
//
// By default, it returns an iterator for prefix seek if prefix_extractor
// is configured in Options.
// arena: If not null, the arena needs to be used to allocate the Iterator.
// Calling ~Iterator of the iterator will destroy all the states but
// those allocated in arena.
InternalIterator* NewIterator(const ReadOptions& read_options, Arena* arena);
FragmentedRangeTombstoneIterator* NewRangeTombstoneIterator(
const ReadOptions& read_options, SequenceNumber read_seq);
// Add an entry into memtable that maps key to value at the
// specified sequence number and with the specified type.
// Typically value will be empty if type==kTypeDeletion.
//
// REQUIRES: if allow_concurrent = false, external synchronization to prevent
// simultaneous operations on the same MemTable.
//
// Returns false if MemTableRepFactory::CanHandleDuplicatedKey() is true and
// the <key, seq> already exists.
bool Add(SequenceNumber seq, ValueType type, const Slice& key,
const Slice& value, bool allow_concurrent = false,
MemTablePostProcessInfo* post_process_info = nullptr);
// Used to Get value associated with key or Get Merge Operands associated
// with key.
// If do_merge = true the default behavior which is Get value for key is
// executed. Expected behavior is described right below.
// If memtable contains a value for key, store it in *value and return true.
// If memtable contains a deletion for key, store a NotFound() error
// in *status and return true.
// If memtable contains Merge operation as the most recent entry for a key,
// and the merge process does not stop (not reaching a value or delete),
// prepend the current merge operand to *operands.
// store MergeInProgress in s, and return false.
// Else, return false.
// If any operation was found, its most recent sequence number
// will be stored in *seq on success (regardless of whether true/false is
// returned). Otherwise, *seq will be set to kMaxSequenceNumber.
// On success, *s may be set to OK, NotFound, or MergeInProgress. Any other
// status returned indicates a corruption or other unexpected error.
// If do_merge = false then any Merge Operands encountered for key are simply
// stored in merge_context.operands_list and never actually merged to get a
// final value. The raw Merge Operands are eventually returned to the user.
bool Get(const LookupKey& key, std::string* value, Status* s,
MergeContext* merge_context,
SequenceNumber* max_covering_tombstone_seq, SequenceNumber* seq,
const ReadOptions& read_opts, ReadCallback* callback = nullptr,
bool* is_blob_index = nullptr, bool do_merge = true);
bool Get(const LookupKey& key, std::string* value, Status* s,
MergeContext* merge_context,
SequenceNumber* max_covering_tombstone_seq,
const ReadOptions& read_opts, ReadCallback* callback = nullptr,
bool* is_blob_index = nullptr, bool do_merge = true) {
SequenceNumber seq;
return Get(key, value, s, merge_context, max_covering_tombstone_seq, &seq,
read_opts, callback, is_blob_index, do_merge);
}
// Attempts to update the new_value inplace, else does normal Add
// Pseudocode
// if key exists in current memtable && prev_value is of type kTypeValue
// if new sizeof(new_value) <= sizeof(prev_value)
// update inplace
// else add(key, new_value)
// else add(key, new_value)
//
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
void Update(SequenceNumber seq,
const Slice& key,
const Slice& value);
// If prev_value for key exists, attempts to update it inplace.
// else returns false
// Pseudocode
// if key exists in current memtable && prev_value is of type kTypeValue
// new_value = delta(prev_value)
// if sizeof(new_value) <= sizeof(prev_value)
// update inplace
// else add(key, new_value)
// else return false
//
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
bool UpdateCallback(SequenceNumber seq,
const Slice& key,
const Slice& delta);
// Returns the number of successive merge entries starting from the newest
// entry for the key up to the last non-merge entry or last entry for the
// key in the memtable.
size_t CountSuccessiveMergeEntries(const LookupKey& key);
// Update counters and flush status after inserting a whole write batch
// Used in concurrent memtable inserts.
void BatchPostProcess(const MemTablePostProcessInfo& update_counters) {
num_entries_.fetch_add(update_counters.num_entries,
std::memory_order_relaxed);
data_size_.fetch_add(update_counters.data_size, std::memory_order_relaxed);
if (update_counters.num_deletes != 0) {
num_deletes_.fetch_add(update_counters.num_deletes,
std::memory_order_relaxed);
}
UpdateFlushState();
}
// Get total number of entries in the mem table.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
uint64_t num_entries() const {
return num_entries_.load(std::memory_order_relaxed);
}
// Get total number of deletes in the mem table.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
uint64_t num_deletes() const {
return num_deletes_.load(std::memory_order_relaxed);
}
uint64_t get_data_size() const {
return data_size_.load(std::memory_order_relaxed);
}
// Dynamically change the memtable's capacity. If set below the current usage,
// the next key added will trigger a flush. Can only increase size when
// memtable prefix bloom is disabled, since we can't easily allocate more
// space.
void UpdateWriteBufferSize(size_t new_write_buffer_size) {
if (bloom_filter_ == nullptr ||
new_write_buffer_size < write_buffer_size_) {
write_buffer_size_.store(new_write_buffer_size,
std::memory_order_relaxed);
}
}
// Returns the edits area that is needed for flushing the memtable
VersionEdit* GetEdits() { return &edit_; }
// Returns if there is no entry inserted to the mem table.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
bool IsEmpty() const { return first_seqno_ == 0; }
// Returns the sequence number of the first element that was inserted
// into the memtable.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable (unless this Memtable is immutable).
SequenceNumber GetFirstSequenceNumber() {
return first_seqno_.load(std::memory_order_relaxed);
}
// Returns the sequence number that is guaranteed to be smaller than or equal
// to the sequence number of any key that could be inserted into this
// memtable. It can then be assumed that any write with a larger(or equal)
// sequence number will be present in this memtable or a later memtable.
//
// If the earliest sequence number could not be determined,
// kMaxSequenceNumber will be returned.
SequenceNumber GetEarliestSequenceNumber() {
return earliest_seqno_.load(std::memory_order_relaxed);
}
// DB's latest sequence ID when the memtable is created. This number
// may be updated to a more recent one before any key is inserted.
SequenceNumber GetCreationSeq() const { return creation_seq_; }
void SetCreationSeq(SequenceNumber sn) { creation_seq_ = sn; }
// Returns the next active logfile number when this memtable is about to
// be flushed to storage
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
uint64_t GetNextLogNumber() { return mem_next_logfile_number_; }
// Sets the next active logfile number when this memtable is about to
// be flushed to storage
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
void SetNextLogNumber(uint64_t num) { mem_next_logfile_number_ = num; }
// if this memtable contains data from a committed
// two phase transaction we must take note of the
// log which contains that data so we can know
// when to relese that log
void RefLogContainingPrepSection(uint64_t log);
uint64_t GetMinLogContainingPrepSection();
// Notify the underlying storage that no more items will be added.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
// After MarkImmutable() is called, you should not attempt to
// write anything to this MemTable(). (Ie. do not call Add() or Update()).
void MarkImmutable() {
table_->MarkReadOnly();
mem_tracker_.DoneAllocating();
}
// Notify the underlying storage that all data it contained has been
// persisted.
// REQUIRES: external synchronization to prevent simultaneous
// operations on the same MemTable.
void MarkFlushed() {
table_->MarkFlushed();
}
// return true if the current MemTableRep supports merge operator.
bool IsMergeOperatorSupported() const {
return table_->IsMergeOperatorSupported();
}
// return true if the current MemTableRep supports snapshots.
// inplace update prevents snapshots,
bool IsSnapshotSupported() const {
return table_->IsSnapshotSupported() && !moptions_.inplace_update_support;
}
struct MemTableStats {
uint64_t size;
uint64_t count;
};
MemTableStats ApproximateStats(const Slice& start_ikey,
const Slice& end_ikey);
// Get the lock associated for the key
port::RWMutex* GetLock(const Slice& key);
const InternalKeyComparator& GetInternalKeyComparator() const {
return comparator_.comparator;
}
const ImmutableMemTableOptions* GetImmutableMemTableOptions() const {
return &moptions_;
}
uint64_t ApproximateOldestKeyTime() const {
return oldest_key_time_.load(std::memory_order_relaxed);
}
// REQUIRES: db_mutex held.
void SetID(uint64_t id) { id_ = id; }
uint64_t GetID() const { return id_; }
void SetFlushCompleted(bool completed) { flush_completed_ = completed; }
uint64_t GetFileNumber() const { return file_number_; }
void SetFileNumber(uint64_t file_num) { file_number_ = file_num; }
void SetFlushInProgress(bool in_progress) {
flush_in_progress_ = in_progress;
}
private:
enum FlushStateEnum { FLUSH_NOT_REQUESTED, FLUSH_REQUESTED, FLUSH_SCHEDULED };
friend class MemTableIterator;
friend class MemTableBackwardIterator;
friend class MemTableList;
KeyComparator comparator_;
const ImmutableMemTableOptions moptions_;
int refs_;
const size_t kArenaBlockSize;
AllocTracker mem_tracker_;
ConcurrentArena arena_;
std::unique_ptr<MemTableRep> table_;
std::unique_ptr<MemTableRep> range_del_table_;
std::atomic_bool is_range_del_table_empty_;
// Total data size of all data inserted
std::atomic<uint64_t> data_size_;
std::atomic<uint64_t> num_entries_;
std::atomic<uint64_t> num_deletes_;
// Dynamically changeable memtable option
std::atomic<size_t> write_buffer_size_;
// These are used to manage memtable flushes to storage
bool flush_in_progress_; // started the flush
bool flush_completed_; // finished the flush
uint64_t file_number_; // filled up after flush is complete
// The updates to be applied to the transaction log when this
// memtable is flushed to storage.
VersionEdit edit_;
// The sequence number of the kv that was inserted first
std::atomic<SequenceNumber> first_seqno_;
// The db sequence number at the time of creation or kMaxSequenceNumber
// if not set.
std::atomic<SequenceNumber> earliest_seqno_;
SequenceNumber creation_seq_;
// The log files earlier than this number can be deleted.
uint64_t mem_next_logfile_number_;
// the earliest log containing a prepared section
// which has been inserted into this memtable.
std::atomic<uint64_t> min_prep_log_referenced_;
// rw locks for inplace updates
std::vector<port::RWMutex> locks_;
const SliceTransform* const prefix_extractor_;
std::unique_ptr<DynamicBloom> bloom_filter_;
std::atomic<FlushStateEnum> flush_state_;
Env* env_;
// Extract sequential insert prefixes.
const SliceTransform* insert_with_hint_prefix_extractor_;
// Insert hints for each prefix.
std::unordered_map<Slice, void*, SliceHasher> insert_hints_;
// Timestamp of oldest key
std::atomic<uint64_t> oldest_key_time_;
// Memtable id to track flush.
uint64_t id_ = 0;
// Sequence number of the atomic flush that is responsible for this memtable.
// The sequence number of atomic flush is a seq, such that no writes with
// sequence numbers greater than or equal to seq are flushed, while all
// writes with sequence number smaller than seq are flushed.
SequenceNumber atomic_flush_seqno_;
// keep track of memory usage in table_, arena_, and range_del_table_.
// Gets refrshed inside `ApproximateMemoryUsage()` or `ShouldFlushNow`
std::atomic<uint64_t> approximate_memory_usage_;
// Returns a heuristic flush decision
bool ShouldFlushNow();
// Updates flush_state_ using ShouldFlushNow()
void UpdateFlushState();
void UpdateOldestKeyTime();
// No copying allowed
MemTable(const MemTable&);
MemTable& operator=(const MemTable&);
};
extern const char* EncodeKey(std::string* scratch, const Slice& target);
} // namespace rocksdb