rocksdb/db/memtable_list.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

403 lines
16 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).
//
#pragma once
#include <deque>
#include <limits>
#include <list>
#include <set>
#include <string>
#include <vector>
#include "db/dbformat.h"
#include "db/logs_with_prep_tracker.h"
#include "db/memtable.h"
#include "db/range_del_aggregator.h"
#include "file/filename.h"
#include "logging/log_buffer.h"
#include "monitoring/instrumented_mutex.h"
#include "rocksdb/db.h"
#include "rocksdb/iterator.h"
#include "rocksdb/options.h"
#include "rocksdb/types.h"
#include "util/autovector.h"
namespace rocksdb {
class ColumnFamilyData;
class InternalKeyComparator;
class InstrumentedMutex;
class MergeIteratorBuilder;
class MemTableList;
// keeps a list of immutable memtables in a vector. the list is immutable
// if refcount is bigger than one. It is used as a state for Get() and
// Iterator code paths
//
// This class is not thread-safe. External synchronization is required
// (such as holding the db mutex or being on the write thread).
class MemTableListVersion {
public:
explicit MemTableListVersion(size_t* parent_memtable_list_memory_usage,
MemTableListVersion* old = nullptr);
explicit MemTableListVersion(size_t* parent_memtable_list_memory_usage,
int max_write_buffer_number_to_maintain,
int64_t max_write_buffer_size_to_maintain);
void Ref();
void Unref(autovector<MemTable*>* to_delete = nullptr);
// Search all the memtables starting from the most recent one.
// Return the most recent value found, if any.
//
// If any operation was found for this key, 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.
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 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) {
SequenceNumber seq;
return Get(key, value, s, merge_context, max_covering_tombstone_seq, &seq,
read_opts, callback, is_blob_index);
}
// Returns all the merge operands corresponding to the key by searching all
// memtables starting from the most recent one.
bool GetMergeOperands(const LookupKey& key, Status* s,
MergeContext* merge_context,
SequenceNumber* max_covering_tombstone_seq,
const ReadOptions& read_opts);
// Similar to Get(), but searches the Memtable history of memtables that
// have already been flushed. Should only be used from in-memory only
// queries (such as Transaction validation) as the history may contain
// writes that are also present in the SST files.
bool GetFromHistory(const LookupKey& key, std::string* value, Status* s,
MergeContext* merge_context,
SequenceNumber* max_covering_tombstone_seq,
SequenceNumber* seq, const ReadOptions& read_opts,
bool* is_blob_index = nullptr);
bool GetFromHistory(const LookupKey& key, std::string* value, Status* s,
MergeContext* merge_context,
SequenceNumber* max_covering_tombstone_seq,
const ReadOptions& read_opts,
bool* is_blob_index = nullptr) {
SequenceNumber seq;
return GetFromHistory(key, value, s, merge_context,
max_covering_tombstone_seq, &seq, read_opts,
is_blob_index);
}
Status AddRangeTombstoneIterators(const ReadOptions& read_opts, Arena* arena,
RangeDelAggregator* range_del_agg);
void AddIterators(const ReadOptions& options,
std::vector<InternalIterator*>* iterator_list,
Arena* arena);
void AddIterators(const ReadOptions& options,
MergeIteratorBuilder* merge_iter_builder);
uint64_t GetTotalNumEntries() const;
uint64_t GetTotalNumDeletes() const;
MemTable::MemTableStats ApproximateStats(const Slice& start_ikey,
const Slice& end_ikey);
// Returns the value of MemTable::GetEarliestSequenceNumber() on the most
// recent MemTable in this list or kMaxSequenceNumber if the list is empty.
// If include_history=true, will also search Memtables in MemTableList
// History.
SequenceNumber GetEarliestSequenceNumber(bool include_history = false) const;
private:
friend class MemTableList;
friend Status InstallMemtableAtomicFlushResults(
const autovector<MemTableList*>* imm_lists,
const autovector<ColumnFamilyData*>& cfds,
const autovector<const MutableCFOptions*>& mutable_cf_options_list,
const autovector<const autovector<MemTable*>*>& mems_list,
VersionSet* vset, InstrumentedMutex* mu,
const autovector<FileMetaData*>& file_meta,
autovector<MemTable*>* to_delete, Directory* db_directory,
LogBuffer* log_buffer);
// REQUIRE: m is an immutable memtable
void Add(MemTable* m, autovector<MemTable*>* to_delete);
// REQUIRE: m is an immutable memtable
void Remove(MemTable* m, autovector<MemTable*>* to_delete);
void TrimHistory(autovector<MemTable*>* to_delete, size_t usage);
bool GetFromList(std::list<MemTable*>* list, 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);
void AddMemTable(MemTable* m);
void UnrefMemTable(autovector<MemTable*>* to_delete, MemTable* m);
// Calculate the total amount of memory used by memlist_ and memlist_history_
// excluding the last MemTable in memlist_history_. The reason for excluding
// the last MemTable is to see if dropping the last MemTable will keep total
// memory usage above or equal to max_write_buffer_size_to_maintain_
size_t ApproximateMemoryUsageExcludingLast();
bool MemtableLimitExceeded(size_t usage);
// Immutable MemTables that have not yet been flushed.
std::list<MemTable*> memlist_;
// MemTables that have already been flushed
// (used during Transaction validation)
std::list<MemTable*> memlist_history_;
// Maximum number of MemTables to keep in memory (including both flushed
const int max_write_buffer_number_to_maintain_;
// Maximum size of MemTables to keep in memory (including both flushed
// and not-yet-flushed tables).
const int64_t max_write_buffer_size_to_maintain_;
int refs_ = 0;
size_t* parent_memtable_list_memory_usage_;
};
// This class stores references to all the immutable memtables.
// The memtables are flushed to L0 as soon as possible and in
// any order. If there are more than one immutable memtable, their
// flushes can occur concurrently. However, they are 'committed'
// to the manifest in FIFO order to maintain correctness and
// recoverability from a crash.
//
//
// Other than imm_flush_needed and imm_trim_needed, this class is not
// thread-safe and requires external synchronization (such as holding the db
// mutex or being on the write thread.)
class MemTableList {
public:
// A list of memtables.
explicit MemTableList(int min_write_buffer_number_to_merge,
int max_write_buffer_number_to_maintain,
int64_t max_write_buffer_size_to_maintain)
: imm_flush_needed(false),
imm_trim_needed(false),
min_write_buffer_number_to_merge_(min_write_buffer_number_to_merge),
current_(new MemTableListVersion(&current_memory_usage_,
max_write_buffer_number_to_maintain,
max_write_buffer_size_to_maintain)),
num_flush_not_started_(0),
commit_in_progress_(false),
flush_requested_(false),
current_memory_usage_(0),
current_memory_usage_excluding_last_(0) {
current_->Ref();
}
// Should not delete MemTableList without making sure MemTableList::current()
// is Unref()'d.
~MemTableList() {}
MemTableListVersion* current() const { return current_; }
// so that background threads can detect non-nullptr pointer to
// determine whether there is anything more to start flushing.
std::atomic<bool> imm_flush_needed;
std::atomic<bool> imm_trim_needed;
// Returns the total number of memtables in the list that haven't yet
// been flushed and logged.
int NumNotFlushed() const;
// Returns total number of memtables in the list that have been
// completely flushed and logged.
int NumFlushed() const;
// Returns true if there is at least one memtable on which flush has
// not yet started.
bool IsFlushPending() const;
// Returns the earliest memtables that needs to be flushed. The returned
// memtables are guaranteed to be in the ascending order of created time.
void PickMemtablesToFlush(const uint64_t* max_memtable_id,
autovector<MemTable*>* mems);
// Reset status of the given memtable list back to pending state so that
// they can get picked up again on the next round of flush.
void RollbackMemtableFlush(const autovector<MemTable*>& mems,
uint64_t file_number);
// Try commit a successful flush in the manifest file. It might just return
// Status::OK letting a concurrent flush to do the actual the recording.
Status TryInstallMemtableFlushResults(
ColumnFamilyData* cfd, const MutableCFOptions& mutable_cf_options,
const autovector<MemTable*>& m, LogsWithPrepTracker* prep_tracker,
VersionSet* vset, InstrumentedMutex* mu, uint64_t file_number,
autovector<MemTable*>* to_delete, Directory* db_directory,
LogBuffer* log_buffer);
// New memtables are inserted at the front of the list.
// Takes ownership of the referenced held on *m by the caller of Add().
void Add(MemTable* m, autovector<MemTable*>* to_delete);
// Returns an estimate of the number of bytes of data in use.
size_t ApproximateMemoryUsage();
// Returns the cached current_memory_usage_excluding_last_ value
size_t ApproximateMemoryUsageExcludingLast();
// Update current_memory_usage_excluding_last_ from MemtableListVersion
void UpdateMemoryUsageExcludingLast();
// `usage` is the current size of the mutable Memtable. When
// max_write_buffer_size_to_maintain is used, total size of mutable and
// immutable memtables is checked against it to decide whether to trim
// memtable list.
void TrimHistory(autovector<MemTable*>* to_delete, size_t usage);
// Returns an estimate of the number of bytes of data used by
// the unflushed mem-tables.
size_t ApproximateUnflushedMemTablesMemoryUsage();
// Returns an estimate of the timestamp of the earliest key.
uint64_t ApproximateOldestKeyTime() const;
// Request a flush of all existing memtables to storage. This will
// cause future calls to IsFlushPending() to return true if this list is
// non-empty (regardless of the min_write_buffer_number_to_merge
// parameter). This flush request will persist until the next time
// PickMemtablesToFlush() is called.
void FlushRequested() { flush_requested_ = true; }
bool HasFlushRequested() { return flush_requested_; }
// Returns true if a trim history should be scheduled and the caller should
// be the one to schedule it
bool MarkTrimHistoryNeeded() {
auto expected = false;
return imm_trim_needed.compare_exchange_strong(
expected, true, std::memory_order_relaxed, std::memory_order_relaxed);
}
void ResetTrimHistoryNeeded() {
auto expected = true;
imm_trim_needed.compare_exchange_strong(
expected, false, std::memory_order_relaxed, std::memory_order_relaxed);
}
// Copying allowed
// MemTableList(const MemTableList&);
// void operator=(const MemTableList&);
size_t* current_memory_usage() { return &current_memory_usage_; }
// Returns the min log containing the prep section after memtables listsed in
// `memtables_to_flush` are flushed and their status is persisted in manifest.
uint64_t PrecomputeMinLogContainingPrepSection(
const autovector<MemTable*>& memtables_to_flush);
uint64_t GetEarliestMemTableID() const {
auto& memlist = current_->memlist_;
if (memlist.empty()) {
return std::numeric_limits<uint64_t>::max();
}
return memlist.back()->GetID();
}
uint64_t GetLatestMemTableID() const {
auto& memlist = current_->memlist_;
if (memlist.empty()) {
return 0;
}
return memlist.front()->GetID();
}
void AssignAtomicFlushSeq(const SequenceNumber& seq) {
const auto& memlist = current_->memlist_;
// Scan the memtable list from new to old
for (auto it = memlist.begin(); it != memlist.end(); ++it) {
MemTable* mem = *it;
if (mem->atomic_flush_seqno_ == kMaxSequenceNumber) {
mem->atomic_flush_seqno_ = seq;
} else {
// Earlier memtables must have been assigned a atomic flush seq, no
// need to continue scan.
break;
}
}
}
// Used only by DBImplSecondary during log replay.
// Remove memtables whose data were written before the WAL with log_number
// was created, i.e. mem->GetNextLogNumber() <= log_number. The memtables are
// not freed, but put into a vector for future deref and reclamation.
void RemoveOldMemTables(uint64_t log_number,
autovector<MemTable*>* to_delete);
private:
friend Status InstallMemtableAtomicFlushResults(
const autovector<MemTableList*>* imm_lists,
const autovector<ColumnFamilyData*>& cfds,
const autovector<const MutableCFOptions*>& mutable_cf_options_list,
const autovector<const autovector<MemTable*>*>& mems_list,
VersionSet* vset, InstrumentedMutex* mu,
const autovector<FileMetaData*>& file_meta,
autovector<MemTable*>* to_delete, Directory* db_directory,
LogBuffer* log_buffer);
// DB mutex held
void InstallNewVersion();
const int min_write_buffer_number_to_merge_;
MemTableListVersion* current_;
// the number of elements that still need flushing
int num_flush_not_started_;
// committing in progress
bool commit_in_progress_;
// Requested a flush of memtables to storage. It's possible to request that
// a subset of memtables be flushed.
bool flush_requested_;
// The current memory usage.
size_t current_memory_usage_;
std::atomic<size_t> current_memory_usage_excluding_last_;
};
// Installs memtable atomic flush results.
// In most cases, imm_lists is nullptr, and the function simply uses the
// immutable memtable lists associated with the cfds. There are unit tests that
// installs flush results for external immutable memtable lists other than the
// cfds' own immutable memtable lists, e.g. MemTableLIstTest. In this case,
// imm_lists parameter is not nullptr.
extern Status InstallMemtableAtomicFlushResults(
const autovector<MemTableList*>* imm_lists,
const autovector<ColumnFamilyData*>& cfds,
const autovector<const MutableCFOptions*>& mutable_cf_options_list,
const autovector<const autovector<MemTable*>*>& mems_list, VersionSet* vset,
InstrumentedMutex* mu, const autovector<FileMetaData*>& file_meta,
autovector<MemTable*>* to_delete, Directory* db_directory,
LogBuffer* log_buffer);
} // namespace rocksdb