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rocksdb/utilities/transactions/write_prepared_txn_db.cc
Manuel Ung a16e00b7b9 WriteUnPrepared Txn: Disable seek to snapshot optimization (#3955) 
Summary:
This is implemented by extending ReadCallback with another function `MaxUnpreparedSequenceNumber` which returns the largest visible sequence number for the current transaction, if there is uncommitted data written to DB. Otherwise, it returns zero, indicating no uncommitted data.

There are the places where reads had to be modified.
- Get and Seek/Next was just updated to seek to max(snapshot_seq, MaxUnpreparedSequenceNumber()) instead, and iterate until a key was visible.
- Prev did not need need updates since it did not use the Seek to sequence number optimization. Assuming that locks were held when writing unprepared keys, and ValidateSnapshot runs, there should only be committed keys and unprepared keys of the current transaction, all of which are visible. Prev will simply iterate to get the last visible key.
- Reseeking to skip keys optimization was also disabled for write unprepared, since it's possible to hit the max_skip condition even while reseeking. There needs to be some way to resolve infinite looping in this case.
Closes https://github.com/facebook/rocksdb/pull/3955

Differential Revision: D8286688

Pulled By: lth

fbshipit-source-id: 25e42f47fdeb5f7accea0f4fd350ef35198caafe
2018-06-27 12:23:07 -07:00

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// 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).
#ifndef ROCKSDB_LITE
#ifndef __STDC_FORMAT_MACROS
#define __STDC_FORMAT_MACROS
#endif
#include "utilities/transactions/write_prepared_txn_db.h"
#include <inttypes.h>
#include <algorithm>
#include <string>
#include <unordered_set>
#include <vector>
#include "db/db_impl.h"
#include "rocksdb/db.h"
#include "rocksdb/options.h"
#include "rocksdb/utilities/transaction_db.h"
#include "util/cast_util.h"
#include "util/mutexlock.h"
#include "util/string_util.h"
#include "util/sync_point.h"
#include "utilities/transactions/pessimistic_transaction.h"
#include "utilities/transactions/transaction_db_mutex_impl.h"
namespace rocksdb {
Status WritePreparedTxnDB::Initialize(
const std::vector<size_t>& compaction_enabled_cf_indices,
const std::vector<ColumnFamilyHandle*>& handles) {
auto dbimpl = reinterpret_cast<DBImpl*>(GetRootDB());
assert(dbimpl != nullptr);
auto rtxns = dbimpl->recovered_transactions();
for (auto rtxn : rtxns) {
auto cnt = rtxn.second->batch_cnt_ ? rtxn.second->batch_cnt_ : 1;
for (size_t i = 0; i < cnt; i++) {
AddPrepared(rtxn.second->seq_ + i);
}
}
SequenceNumber prev_max = max_evicted_seq_;
SequenceNumber last_seq = db_impl_->GetLatestSequenceNumber();
AdvanceMaxEvictedSeq(prev_max, last_seq);
db_impl_->SetSnapshotChecker(new WritePreparedSnapshotChecker(this));
// A callback to commit a single sub-batch
class CommitSubBatchPreReleaseCallback : public PreReleaseCallback {
public:
explicit CommitSubBatchPreReleaseCallback(WritePreparedTxnDB* db)
: db_(db) {}
virtual Status Callback(SequenceNumber commit_seq,
bool is_mem_disabled) override {
#ifdef NDEBUG
(void)is_mem_disabled;
#endif
assert(!is_mem_disabled);
db_->AddCommitted(commit_seq, commit_seq);
return Status::OK();
}
private:
WritePreparedTxnDB* db_;
};
db_impl_->SetRecoverableStatePreReleaseCallback(
new CommitSubBatchPreReleaseCallback(this));
auto s = PessimisticTransactionDB::Initialize(compaction_enabled_cf_indices,
handles);
return s;
}
Status WritePreparedTxnDB::VerifyCFOptions(
const ColumnFamilyOptions& cf_options) {
Status s = PessimisticTransactionDB::VerifyCFOptions(cf_options);
if (!s.ok()) {
return s;
}
if (!cf_options.memtable_factory->CanHandleDuplicatedKey()) {
return Status::InvalidArgument(
"memtable_factory->CanHandleDuplicatedKey() cannot be false with "
"WritePrpeared transactions");
}
return Status::OK();
}
Transaction* WritePreparedTxnDB::BeginTransaction(
const WriteOptions& write_options, const TransactionOptions& txn_options,
Transaction* old_txn) {
if (old_txn != nullptr) {
ReinitializeTransaction(old_txn, write_options, txn_options);
return old_txn;
} else {
return new WritePreparedTxn(this, write_options, txn_options);
}
}
Status WritePreparedTxnDB::Write(
const WriteOptions& opts,
const TransactionDBWriteOptimizations& optimizations, WriteBatch* updates) {
if (optimizations.skip_concurrency_control) {
// Skip locking the rows
const size_t UNKNOWN_BATCH_CNT = 0;
const size_t ONE_BATCH_CNT = 1;
const size_t batch_cnt = optimizations.skip_duplicate_key_check
? ONE_BATCH_CNT
: UNKNOWN_BATCH_CNT;
WritePreparedTxn* NO_TXN = nullptr;
return WriteInternal(opts, updates, batch_cnt, NO_TXN);
} else {
// TODO(myabandeh): Make use of skip_duplicate_key_check hint
// Fall back to unoptimized version
return PessimisticTransactionDB::Write(opts, updates);
}
}
Status WritePreparedTxnDB::WriteInternal(const WriteOptions& write_options_orig,
WriteBatch* batch, size_t batch_cnt,
WritePreparedTxn* txn) {
ROCKS_LOG_DETAILS(db_impl_->immutable_db_options().info_log,
"CommitBatchInternal");
if (batch->Count() == 0) {
// Otherwise our 1 seq per batch logic will break since there is no seq
// increased for this batch.
return Status::OK();
}
if (batch_cnt == 0) { // not provided, then compute it
// TODO(myabandeh): add an option to allow user skipping this cost
SubBatchCounter counter(*GetCFComparatorMap());
auto s = batch->Iterate(&counter);
assert(s.ok());
batch_cnt = counter.BatchCount();
WPRecordTick(TXN_DUPLICATE_KEY_OVERHEAD);
ROCKS_LOG_DETAILS(info_log_, "Duplicate key overhead: %" PRIu64 " batches",
static_cast<uint64_t>(batch_cnt));
}
assert(batch_cnt);
bool do_one_write = !db_impl_->immutable_db_options().two_write_queues;
WriteOptions write_options(write_options_orig);
bool sync = write_options.sync;
if (!do_one_write) {
// No need to sync on the first write
write_options.sync = false;
}
// In the absence of Prepare markers, use Noop as a batch separator
WriteBatchInternal::InsertNoop(batch);
const bool DISABLE_MEMTABLE = true;
const uint64_t no_log_ref = 0;
uint64_t seq_used = kMaxSequenceNumber;
const size_t ZERO_PREPARES = 0;
// Since this is not 2pc, there is no need for AddPrepared but having it in
// the PreReleaseCallback enables an optimization. Refer to
// SmallestUnCommittedSeq for more details.
AddPreparedCallback add_prepared_callback(
this, batch_cnt, db_impl_->immutable_db_options().two_write_queues);
WritePreparedCommitEntryPreReleaseCallback update_commit_map(
this, db_impl_, kMaxSequenceNumber, ZERO_PREPARES, batch_cnt);
PreReleaseCallback* pre_release_callback;
if (do_one_write) {
pre_release_callback = &update_commit_map;
} else {
pre_release_callback = &add_prepared_callback;
}
auto s = db_impl_->WriteImpl(write_options, batch, nullptr, nullptr,
no_log_ref, !DISABLE_MEMTABLE, &seq_used,
batch_cnt, pre_release_callback);
assert(!s.ok() || seq_used != kMaxSequenceNumber);
uint64_t prepare_seq = seq_used;
if (txn != nullptr) {
txn->SetId(prepare_seq);
}
if (!s.ok()) {
return s;
}
if (do_one_write) {
return s;
} // else do the 2nd write for commit
// Set the original value of sync
write_options.sync = sync;
ROCKS_LOG_DETAILS(db_impl_->immutable_db_options().info_log,
"CommitBatchInternal 2nd write prepare_seq: %" PRIu64,
prepare_seq);
// Commit the batch by writing an empty batch to the 2nd queue that will
// release the commit sequence number to readers.
const size_t ZERO_COMMITS = 0;
WritePreparedCommitEntryPreReleaseCallback update_commit_map_with_prepare(
this, db_impl_, prepare_seq, batch_cnt, ZERO_COMMITS);
WriteBatch empty_batch;
empty_batch.PutLogData(Slice());
const size_t ONE_BATCH = 1;
// In the absence of Prepare markers, use Noop as a batch separator
WriteBatchInternal::InsertNoop(&empty_batch);
s = db_impl_->WriteImpl(write_options, &empty_batch, nullptr, nullptr,
no_log_ref, DISABLE_MEMTABLE, &seq_used, ONE_BATCH,
&update_commit_map_with_prepare);
assert(!s.ok() || seq_used != kMaxSequenceNumber);
// Note RemovePrepared should be called after WriteImpl that publishsed the
// seq. Otherwise SmallestUnCommittedSeq optimization breaks.
RemovePrepared(prepare_seq, batch_cnt);
return s;
}
Status WritePreparedTxnDB::Get(const ReadOptions& options,
ColumnFamilyHandle* column_family,
const Slice& key, PinnableSlice* value) {
// We are fine with the latest committed value. This could be done by
// specifying the snapshot as kMaxSequenceNumber.
SequenceNumber seq = kMaxSequenceNumber;
SequenceNumber min_uncommitted = 0;
if (options.snapshot != nullptr) {
seq = options.snapshot->GetSequenceNumber();
min_uncommitted = static_cast_with_check<const SnapshotImpl, const Snapshot>(
options.snapshot)
->min_uncommitted_;
} else {
min_uncommitted = SmallestUnCommittedSeq();
}
WritePreparedTxnReadCallback callback(this, seq, min_uncommitted);
bool* dont_care = nullptr;
// Note: no need to specify a snapshot for read options as no specific
// snapshot is requested by the user.
return db_impl_->GetImpl(options, column_family, key, value, dont_care,
&callback);
}
void WritePreparedTxnDB::UpdateCFComparatorMap(
const std::vector<ColumnFamilyHandle*>& handles) {
auto cf_map = new std::map<uint32_t, const Comparator*>();
auto handle_map = new std::map<uint32_t, ColumnFamilyHandle*>();
for (auto h : handles) {
auto id = h->GetID();
const Comparator* comparator = h->GetComparator();
(*cf_map)[id] = comparator;
if (id != 0) {
(*handle_map)[id] = h;
} else {
// The pointer to the default cf handle in the handles will be deleted.
// Use the pointer maintained by the db instead.
(*handle_map)[id] = DefaultColumnFamily();
}
}
cf_map_.reset(cf_map);
handle_map_.reset(handle_map);
}
void WritePreparedTxnDB::UpdateCFComparatorMap(ColumnFamilyHandle* h) {
auto old_cf_map_ptr = cf_map_.get();
assert(old_cf_map_ptr);
auto cf_map = new std::map<uint32_t, const Comparator*>(*old_cf_map_ptr);
auto old_handle_map_ptr = handle_map_.get();
assert(old_handle_map_ptr);
auto handle_map =
new std::map<uint32_t, ColumnFamilyHandle*>(*old_handle_map_ptr);
auto id = h->GetID();
const Comparator* comparator = h->GetComparator();
(*cf_map)[id] = comparator;
(*handle_map)[id] = h;
cf_map_.reset(cf_map);
handle_map_.reset(handle_map);
}
std::vector<Status> WritePreparedTxnDB::MultiGet(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_family,
const std::vector<Slice>& keys, std::vector<std::string>* values) {
assert(values);
size_t num_keys = keys.size();
values->resize(num_keys);
std::vector<Status> stat_list(num_keys);
for (size_t i = 0; i < num_keys; ++i) {
std::string* value = values ? &(*values)[i] : nullptr;
stat_list[i] = this->Get(options, column_family[i], keys[i], value);
}
return stat_list;
}
// Struct to hold ownership of snapshot and read callback for iterator cleanup.
struct WritePreparedTxnDB::IteratorState {
IteratorState(WritePreparedTxnDB* txn_db, SequenceNumber sequence,
std::shared_ptr<ManagedSnapshot> s,
SequenceNumber min_uncommitted)
: callback(txn_db, sequence, min_uncommitted), snapshot(s) {}
WritePreparedTxnReadCallback callback;
std::shared_ptr<ManagedSnapshot> snapshot;
};
namespace {
static void CleanupWritePreparedTxnDBIterator(void* arg1, void* /*arg2*/) {
delete reinterpret_cast<WritePreparedTxnDB::IteratorState*>(arg1);
}
} // anonymous namespace
Iterator* WritePreparedTxnDB::NewIterator(const ReadOptions& options,
ColumnFamilyHandle* column_family) {
constexpr bool ALLOW_BLOB = true;
constexpr bool ALLOW_REFRESH = true;
std::shared_ptr<ManagedSnapshot> own_snapshot = nullptr;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
SequenceNumber min_uncommitted = 0;
if (options.snapshot != nullptr) {
snapshot_seq = options.snapshot->GetSequenceNumber();
min_uncommitted =
static_cast_with_check<const SnapshotImpl, const Snapshot>(
options.snapshot)
->min_uncommitted_;
} else {
auto* snapshot = GetSnapshot();
// We take a snapshot to make sure that the related data in the commit map
// are not deleted.
snapshot_seq = snapshot->GetSequenceNumber();
min_uncommitted =
static_cast_with_check<const SnapshotImpl, const Snapshot>(snapshot)
->min_uncommitted_;
own_snapshot = std::make_shared<ManagedSnapshot>(db_impl_, snapshot);
}
assert(snapshot_seq != kMaxSequenceNumber);
auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
auto* state =
new IteratorState(this, snapshot_seq, own_snapshot, min_uncommitted);
auto* db_iter =
db_impl_->NewIteratorImpl(options, cfd, snapshot_seq, &state->callback,
!ALLOW_BLOB, !ALLOW_REFRESH);
db_iter->RegisterCleanup(CleanupWritePreparedTxnDBIterator, state, nullptr);
return db_iter;
}
Status WritePreparedTxnDB::NewIterators(
const ReadOptions& options,
const std::vector<ColumnFamilyHandle*>& column_families,
std::vector<Iterator*>* iterators) {
constexpr bool ALLOW_BLOB = true;
constexpr bool ALLOW_REFRESH = true;
std::shared_ptr<ManagedSnapshot> own_snapshot = nullptr;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
SequenceNumber min_uncommitted = 0;
if (options.snapshot != nullptr) {
snapshot_seq = options.snapshot->GetSequenceNumber();
min_uncommitted = static_cast_with_check<const SnapshotImpl, const Snapshot>(
options.snapshot)
->min_uncommitted_;
} else {
auto* snapshot = GetSnapshot();
// We take a snapshot to make sure that the related data in the commit map
// are not deleted.
snapshot_seq = snapshot->GetSequenceNumber();
own_snapshot = std::make_shared<ManagedSnapshot>(db_impl_, snapshot);
min_uncommitted =
static_cast_with_check<const SnapshotImpl, const Snapshot>(snapshot)
->min_uncommitted_;
}
iterators->clear();
iterators->reserve(column_families.size());
for (auto* column_family : column_families) {
auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
auto* state =
new IteratorState(this, snapshot_seq, own_snapshot, min_uncommitted);
auto* db_iter =
db_impl_->NewIteratorImpl(options, cfd, snapshot_seq, &state->callback,
!ALLOW_BLOB, !ALLOW_REFRESH);
db_iter->RegisterCleanup(CleanupWritePreparedTxnDBIterator, state, nullptr);
iterators->push_back(db_iter);
}
return Status::OK();
}
void WritePreparedTxnDB::Init(const TransactionDBOptions& /* unused */) {
// Adcance max_evicted_seq_ no more than 100 times before the cache wraps
// around.
INC_STEP_FOR_MAX_EVICTED =
std::max(COMMIT_CACHE_SIZE / 100, static_cast<size_t>(1));
snapshot_cache_ = unique_ptr<std::atomic<SequenceNumber>[]>(
new std::atomic<SequenceNumber>[SNAPSHOT_CACHE_SIZE] {});
commit_cache_ = unique_ptr<std::atomic<CommitEntry64b>[]>(
new std::atomic<CommitEntry64b>[COMMIT_CACHE_SIZE] {});
}
void WritePreparedTxnDB::AddPrepared(uint64_t seq) {
ROCKS_LOG_DETAILS(info_log_, "Txn %" PRIu64 " Prepareing", seq);
assert(seq > max_evicted_seq_);
if (seq <= max_evicted_seq_) {
throw std::runtime_error(
"Added prepare_seq is larger than max_evicted_seq_: " + ToString(seq) +
" <= " + ToString(max_evicted_seq_.load()));
}
WriteLock wl(&prepared_mutex_);
prepared_txns_.push(seq);
}
void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
uint8_t loop_cnt) {
ROCKS_LOG_DETAILS(info_log_, "Txn %" PRIu64 " Committing with %" PRIu64,
prepare_seq, commit_seq);
TEST_SYNC_POINT("WritePreparedTxnDB::AddCommitted:start");
TEST_SYNC_POINT("WritePreparedTxnDB::AddCommitted:start:pause");
auto indexed_seq = prepare_seq % COMMIT_CACHE_SIZE;
CommitEntry64b evicted_64b;
CommitEntry evicted;
bool to_be_evicted = GetCommitEntry(indexed_seq, &evicted_64b, &evicted);
if (LIKELY(to_be_evicted)) {
assert(evicted.prep_seq != prepare_seq);
auto prev_max = max_evicted_seq_.load(std::memory_order_acquire);
ROCKS_LOG_DETAILS(info_log_,
"Evicting %" PRIu64 ",%" PRIu64 " with max %" PRIu64,
evicted.prep_seq, evicted.commit_seq, prev_max);
if (prev_max < evicted.commit_seq) {
// Inc max in larger steps to avoid frequent updates
auto max_evicted_seq = evicted.commit_seq + INC_STEP_FOR_MAX_EVICTED;
AdvanceMaxEvictedSeq(prev_max, max_evicted_seq);
}
// After each eviction from commit cache, check if the commit entry should
// be kept around because it overlaps with a live snapshot.
CheckAgainstSnapshots(evicted);
}
bool succ =
ExchangeCommitEntry(indexed_seq, evicted_64b, {prepare_seq, commit_seq});
if (UNLIKELY(!succ)) {
ROCKS_LOG_ERROR(info_log_,
"ExchangeCommitEntry failed on [%" PRIu64 "] %" PRIu64
",%" PRIu64 " retrying...",
indexed_seq, prepare_seq, commit_seq);
// A very rare event, in which the commit entry is updated before we do.
// Here we apply a very simple solution of retrying.
if (loop_cnt > 100) {
throw std::runtime_error("Infinite loop in AddCommitted!");
}
AddCommitted(prepare_seq, commit_seq, ++loop_cnt);
return;
}
TEST_SYNC_POINT("WritePreparedTxnDB::AddCommitted:end");
TEST_SYNC_POINT("WritePreparedTxnDB::AddCommitted:end:pause");
}
void WritePreparedTxnDB::RemovePrepared(const uint64_t prepare_seq,
const size_t batch_cnt) {
WriteLock wl(&prepared_mutex_);
for (size_t i = 0; i < batch_cnt; i++) {
prepared_txns_.erase(prepare_seq + i);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prepare_seq + i);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
}
bool WritePreparedTxnDB::GetCommitEntry(const uint64_t indexed_seq,
CommitEntry64b* entry_64b,
CommitEntry* entry) const {
*entry_64b = commit_cache_[indexed_seq].load(std::memory_order_acquire);
bool valid = entry_64b->Parse(indexed_seq, entry, FORMAT);
return valid;
}
bool WritePreparedTxnDB::AddCommitEntry(const uint64_t indexed_seq,
const CommitEntry& new_entry,
CommitEntry* evicted_entry) {
CommitEntry64b new_entry_64b(new_entry, FORMAT);
CommitEntry64b evicted_entry_64b = commit_cache_[indexed_seq].exchange(
new_entry_64b, std::memory_order_acq_rel);
bool valid = evicted_entry_64b.Parse(indexed_seq, evicted_entry, FORMAT);
return valid;
}
bool WritePreparedTxnDB::ExchangeCommitEntry(const uint64_t indexed_seq,
CommitEntry64b& expected_entry_64b,
const CommitEntry& new_entry) {
auto& atomic_entry = commit_cache_[indexed_seq];
CommitEntry64b new_entry_64b(new_entry, FORMAT);
bool succ = atomic_entry.compare_exchange_strong(
expected_entry_64b, new_entry_64b, std::memory_order_acq_rel,
std::memory_order_acquire);
return succ;
}
void WritePreparedTxnDB::AdvanceMaxEvictedSeq(const SequenceNumber& prev_max,
const SequenceNumber& new_max) {
ROCKS_LOG_DETAILS(info_log_,
"AdvanceMaxEvictedSeq overhead %" PRIu64 " => %" PRIu64,
prev_max, new_max);
// When max_evicted_seq_ advances, move older entries from prepared_txns_
// to delayed_prepared_. This guarantees that if a seq is lower than max,
// then it is not in prepared_txns_ ans save an expensive, synchronized
// lookup from a shared set. delayed_prepared_ is expected to be empty in
// normal cases.
{
WriteLock wl(&prepared_mutex_);
while (!prepared_txns_.empty() && prepared_txns_.top() <= new_max) {
auto to_be_popped = prepared_txns_.top();
delayed_prepared_.insert(to_be_popped);
ROCKS_LOG_WARN(info_log_,
"prepared_mutex_ overhead %" PRIu64 " (prep=%" PRIu64
" new_max=%" PRIu64 " oldmax=%" PRIu64,
static_cast<uint64_t>(delayed_prepared_.size()),
to_be_popped, new_max, prev_max);
prepared_txns_.pop();
delayed_prepared_empty_.store(false, std::memory_order_release);
}
}
// With each change to max_evicted_seq_ fetch the live snapshots behind it.
// We use max as the version of snapshots to identify how fresh are the
// snapshot list. This works because the snapshots are between 0 and
// max, so the larger the max, the more complete they are.
SequenceNumber new_snapshots_version = new_max;
std::vector<SequenceNumber> snapshots;
bool update_snapshots = false;
if (new_snapshots_version > snapshots_version_) {
// This is to avoid updating the snapshots_ if it already updated
// with a more recent vesion by a concrrent thread
update_snapshots = true;
// We only care about snapshots lower then max
snapshots = GetSnapshotListFromDB(new_max);
}
if (update_snapshots) {
UpdateSnapshots(snapshots, new_snapshots_version);
}
auto updated_prev_max = prev_max;
while (updated_prev_max < new_max &&
!max_evicted_seq_.compare_exchange_weak(updated_prev_max, new_max,
std::memory_order_acq_rel,
std::memory_order_relaxed)) {
};
}
const Snapshot* WritePreparedTxnDB::GetSnapshot() {
// Note: SmallestUnCommittedSeq must be called before GetSnapshotImpl. Refer
// to WritePreparedTxn::SetSnapshot for more explanation.
auto min_uncommitted = WritePreparedTxnDB::SmallestUnCommittedSeq();
const bool FOR_WW_CONFLICT_CHECK = true;
SnapshotImpl* snap_impl = db_impl_->GetSnapshotImpl(!FOR_WW_CONFLICT_CHECK);
assert(snap_impl);
EnhanceSnapshot(snap_impl, min_uncommitted);
return snap_impl;
}
const std::vector<SequenceNumber> WritePreparedTxnDB::GetSnapshotListFromDB(
SequenceNumber max) {
ROCKS_LOG_DETAILS(info_log_, "GetSnapshotListFromDB with max %" PRIu64, max);
InstrumentedMutex(db_impl_->mutex());
return db_impl_->snapshots().GetAll(nullptr, max);
}
void WritePreparedTxnDB::ReleaseSnapshot(const Snapshot* snapshot) {
auto snap_seq = snapshot->GetSequenceNumber();
ReleaseSnapshotInternal(snap_seq);
db_impl_->ReleaseSnapshot(snapshot);
}
void WritePreparedTxnDB::ReleaseSnapshotInternal(
const SequenceNumber snap_seq) {
// relax is enough since max increases monotonically, i.e., if snap_seq <
// old_max => snap_seq < new_max as well.
if (snap_seq < max_evicted_seq_.load(std::memory_order_relaxed)) {
// Then this is a rare case that transaction did not finish before max
// advances. It is expected for a few read-only backup snapshots. For such
// snapshots we might have kept around a couple of entries in the
// old_commit_map_. Check and do garbage collection if that is the case.
bool need_gc = false;
{
WPRecordTick(TXN_OLD_COMMIT_MAP_MUTEX_OVERHEAD);
ROCKS_LOG_WARN(info_log_, "old_commit_map_mutex_ overhead");
ReadLock rl(&old_commit_map_mutex_);
auto prep_set_entry = old_commit_map_.find(snap_seq);
need_gc = prep_set_entry != old_commit_map_.end();
}
if (need_gc) {
WPRecordTick(TXN_OLD_COMMIT_MAP_MUTEX_OVERHEAD);
ROCKS_LOG_WARN(info_log_, "old_commit_map_mutex_ overhead");
WriteLock wl(&old_commit_map_mutex_);
old_commit_map_.erase(snap_seq);
old_commit_map_empty_.store(old_commit_map_.empty(),
std::memory_order_release);
}
}
}
void WritePreparedTxnDB::UpdateSnapshots(
const std::vector<SequenceNumber>& snapshots,
const SequenceNumber& version) {
ROCKS_LOG_DETAILS(info_log_, "UpdateSnapshots with version %" PRIu64,
version);
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:start");
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:start");
#ifndef NDEBUG
size_t sync_i = 0;
#endif
ROCKS_LOG_DETAILS(info_log_, "snapshots_mutex_ overhead");
WriteLock wl(&snapshots_mutex_);
snapshots_version_ = version;
// We update the list concurrently with the readers.
// Both new and old lists are sorted and the new list is subset of the
// previous list plus some new items. Thus if a snapshot repeats in
// both new and old lists, it will appear upper in the new list. So if
// we simply insert the new snapshots in order, if an overwritten item
// is still valid in the new list is either written to the same place in
// the array or it is written in a higher palce before it gets
// overwritten by another item. This guarantess a reader that reads the
// list bottom-up will eventaully see a snapshot that repeats in the
// update, either before it gets overwritten by the writer or
// afterwards.
size_t i = 0;
auto it = snapshots.begin();
for (; it != snapshots.end() && i < SNAPSHOT_CACHE_SIZE; it++, i++) {
snapshot_cache_[i].store(*it, std::memory_order_release);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:", ++sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:", sync_i);
}
#ifndef NDEBUG
// Release the remaining sync points since they are useless given that the
// reader would also use lock to access snapshots
for (++sync_i; sync_i <= 10; ++sync_i) {
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:", sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:", sync_i);
}
#endif
snapshots_.clear();
for (; it != snapshots.end(); it++) {
// Insert them to a vector that is less efficient to access
// concurrently
snapshots_.push_back(*it);
}
// Update the size at the end. Otherwise a parallel reader might read
// items that are not set yet.
snapshots_total_.store(snapshots.size(), std::memory_order_release);
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:p:end");
TEST_SYNC_POINT("WritePreparedTxnDB::UpdateSnapshots:s:end");
}
void WritePreparedTxnDB::CheckAgainstSnapshots(const CommitEntry& evicted) {
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:start");
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:start");
#ifndef NDEBUG
size_t sync_i = 0;
#endif
// First check the snapshot cache that is efficient for concurrent access
auto cnt = snapshots_total_.load(std::memory_order_acquire);
// The list might get updated concurrently as we are reading from it. The
// reader should be able to read all the snapshots that are still valid
// after the update. Since the survived snapshots are written in a higher
// place before gets overwritten the reader that reads bottom-up will
// eventully see it.
const bool next_is_larger = true;
SequenceNumber snapshot_seq = kMaxSequenceNumber;
size_t ip1 = std::min(cnt, SNAPSHOT_CACHE_SIZE);
for (; 0 < ip1; ip1--) {
snapshot_seq = snapshot_cache_[ip1 - 1].load(std::memory_order_acquire);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:",
++sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:", sync_i);
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq, !next_is_larger)) {
break;
}
}
#ifndef NDEBUG
// Release the remaining sync points before accquiring the lock
for (++sync_i; sync_i <= 10; ++sync_i) {
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:", sync_i);
TEST_IDX_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:", sync_i);
}
#endif
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:p:end");
TEST_SYNC_POINT("WritePreparedTxnDB::CheckAgainstSnapshots:s:end");
if (UNLIKELY(SNAPSHOT_CACHE_SIZE < cnt && ip1 == SNAPSHOT_CACHE_SIZE &&
snapshot_seq < evicted.prep_seq)) {
// Then access the less efficient list of snapshots_
WPRecordTick(TXN_SNAPSHOT_MUTEX_OVERHEAD);
ROCKS_LOG_WARN(info_log_, "snapshots_mutex_ overhead");
ReadLock rl(&snapshots_mutex_);
// Items could have moved from the snapshots_ to snapshot_cache_ before
// accquiring the lock. To make sure that we do not miss a valid snapshot,
// read snapshot_cache_ again while holding the lock.
for (size_t i = 0; i < SNAPSHOT_CACHE_SIZE; i++) {
snapshot_seq = snapshot_cache_[i].load(std::memory_order_acquire);
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq, next_is_larger)) {
break;
}
}
for (auto snapshot_seq_2 : snapshots_) {
if (!MaybeUpdateOldCommitMap(evicted.prep_seq, evicted.commit_seq,
snapshot_seq_2, next_is_larger)) {
break;
}
}
}
}
bool WritePreparedTxnDB::MaybeUpdateOldCommitMap(
const uint64_t& prep_seq, const uint64_t& commit_seq,
const uint64_t& snapshot_seq, const bool next_is_larger = true) {
// If we do not store an entry in old_commit_map_ we assume it is committed in
// all snapshots. If commit_seq <= snapshot_seq, it is considered already in
// the snapshot so we need not to keep the entry around for this snapshot.
if (commit_seq <= snapshot_seq) {
// continue the search if the next snapshot could be smaller than commit_seq
return !next_is_larger;
}
// then snapshot_seq < commit_seq
if (prep_seq <= snapshot_seq) { // overlapping range
WPRecordTick(TXN_OLD_COMMIT_MAP_MUTEX_OVERHEAD);
ROCKS_LOG_WARN(info_log_, "old_commit_map_mutex_ overhead");
WriteLock wl(&old_commit_map_mutex_);
old_commit_map_empty_.store(false, std::memory_order_release);
auto& vec = old_commit_map_[snapshot_seq];
vec.insert(std::upper_bound(vec.begin(), vec.end(), prep_seq), prep_seq);
// We need to store it once for each overlapping snapshot. Returning true to
// continue the search if there is more overlapping snapshot.
return true;
}
// continue the search if the next snapshot could be larger than prep_seq
return next_is_larger;
}
WritePreparedTxnDB::~WritePreparedTxnDB() {
// At this point there could be running compaction/flush holding a
// SnapshotChecker, which holds a pointer back to WritePreparedTxnDB.
// Make sure those jobs finished before destructing WritePreparedTxnDB.
db_impl_->CancelAllBackgroundWork(true /*wait*/);
}
void SubBatchCounter::InitWithComp(const uint32_t cf) {
auto cmp = comparators_[cf];
keys_[cf] = CFKeys(SetComparator(cmp));
}
void SubBatchCounter::AddKey(const uint32_t cf, const Slice& key) {
CFKeys& cf_keys = keys_[cf];
if (cf_keys.size() == 0) { // just inserted
InitWithComp(cf);
}
auto it = cf_keys.insert(key);
if (it.second == false) { // second is false if a element already existed.
batches_++;
keys_.clear();
InitWithComp(cf);
keys_[cf].insert(key);
}
}
} // namespace rocksdb
#endif // ROCKSDB_LITE
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