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b225de7e10
rocksdb/utilities/transactions/write_prepared_txn_db.cc
Maysam Yabandeh b225de7e10 WritePrepared Txn: smallest_prepare optimization 
Summary:
The is an optimization to reduce lookup in the CommitCache when querying IsInSnapshot. The optimization takes the smallest uncommitted data at the time that the snapshot was taken and if the sequence number of the read data is lower than that number it assumes the data as committed.
To implement this optimization two changes are required: i) The AddPrepared function must be called sequentially to avoid out of order insertion in the PrepareHeap (otherwise the top of the heap does not indicate the smallest prepare in future too), ii) non-2PC transactions also call AddPrepared if they do not commit in one step.
Closes https://github.com/facebook/rocksdb/pull/3649

Differential Revision: D7388630

Pulled By: maysamyabandeh

fbshipit-source-id: b79506238c17467d590763582960d4d90181c600
2018-04-02 20:27:41 -07:00

761 lines
31 KiB
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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 {
assert(!is_mem_disabled);
const bool PREPARE_SKIPPED = true;
db_->AddCommitted(commit_seq, commit_seq, PREPARE_SKIPPED);
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();
// TODO(myabandeh): replace me with a stat
ROCKS_LOG_WARN(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;
const bool PREP_HEAP_SKIPPED = true;
WritePreparedCommitEntryPreReleaseCallback update_commit_map_with_prepare(
this, db_impl_, prepare_seq, batch_cnt, ZERO_COMMITS, !PREP_HEAP_SKIPPED);
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);
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*>();
for (auto h : handles) {
auto id = h->GetID();
const Comparator* comparator = h->GetComparator();
(*cf_map)[id] = comparator;
}
cf_map_.store(cf_map);
cf_map_gc_.reset(cf_map);
}
void WritePreparedTxnDB::UpdateCFComparatorMap(
const ColumnFamilyHandle* h) {
auto old_cf_map_ptr = cf_map_.load();
assert(old_cf_map_ptr);
auto cf_map = new std::map<uint32_t, const Comparator*>(*old_cf_map_ptr);
auto id = h->GetID();
const Comparator* comparator = h->GetComparator();
(*cf_map)[id] = comparator;
cf_map_.store(cf_map);
cf_map_gc_.reset(cf_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::RollbackPrepared(uint64_t prep_seq,
uint64_t /*rollback_seq*/) {
ROCKS_LOG_DETAILS(
info_log_, "Txn %" PRIu64 " rolling back with rollback seq of " PRIu64 "",
prep_seq, rollback_seq);
std::vector<SequenceNumber> snapshots =
GetSnapshotListFromDB(kMaxSequenceNumber);
// TODO(myabandeh): currently we are assuming that there is no snapshot taken
// when a transaciton is rolled back. This is the case the way MySQL does
// rollback which is after recovery. We should extend it to be able to
// rollback txns that overlap with exsiting snapshots.
assert(snapshots.size() == 0);
if (snapshots.size()) {
throw std::runtime_error(
"Rollback reqeust while there are live snapshots.");
}
WriteLock wl(&prepared_mutex_);
prepared_txns_.erase(prep_seq);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prep_seq);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
void WritePreparedTxnDB::AddCommitted(uint64_t prepare_seq, uint64_t commit_seq,
bool prepare_skipped, uint8_t loop_cnt) {
ROCKS_LOG_DETAILS(info_log_,
"Txn %" PRIu64 " Committing with %" PRIu64
"(prepare_skipped=%d)",
prepare_seq, commit_seq, prepare_skipped);
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, prepare_skipped, ++loop_cnt);
return;
}
if (!prepare_skipped) {
WriteLock wl(&prepared_mutex_);
prepared_txns_.erase(prepare_seq);
bool was_empty = delayed_prepared_.empty();
if (!was_empty) {
delayed_prepared_.erase(prepare_seq);
bool is_empty = delayed_prepared_.empty();
if (was_empty != is_empty) {
delayed_prepared_empty_.store(is_empty, std::memory_order_release);
}
}
}
TEST_SYNC_POINT("WritePreparedTxnDB::AddCommitted:end");
TEST_SYNC_POINT("WritePreparedTxnDB::AddCommitted:end:pause");
}
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);
// TODO(myabandeh): also add a stat
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() {
const bool FOR_WW_CONFLICT_CHECK = true;
SnapshotImpl* snap_impl = db_impl_->GetSnapshotImpl(!FOR_WW_CONFLICT_CHECK);
assert(snap_impl);
EnhanceSnapshot(snap_impl);
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;
{
// TODO(myabandeh): also add a stat
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) {
// TODO(myabandeh): also add a stat
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
// TODO(myabandeh): replace me with a stat
ROCKS_LOG_WARN(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_
// TODO(myabandeh): also add a stat
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
ROCKS_LOG_WARN(info_log_, "old_commit_map_mutex_ overhead");
// TODO(myabandeh): also add a stat
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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