rocksdb/utilities/transactions/transaction_lock_mgr.cc
Manuel Ung e03377c7fd Add lock wait time as a perf context counter
Summary:
Adds two new counters:

`key_lock_wait_count` counts how many times a lock was blocked by another transaction and had to wait, instead of being granted the lock immediately.
`key_lock_wait_time` counts the time spent acquiring locks.
Closes https://github.com/facebook/rocksdb/pull/3107

Differential Revision: D6217332

Pulled By: lth

fbshipit-source-id: 55d4f46da5550c333e523263422fd61d6a46deb9
2017-11-06 10:57:19 -08:00

746 lines
24 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).
#ifndef ROCKSDB_LITE
#ifndef __STDC_FORMAT_MACROS
#define __STDC_FORMAT_MACROS
#endif
#include "utilities/transactions/transaction_lock_mgr.h"
#include <inttypes.h>
#include <algorithm>
#include <condition_variable>
#include <functional>
#include <mutex>
#include <string>
#include <vector>
#include "monitoring/perf_context_imp.h"
#include "rocksdb/slice.h"
#include "rocksdb/utilities/transaction_db_mutex.h"
#include "util/cast_util.h"
#include "util/murmurhash.h"
#include "util/sync_point.h"
#include "util/thread_local.h"
#include "utilities/transactions/pessimistic_transaction_db.h"
namespace rocksdb {
struct LockInfo {
bool exclusive;
autovector<TransactionID> txn_ids;
// Transaction locks are not valid after this time in us
uint64_t expiration_time;
LockInfo(TransactionID id, uint64_t time, bool ex)
: exclusive(ex), expiration_time(time) {
txn_ids.push_back(id);
}
LockInfo(const LockInfo& lock_info)
: exclusive(lock_info.exclusive),
txn_ids(lock_info.txn_ids),
expiration_time(lock_info.expiration_time) {}
};
struct LockMapStripe {
explicit LockMapStripe(std::shared_ptr<TransactionDBMutexFactory> factory) {
stripe_mutex = factory->AllocateMutex();
stripe_cv = factory->AllocateCondVar();
assert(stripe_mutex);
assert(stripe_cv);
}
// Mutex must be held before modifying keys map
std::shared_ptr<TransactionDBMutex> stripe_mutex;
// Condition Variable per stripe for waiting on a lock
std::shared_ptr<TransactionDBCondVar> stripe_cv;
// Locked keys mapped to the info about the transactions that locked them.
// TODO(agiardullo): Explore performance of other data structures.
std::unordered_map<std::string, LockInfo> keys;
};
// Map of #num_stripes LockMapStripes
struct LockMap {
explicit LockMap(size_t num_stripes,
std::shared_ptr<TransactionDBMutexFactory> factory)
: num_stripes_(num_stripes) {
lock_map_stripes_.reserve(num_stripes);
for (size_t i = 0; i < num_stripes; i++) {
LockMapStripe* stripe = new LockMapStripe(factory);
lock_map_stripes_.push_back(stripe);
}
}
~LockMap() {
for (auto stripe : lock_map_stripes_) {
delete stripe;
}
}
// Number of sepearate LockMapStripes to create, each with their own Mutex
const size_t num_stripes_;
// Count of keys that are currently locked in this column family.
// (Only maintained if TransactionLockMgr::max_num_locks_ is positive.)
std::atomic<int64_t> lock_cnt{0};
std::vector<LockMapStripe*> lock_map_stripes_;
size_t GetStripe(const std::string& key) const;
};
void DeadlockInfoBuffer::AddNewPath(DeadlockPath path) {
std::lock_guard<std::mutex> lock(paths_buffer_mutex_);
if (paths_buffer_.empty()) {
return;
}
paths_buffer_[buffer_idx_] = path;
buffer_idx_ = (buffer_idx_ + 1) % paths_buffer_.size();
}
void DeadlockInfoBuffer::Resize(uint32_t target_size) {
std::lock_guard<std::mutex> lock(paths_buffer_mutex_);
paths_buffer_ = Normalize();
// Drop the deadlocks that will no longer be needed ater the normalize
if (target_size < paths_buffer_.size()) {
paths_buffer_.erase(
paths_buffer_.begin(),
paths_buffer_.begin() + (paths_buffer_.size() - target_size));
buffer_idx_ = 0;
}
// Resize the buffer to the target size and restore the buffer's idx
else {
auto prev_size = paths_buffer_.size();
paths_buffer_.resize(target_size);
buffer_idx_ = (uint32_t)prev_size;
}
}
std::vector<DeadlockPath> DeadlockInfoBuffer::Normalize() {
auto working = paths_buffer_;
if (working.empty()) {
return working;
}
// Next write occurs at a nonexistent path's slot
if (paths_buffer_[buffer_idx_].empty()) {
working.resize(buffer_idx_);
} else {
std::rotate(working.begin(), working.begin() + buffer_idx_, working.end());
}
return working;
}
std::vector<DeadlockPath> DeadlockInfoBuffer::PrepareBuffer() {
std::lock_guard<std::mutex> lock(paths_buffer_mutex_);
// Reversing the normalized vector returns the latest deadlocks first
auto working = Normalize();
std::reverse(working.begin(), working.end());
return working;
}
namespace {
void UnrefLockMapsCache(void* ptr) {
// Called when a thread exits or a ThreadLocalPtr gets destroyed.
auto lock_maps_cache =
static_cast<std::unordered_map<uint32_t, std::shared_ptr<LockMap>>*>(ptr);
delete lock_maps_cache;
}
} // anonymous namespace
TransactionLockMgr::TransactionLockMgr(
TransactionDB* txn_db, size_t default_num_stripes, int64_t max_num_locks,
uint32_t max_num_deadlocks,
std::shared_ptr<TransactionDBMutexFactory> mutex_factory)
: txn_db_impl_(nullptr),
default_num_stripes_(default_num_stripes),
max_num_locks_(max_num_locks),
lock_maps_cache_(new ThreadLocalPtr(&UnrefLockMapsCache)),
dlock_buffer_(max_num_deadlocks),
mutex_factory_(mutex_factory) {
assert(txn_db);
txn_db_impl_ =
static_cast_with_check<PessimisticTransactionDB, TransactionDB>(txn_db);
}
TransactionLockMgr::~TransactionLockMgr() {}
size_t LockMap::GetStripe(const std::string& key) const {
assert(num_stripes_ > 0);
static murmur_hash hash;
size_t stripe = hash(key) % num_stripes_;
return stripe;
}
void TransactionLockMgr::AddColumnFamily(uint32_t column_family_id) {
InstrumentedMutexLock l(&lock_map_mutex_);
if (lock_maps_.find(column_family_id) == lock_maps_.end()) {
lock_maps_.emplace(column_family_id,
std::shared_ptr<LockMap>(
new LockMap(default_num_stripes_, mutex_factory_)));
} else {
// column_family already exists in lock map
assert(false);
}
}
void TransactionLockMgr::RemoveColumnFamily(uint32_t column_family_id) {
// Remove lock_map for this column family. Since the lock map is stored
// as a shared ptr, concurrent transactions can still keep using it
// until they release their references to it.
{
InstrumentedMutexLock l(&lock_map_mutex_);
auto lock_maps_iter = lock_maps_.find(column_family_id);
assert(lock_maps_iter != lock_maps_.end());
lock_maps_.erase(lock_maps_iter);
} // lock_map_mutex_
// Clear all thread-local caches
autovector<void*> local_caches;
lock_maps_cache_->Scrape(&local_caches, nullptr);
for (auto cache : local_caches) {
delete static_cast<LockMaps*>(cache);
}
}
// Look up the LockMap shared_ptr for a given column_family_id.
// Note: The LockMap is only valid as long as the caller is still holding on
// to the returned shared_ptr.
std::shared_ptr<LockMap> TransactionLockMgr::GetLockMap(
uint32_t column_family_id) {
// First check thread-local cache
if (lock_maps_cache_->Get() == nullptr) {
lock_maps_cache_->Reset(new LockMaps());
}
auto lock_maps_cache = static_cast<LockMaps*>(lock_maps_cache_->Get());
auto lock_map_iter = lock_maps_cache->find(column_family_id);
if (lock_map_iter != lock_maps_cache->end()) {
// Found lock map for this column family.
return lock_map_iter->second;
}
// Not found in local cache, grab mutex and check shared LockMaps
InstrumentedMutexLock l(&lock_map_mutex_);
lock_map_iter = lock_maps_.find(column_family_id);
if (lock_map_iter == lock_maps_.end()) {
return std::shared_ptr<LockMap>(nullptr);
} else {
// Found lock map. Store in thread-local cache and return.
std::shared_ptr<LockMap>& lock_map = lock_map_iter->second;
lock_maps_cache->insert({column_family_id, lock_map});
return lock_map;
}
}
// Returns true if this lock has expired and can be acquired by another
// transaction.
// If false, sets *expire_time to the expiration time of the lock according
// to Env->GetMicros() or 0 if no expiration.
bool TransactionLockMgr::IsLockExpired(TransactionID txn_id,
const LockInfo& lock_info, Env* env,
uint64_t* expire_time) {
auto now = env->NowMicros();
bool expired =
(lock_info.expiration_time > 0 && lock_info.expiration_time <= now);
if (!expired && lock_info.expiration_time > 0) {
// return how many microseconds until lock will be expired
*expire_time = lock_info.expiration_time;
} else {
for (auto id : lock_info.txn_ids) {
if (txn_id == id) {
continue;
}
bool success = txn_db_impl_->TryStealingExpiredTransactionLocks(id);
if (!success) {
expired = false;
break;
}
*expire_time = 0;
}
}
return expired;
}
Status TransactionLockMgr::TryLock(PessimisticTransaction* txn,
uint32_t column_family_id,
const std::string& key, Env* env,
bool exclusive) {
// Lookup lock map for this column family id
std::shared_ptr<LockMap> lock_map_ptr = GetLockMap(column_family_id);
LockMap* lock_map = lock_map_ptr.get();
if (lock_map == nullptr) {
char msg[255];
snprintf(msg, sizeof(msg), "Column family id not found: %" PRIu32,
column_family_id);
return Status::InvalidArgument(msg);
}
// Need to lock the mutex for the stripe that this key hashes to
size_t stripe_num = lock_map->GetStripe(key);
assert(lock_map->lock_map_stripes_.size() > stripe_num);
LockMapStripe* stripe = lock_map->lock_map_stripes_.at(stripe_num);
LockInfo lock_info(txn->GetID(), txn->GetExpirationTime(), exclusive);
int64_t timeout = txn->GetLockTimeout();
return AcquireWithTimeout(txn, lock_map, stripe, column_family_id, key, env,
timeout, lock_info);
}
// Helper function for TryLock().
Status TransactionLockMgr::AcquireWithTimeout(
PessimisticTransaction* txn, LockMap* lock_map, LockMapStripe* stripe,
uint32_t column_family_id, const std::string& key, Env* env,
int64_t timeout, const LockInfo& lock_info) {
Status result;
uint64_t start_time = 0;
uint64_t end_time = 0;
if (timeout > 0) {
start_time = env->NowMicros();
end_time = start_time + timeout;
}
if (timeout < 0) {
// If timeout is negative, we wait indefinitely to acquire the lock
result = stripe->stripe_mutex->Lock();
} else {
result = stripe->stripe_mutex->TryLockFor(timeout);
}
if (!result.ok()) {
// failed to acquire mutex
return result;
}
// Acquire lock if we are able to
uint64_t expire_time_hint = 0;
autovector<TransactionID> wait_ids;
result = AcquireLocked(lock_map, stripe, key, env, lock_info,
&expire_time_hint, &wait_ids);
if (!result.ok() && timeout != 0) {
PERF_TIMER_GUARD(key_lock_wait_time);
PERF_COUNTER_ADD(key_lock_wait_count, 1);
// If we weren't able to acquire the lock, we will keep retrying as long
// as the timeout allows.
bool timed_out = false;
do {
// Decide how long to wait
int64_t cv_end_time = -1;
// Check if held lock's expiration time is sooner than our timeout
if (expire_time_hint > 0 &&
(timeout < 0 || (timeout > 0 && expire_time_hint < end_time))) {
// expiration time is sooner than our timeout
cv_end_time = expire_time_hint;
} else if (timeout >= 0) {
cv_end_time = end_time;
}
assert(result.IsBusy() || wait_ids.size() != 0);
// We are dependent on a transaction to finish, so perform deadlock
// detection.
if (wait_ids.size() != 0) {
if (txn->IsDeadlockDetect()) {
if (IncrementWaiters(txn, wait_ids, key, column_family_id,
lock_info.exclusive)) {
result = Status::Busy(Status::SubCode::kDeadlock);
stripe->stripe_mutex->UnLock();
return result;
}
}
txn->SetWaitingTxn(wait_ids, column_family_id, &key);
}
TEST_SYNC_POINT("TransactionLockMgr::AcquireWithTimeout:WaitingTxn");
if (cv_end_time < 0) {
// Wait indefinitely
result = stripe->stripe_cv->Wait(stripe->stripe_mutex);
} else {
uint64_t now = env->NowMicros();
if (static_cast<uint64_t>(cv_end_time) > now) {
result = stripe->stripe_cv->WaitFor(stripe->stripe_mutex,
cv_end_time - now);
}
}
if (wait_ids.size() != 0) {
txn->ClearWaitingTxn();
if (txn->IsDeadlockDetect()) {
DecrementWaiters(txn, wait_ids);
}
}
if (result.IsTimedOut()) {
timed_out = true;
// Even though we timed out, we will still make one more attempt to
// acquire lock below (it is possible the lock expired and we
// were never signaled).
}
if (result.ok() || result.IsTimedOut()) {
result = AcquireLocked(lock_map, stripe, key, env, lock_info,
&expire_time_hint, &wait_ids);
}
} while (!result.ok() && !timed_out);
}
stripe->stripe_mutex->UnLock();
return result;
}
void TransactionLockMgr::DecrementWaiters(
const PessimisticTransaction* txn,
const autovector<TransactionID>& wait_ids) {
std::lock_guard<std::mutex> lock(wait_txn_map_mutex_);
DecrementWaitersImpl(txn, wait_ids);
}
void TransactionLockMgr::DecrementWaitersImpl(
const PessimisticTransaction* txn,
const autovector<TransactionID>& wait_ids) {
auto id = txn->GetID();
assert(wait_txn_map_.Contains(id));
wait_txn_map_.Delete(id);
for (auto wait_id : wait_ids) {
rev_wait_txn_map_.Get(wait_id)--;
if (rev_wait_txn_map_.Get(wait_id) == 0) {
rev_wait_txn_map_.Delete(wait_id);
}
}
}
bool TransactionLockMgr::IncrementWaiters(
const PessimisticTransaction* txn,
const autovector<TransactionID>& wait_ids, const std::string& key,
const uint32_t& cf_id, const bool& exclusive) {
auto id = txn->GetID();
std::vector<int> queue_parents(txn->GetDeadlockDetectDepth());
std::vector<TransactionID> queue_values(txn->GetDeadlockDetectDepth());
std::lock_guard<std::mutex> lock(wait_txn_map_mutex_);
assert(!wait_txn_map_.Contains(id));
wait_txn_map_.Insert(id, {wait_ids, cf_id, key, exclusive});
for (auto wait_id : wait_ids) {
if (rev_wait_txn_map_.Contains(wait_id)) {
rev_wait_txn_map_.Get(wait_id)++;
} else {
rev_wait_txn_map_.Insert(wait_id, 1);
}
}
// No deadlock if nobody is waiting on self.
if (!rev_wait_txn_map_.Contains(id)) {
return false;
}
const auto* next_ids = &wait_ids;
int parent = -1;
for (int tail = 0, head = 0; head < txn->GetDeadlockDetectDepth(); head++) {
int i = 0;
if (next_ids) {
for (; i < static_cast<int>(next_ids->size()) &&
tail + i < txn->GetDeadlockDetectDepth();
i++) {
queue_values[tail + i] = (*next_ids)[i];
queue_parents[tail + i] = parent;
}
tail += i;
}
// No more items in the list, meaning no deadlock.
if (tail == head) {
return false;
}
auto next = queue_values[head];
if (next == id) {
std::vector<DeadlockInfo> path;
while (head != -1) {
assert(wait_txn_map_.Contains(queue_values[head]));
auto extracted_info = wait_txn_map_.Get(queue_values[head]);
path.push_back({queue_values[head], extracted_info.m_cf_id,
extracted_info.m_waiting_key,
extracted_info.m_exclusive});
head = queue_parents[head];
}
std::reverse(path.begin(), path.end());
dlock_buffer_.AddNewPath(DeadlockPath(path));
DecrementWaitersImpl(txn, wait_ids);
return true;
} else if (!wait_txn_map_.Contains(next)) {
next_ids = nullptr;
continue;
} else {
parent = head;
next_ids = &(wait_txn_map_.Get(next).m_neighbors);
}
}
// Wait cycle too big, just assume deadlock.
dlock_buffer_.AddNewPath(DeadlockPath(true));
DecrementWaitersImpl(txn, wait_ids);
return true;
}
// Try to lock this key after we have acquired the mutex.
// Sets *expire_time to the expiration time in microseconds
// or 0 if no expiration.
// REQUIRED: Stripe mutex must be held.
Status TransactionLockMgr::AcquireLocked(LockMap* lock_map,
LockMapStripe* stripe,
const std::string& key, Env* env,
const LockInfo& txn_lock_info,
uint64_t* expire_time,
autovector<TransactionID>* txn_ids) {
assert(txn_lock_info.txn_ids.size() == 1);
Status result;
// Check if this key is already locked
if (stripe->keys.find(key) != stripe->keys.end()) {
// Lock already held
LockInfo& lock_info = stripe->keys.at(key);
assert(lock_info.txn_ids.size() == 1 || !lock_info.exclusive);
if (lock_info.exclusive || txn_lock_info.exclusive) {
if (lock_info.txn_ids.size() == 1 &&
lock_info.txn_ids[0] == txn_lock_info.txn_ids[0]) {
// The list contains one txn and we're it, so just take it.
lock_info.exclusive = txn_lock_info.exclusive;
lock_info.expiration_time = txn_lock_info.expiration_time;
} else {
// Check if it's expired. Skips over txn_lock_info.txn_ids[0] in case
// it's there for a shared lock with multiple holders which was not
// caught in the first case.
if (IsLockExpired(txn_lock_info.txn_ids[0], lock_info, env,
expire_time)) {
// lock is expired, can steal it
lock_info.txn_ids = txn_lock_info.txn_ids;
lock_info.exclusive = txn_lock_info.exclusive;
lock_info.expiration_time = txn_lock_info.expiration_time;
// lock_cnt does not change
} else {
result = Status::TimedOut(Status::SubCode::kLockTimeout);
*txn_ids = lock_info.txn_ids;
}
}
} else {
// We are requesting shared access to a shared lock, so just grant it.
lock_info.txn_ids.push_back(txn_lock_info.txn_ids[0]);
// Using std::max means that expiration time never goes down even when
// a transaction is removed from the list. The correct solution would be
// to track expiry for every transaction, but this would also work for
// now.
lock_info.expiration_time =
std::max(lock_info.expiration_time, txn_lock_info.expiration_time);
}
} else { // Lock not held.
// Check lock limit
if (max_num_locks_ > 0 &&
lock_map->lock_cnt.load(std::memory_order_acquire) >= max_num_locks_) {
result = Status::Busy(Status::SubCode::kLockLimit);
} else {
// acquire lock
stripe->keys.insert({key, txn_lock_info});
// Maintain lock count if there is a limit on the number of locks
if (max_num_locks_) {
lock_map->lock_cnt++;
}
}
}
return result;
}
void TransactionLockMgr::UnLockKey(const PessimisticTransaction* txn,
const std::string& key,
LockMapStripe* stripe, LockMap* lock_map,
Env* env) {
TransactionID txn_id = txn->GetID();
auto stripe_iter = stripe->keys.find(key);
if (stripe_iter != stripe->keys.end()) {
auto& txns = stripe_iter->second.txn_ids;
auto txn_it = std::find(txns.begin(), txns.end(), txn_id);
// Found the key we locked. unlock it.
if (txn_it != txns.end()) {
if (txns.size() == 1) {
stripe->keys.erase(stripe_iter);
} else {
auto last_it = txns.end() - 1;
if (txn_it != last_it) {
*txn_it = *last_it;
}
txns.pop_back();
}
if (max_num_locks_ > 0) {
// Maintain lock count if there is a limit on the number of locks.
assert(lock_map->lock_cnt.load(std::memory_order_relaxed) > 0);
lock_map->lock_cnt--;
}
}
} else {
// This key is either not locked or locked by someone else. This should
// only happen if the unlocking transaction has expired.
assert(txn->GetExpirationTime() > 0 &&
txn->GetExpirationTime() < env->NowMicros());
}
}
void TransactionLockMgr::UnLock(PessimisticTransaction* txn,
uint32_t column_family_id,
const std::string& key, Env* env) {
std::shared_ptr<LockMap> lock_map_ptr = GetLockMap(column_family_id);
LockMap* lock_map = lock_map_ptr.get();
if (lock_map == nullptr) {
// Column Family must have been dropped.
return;
}
// Lock the mutex for the stripe that this key hashes to
size_t stripe_num = lock_map->GetStripe(key);
assert(lock_map->lock_map_stripes_.size() > stripe_num);
LockMapStripe* stripe = lock_map->lock_map_stripes_.at(stripe_num);
stripe->stripe_mutex->Lock();
UnLockKey(txn, key, stripe, lock_map, env);
stripe->stripe_mutex->UnLock();
// Signal waiting threads to retry locking
stripe->stripe_cv->NotifyAll();
}
void TransactionLockMgr::UnLock(const PessimisticTransaction* txn,
const TransactionKeyMap* key_map, Env* env) {
for (auto& key_map_iter : *key_map) {
uint32_t column_family_id = key_map_iter.first;
auto& keys = key_map_iter.second;
std::shared_ptr<LockMap> lock_map_ptr = GetLockMap(column_family_id);
LockMap* lock_map = lock_map_ptr.get();
if (lock_map == nullptr) {
// Column Family must have been dropped.
return;
}
// Bucket keys by lock_map_ stripe
std::unordered_map<size_t, std::vector<const std::string*>> keys_by_stripe(
std::max(keys.size(), lock_map->num_stripes_));
for (auto& key_iter : keys) {
const std::string& key = key_iter.first;
size_t stripe_num = lock_map->GetStripe(key);
keys_by_stripe[stripe_num].push_back(&key);
}
// For each stripe, grab the stripe mutex and unlock all keys in this stripe
for (auto& stripe_iter : keys_by_stripe) {
size_t stripe_num = stripe_iter.first;
auto& stripe_keys = stripe_iter.second;
assert(lock_map->lock_map_stripes_.size() > stripe_num);
LockMapStripe* stripe = lock_map->lock_map_stripes_.at(stripe_num);
stripe->stripe_mutex->Lock();
for (const std::string* key : stripe_keys) {
UnLockKey(txn, *key, stripe, lock_map, env);
}
stripe->stripe_mutex->UnLock();
// Signal waiting threads to retry locking
stripe->stripe_cv->NotifyAll();
}
}
}
TransactionLockMgr::LockStatusData TransactionLockMgr::GetLockStatusData() {
LockStatusData data;
// Lock order here is important. The correct order is lock_map_mutex_, then
// for every column family ID in ascending order lock every stripe in
// ascending order.
InstrumentedMutexLock l(&lock_map_mutex_);
std::vector<uint32_t> cf_ids;
for (const auto& map : lock_maps_) {
cf_ids.push_back(map.first);
}
std::sort(cf_ids.begin(), cf_ids.end());
for (auto i : cf_ids) {
const auto& stripes = lock_maps_[i]->lock_map_stripes_;
// Iterate and lock all stripes in ascending order.
for (const auto& j : stripes) {
j->stripe_mutex->Lock();
for (const auto& it : j->keys) {
struct KeyLockInfo info;
info.exclusive = it.second.exclusive;
info.key = it.first;
for (const auto& id : it.second.txn_ids) {
info.ids.push_back(id);
}
data.insert({i, info});
}
}
}
// Unlock everything. Unlocking order is not important.
for (auto i : cf_ids) {
const auto& stripes = lock_maps_[i]->lock_map_stripes_;
for (const auto& j : stripes) {
j->stripe_mutex->UnLock();
}
}
return data;
}
std::vector<DeadlockPath> TransactionLockMgr::GetDeadlockInfoBuffer() {
return dlock_buffer_.PrepareBuffer();
}
void TransactionLockMgr::Resize(uint32_t target_size) {
dlock_buffer_.Resize(target_size);
}
} // namespace rocksdb
#endif // ROCKSDB_LITE