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rocksdb/cache/fast_lru_cache.cc

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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).
//
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#include "cache/fast_lru_cache.h"
#include <cassert>
#include <cstdint>
#include <cstdio>
#include <functional>
#include "monitoring/perf_context_imp.h"
#include "monitoring/statistics.h"
#include "port/lang.h"
#include "util/distributed_mutex.h"
#include "util/hash.h"
#include "util/math.h"
#include "util/random.h"
namespace ROCKSDB_NAMESPACE {
namespace fast_lru_cache {
LRUHandleTable::LRUHandleTable(int hash_bits)
: length_bits_(hash_bits),
length_bits_mask_((uint32_t{1} << length_bits_) - 1),
occupancy_(0),
occupancy_limit_(static_cast<uint32_t>((uint32_t{1} << length_bits_) *
kStrictLoadFactor)),
array_(new LRUHandle[size_t{1} << length_bits_]) {
assert(hash_bits <= 32);
}
LRUHandleTable::~LRUHandleTable() {
ApplyToEntriesRange([](LRUHandle* h) { h->FreeData(); }, 0, GetTableSize());
}
LRUHandle* LRUHandleTable::Lookup(const Slice& key, uint32_t hash) {
int probe = 0;
int slot = FindVisibleElement(key, hash, probe, 0);
return (slot == -1) ? nullptr : &array_[slot];
}
LRUHandle* LRUHandleTable::Insert(LRUHandle* h, LRUHandle** old) {
int probe = 0;
int slot = FindVisibleElementOrAvailableSlot(h->key(), h->hash, probe,
1 /*displacement*/);
*old = nullptr;
if (slot == -1) {
// TODO(Guido) Don't we need to roll back displacements here?
return nullptr;
}
if (array_[slot].IsEmpty() || array_[slot].IsTombstone()) {
bool empty = array_[slot].IsEmpty();
Assign(slot, h);
LRUHandle* new_entry = &array_[slot];
if (empty) {
// This used to be an empty slot.
return new_entry;
}
// It used to be a tombstone, so there may already be a copy of the
// key in the table.
slot = FindVisibleElement(h->key(), h->hash, probe, 0 /*displacement*/);
if (slot == -1) {
// No existing copy of the key.
return new_entry;
}
*old = &array_[slot];
return new_entry;
} else {
// There is an existing copy of the key.
*old = &array_[slot];
// Find an available slot for the new element.
array_[slot].displacements++;
slot = FindAvailableSlot(h->key(), probe, 1 /*displacement*/);
if (slot == -1) {
// No available slots. Roll back displacements.
probe = 0;
slot = FindVisibleElement(h->key(), h->hash, probe, -1);
array_[slot].displacements--;
FindAvailableSlot(h->key(), probe, -1);
return nullptr;
}
Assign(slot, h);
return &array_[slot];
}
}
void LRUHandleTable::Remove(LRUHandle* h) {
assert(h->next == nullptr &&
h->prev == nullptr); // Already off the LRU list.
int probe = 0;
FindSlot(
h->key(), [&h](LRUHandle* e) { return e == h; }, probe,
-1 /*displacement*/);
h->SetIsVisible(false);
h->SetIsElement(false);
occupancy_--;
}
void LRUHandleTable::Assign(int slot, LRUHandle* h) {
LRUHandle* dst = &array_[slot];
uint32_t disp = dst->displacements;
*dst = *h;
dst->displacements = disp;
dst->SetIsVisible(true);
dst->SetIsElement(true);
occupancy_++;
}
void LRUHandleTable::Exclude(LRUHandle* h) { h->SetIsVisible(false); }
int LRUHandleTable::FindVisibleElement(const Slice& key, uint32_t hash,
int& probe, int displacement) {
return FindSlot(
key,
[&](LRUHandle* h) { return h->Matches(key, hash) && h->IsVisible(); },
probe, displacement);
}
int LRUHandleTable::FindAvailableSlot(const Slice& key, int& probe,
int displacement) {
return FindSlot(
key, [](LRUHandle* h) { return h->IsEmpty() || h->IsTombstone(); }, probe,
displacement);
}
int LRUHandleTable::FindVisibleElementOrAvailableSlot(const Slice& key,
uint32_t hash, int& probe,
int displacement) {
return FindSlot(
key,
[&](LRUHandle* h) {
return h->IsEmpty() || h->IsTombstone() ||
(h->Matches(key, hash) && h->IsVisible());
},
probe, displacement);
}
inline int LRUHandleTable::FindSlot(const Slice& key,
std::function<bool(LRUHandle*)> cond,
int& probe, int displacement) {
uint32_t base = ModTableSize(Hash(key.data(), key.size(), kProbingSeed1));
uint32_t increment =
ModTableSize((Hash(key.data(), key.size(), kProbingSeed2) << 1) | 1);
uint32_t current = ModTableSize(base + probe * increment);
while (true) {
LRUHandle* h = &array_[current];
probe++;
if (current == base && probe > 1) {
// We looped back.
return -1;
}
if (cond(h)) {
return current;
}
if (h->IsEmpty()) {
// We check emptyness after the condition, because
// the condition may be emptyness.
return -1;
}
h->displacements += displacement;
current = ModTableSize(current + increment);
}
}
LRUCacheShard::LRUCacheShard(size_t capacity, size_t estimated_value_size,
bool strict_capacity_limit,
CacheMetadataChargePolicy metadata_charge_policy)
: CacheShardBase(metadata_charge_policy),
capacity_(capacity),
strict_capacity_limit_(strict_capacity_limit),
table_(
CalcHashBits(capacity, estimated_value_size, metadata_charge_policy)),
usage_(0),
lru_usage_(0) {
// Make empty circular linked list.
lru_.next = &lru_;
lru_.prev = &lru_;
lru_low_pri_ = &lru_;
}
void LRUCacheShard::EraseUnRefEntries() {
autovector<LRUHandle> last_reference_list;
{
DMutexLock l(mutex_);
while (lru_.next != &lru_) {
LRUHandle* old = lru_.next;
// LRU list contains only elements which can be evicted.
assert(old->IsVisible() && !old->HasRefs());
LRU_Remove(old);
table_.Remove(old);
assert(usage_ >= old->total_charge);
usage_ -= old->total_charge;
last_reference_list.push_back(*old);
}
}
// Free the entries here outside of mutex for performance reasons.
for (auto& h : last_reference_list) {
h.FreeData();
}
}
void LRUCacheShard::ApplyToSomeEntries(
const std::function<void(const Slice& key, void* value, size_t charge,
DeleterFn deleter)>& callback,
size_t average_entries_per_lock, size_t* state) {
// The state is essentially going to be the starting hash, which works
// nicely even if we resize between calls because we use upper-most
// hash bits for table indexes.
DMutexLock l(mutex_);
size_t length_bits = table_.GetLengthBits();
size_t length = table_.GetTableSize();
assert(average_entries_per_lock > 0);
// Assuming we are called with same average_entries_per_lock repeatedly,
// this simplifies some logic (index_end will not overflow).
assert(average_entries_per_lock < length || *state == 0);
size_t index_begin = *state >> (sizeof(size_t) * 8u - length_bits);
size_t index_end = index_begin + average_entries_per_lock;
if (index_end >= length) {
// Going to end
index_end = length;
*state = SIZE_MAX;
} else {
*state = index_end << (sizeof(size_t) * 8u - length_bits);
}
table_.ApplyToEntriesRange(
[callback,
metadata_charge_policy = metadata_charge_policy_](LRUHandle* h) {
callback(h->key(), h->value, h->GetCharge(metadata_charge_policy),
h->deleter);
},
index_begin, index_end);
}
void LRUCacheShard::LRU_Remove(LRUHandle* h) {
assert(h->next != nullptr);
assert(h->prev != nullptr);
h->next->prev = h->prev;
h->prev->next = h->next;
h->prev = h->next = nullptr;
assert(lru_usage_ >= h->total_charge);
lru_usage_ -= h->total_charge;
}
void LRUCacheShard::LRU_Insert(LRUHandle* h) {
assert(h->next == nullptr);
assert(h->prev == nullptr);
// Insert h to head of LRU list.
h->next = &lru_;
h->prev = lru_.prev;
h->prev->next = h;
h->next->prev = h;
lru_usage_ += h->total_charge;
}
void LRUCacheShard::EvictFromLRU(size_t charge,
autovector<LRUHandle>* deleted) {
while ((usage_ + charge) > capacity_ && lru_.next != &lru_) {
LRUHandle* old = lru_.next;
// LRU list contains only elements which can be evicted.
assert(old->IsVisible() && !old->HasRefs());
LRU_Remove(old);
table_.Remove(old);
assert(usage_ >= old->total_charge);
usage_ -= old->total_charge;
deleted->push_back(*old);
}
}
size_t LRUCacheShard::CalcEstimatedHandleCharge(
size_t estimated_value_size,
CacheMetadataChargePolicy metadata_charge_policy) {
LRUHandle h;
h.CalcTotalCharge(estimated_value_size, metadata_charge_policy);
return h.total_charge;
}
int LRUCacheShard::CalcHashBits(
size_t capacity, size_t estimated_value_size,
CacheMetadataChargePolicy metadata_charge_policy) {
size_t handle_charge =
CalcEstimatedHandleCharge(estimated_value_size, metadata_charge_policy);
assert(handle_charge > 0);
uint32_t num_entries =
static_cast<uint32_t>(capacity / (kLoadFactor * handle_charge)) + 1;
assert(num_entries <= uint32_t{1} << 31);
return FloorLog2((num_entries << 1) - 1);
}
void LRUCacheShard::SetCapacity(size_t capacity) {
autovector<LRUHandle> last_reference_list;
{
DMutexLock l(mutex_);
if (capacity > capacity_) {
assert(false); // Not supported.
}
capacity_ = capacity;
EvictFromLRU(0, &last_reference_list);
}
// Free the entries here outside of mutex for performance reasons.
for (auto& h : last_reference_list) {
h.FreeData();
}
}
void LRUCacheShard::SetStrictCapacityLimit(bool strict_capacity_limit) {
DMutexLock l(mutex_);
strict_capacity_limit_ = strict_capacity_limit;
}
Status LRUCacheShard::Insert(const Slice& key, uint32_t hash, void* value,
size_t charge, Cache::DeleterFn deleter,
LRUHandle** handle, Cache::Priority /*priority*/) {
if (key.size() != kCacheKeySize) {
return Status::NotSupported("FastLRUCache only supports key size " +
std::to_string(kCacheKeySize) + "B");
}
LRUHandle tmp;
tmp.value = value;
tmp.deleter = deleter;
tmp.hash = hash;
tmp.CalcTotalCharge(charge, metadata_charge_policy_);
for (int i = 0; i < kCacheKeySize; i++) {
tmp.key_data[i] = key.data()[i];
}
Status s = Status::OK();
autovector<LRUHandle> last_reference_list;
{
DMutexLock l(mutex_);
assert(table_.GetOccupancy() <= table_.GetOccupancyLimit());
// Free the space following strict LRU policy until enough space
// is freed or the lru list is empty.
EvictFromLRU(tmp.total_charge, &last_reference_list);
if ((usage_ + tmp.total_charge > capacity_ &&
(strict_capacity_limit_ || handle == nullptr)) ||
table_.GetOccupancy() == table_.GetOccupancyLimit()) {
// There are two measures of capacity:
// - Space (or charge) capacity: The maximum possible sum of the charges
// of the elements.
// - Table capacity: The number of slots in the hash table.
// These are incomparable, in the sense that one doesn't imply the other.
// Typically we will reach space capacity before table capacity---
// if the user always inserts values with size equal to
// estimated_value_size, then at most a kLoadFactor fraction of slots
// will ever be occupied. But in some cases we may reach table capacity
// before space capacity---if the user initially claims a very large
// estimated_value_size but then inserts tiny values, more elements than
// initially estimated will be inserted.
// TODO(Guido) Some tests (at least two from cache_test, as well as the
// stress tests) currently assume the table capacity is unbounded.
if (handle == nullptr) {
// Don't insert the entry but still return ok, as if the entry inserted
// into cache and get evicted immediately.
last_reference_list.push_back(tmp);
} else {
if (table_.GetOccupancy() == table_.GetOccupancyLimit()) {
// TODO: Consider using a distinct status for this case, but usually
// it will be handled the same way as reaching charge capacity limit
s = Status::MemoryLimit(
"Insert failed because all slots in the hash table are full.");
} else {
s = Status::MemoryLimit(
"Insert failed because the total charge has exceeded the "
"capacity.");
}
}
} else {
// Insert into the cache. Note that the cache might get larger than its
// capacity if not enough space was freed up.
LRUHandle* old;
LRUHandle* h = table_.Insert(&tmp, &old);
assert(h != nullptr); // We're below occupancy, so this insertion should
// never fail.
usage_ += h->total_charge;
if (old != nullptr) {
s = Status::OkOverwritten();
assert(old->IsVisible());
table_.Exclude(old);
if (!old->HasRefs()) {
// old is on LRU because it's in cache and its reference count is 0.
LRU_Remove(old);
table_.Remove(old);
assert(usage_ >= old->total_charge);
usage_ -= old->total_charge;
last_reference_list.push_back(*old);
}
}
if (handle == nullptr) {
LRU_Insert(h);
} else {
// If caller already holds a ref, no need to take one here.
if (!h->HasRefs()) {
h->Ref();
}
*handle = h;
}
}
}
// Free the entries here outside of mutex for performance reasons.
for (auto& h : last_reference_list) {
h.FreeData();
}
return s;
}
LRUHandle* LRUCacheShard::Lookup(const Slice& key, uint32_t hash) {
LRUHandle* h = nullptr;
{
DMutexLock l(mutex_);
h = table_.Lookup(key, hash);
if (h != nullptr) {
assert(h->IsVisible());
if (!h->HasRefs()) {
// The entry is in LRU since it's in hash and has no external
// references.
LRU_Remove(h);
}
h->Ref();
}
}
return h;
}
bool LRUCacheShard::Ref(LRUHandle* h) {
DMutexLock l(mutex_);
// To create another reference - entry must be already externally referenced.
assert(h->HasRefs());
h->Ref();
return true;
}
bool LRUCacheShard::Release(LRUHandle* h, bool erase_if_last_ref) {
if (h == nullptr) {
return false;
}
LRUHandle copy;
bool last_reference = false;
{
DMutexLock l(mutex_);
last_reference = h->Unref();
if (last_reference && h->IsVisible()) {
// The item is still in cache, and nobody else holds a reference to it.
if (usage_ > capacity_ || erase_if_last_ref) {
// The LRU list must be empty since the cache is full.
assert(lru_.next == &lru_ || erase_if_last_ref);
// Take this opportunity and remove the item.
table_.Remove(h);
} else {
// Put the item back on the LRU list, and don't free it.
LRU_Insert(h);
last_reference = false;
}
}
// If it was the last reference, then decrement the cache usage.
if (last_reference) {
assert(usage_ >= h->total_charge);
usage_ -= h->total_charge;
copy = *h;
}
}
// Free the entry here outside of mutex for performance reasons.
if (last_reference) {
copy.FreeData();
}
return last_reference;
}
void LRUCacheShard::Erase(const Slice& key, uint32_t hash) {
LRUHandle copy;
bool last_reference = false;
{
DMutexLock l(mutex_);
LRUHandle* h = table_.Lookup(key, hash);
if (h != nullptr) {
table_.Exclude(h);
if (!h->HasRefs()) {
// The entry is in LRU since it's in cache and has no external
// references.
LRU_Remove(h);
table_.Remove(h);
assert(usage_ >= h->total_charge);
usage_ -= h->total_charge;
last_reference = true;
copy = *h;
}
}
}
// Free the entry here outside of mutex for performance reasons.
// last_reference will only be true if e != nullptr.
if (last_reference) {
copy.FreeData();
}
}
size_t LRUCacheShard::GetUsage() const {
DMutexLock l(mutex_);
return usage_;
}
size_t LRUCacheShard::GetPinnedUsage() const {
DMutexLock l(mutex_);
assert(usage_ >= lru_usage_);
return usage_ - lru_usage_;
}
size_t LRUCacheShard::GetOccupancyCount() const {
DMutexLock l(mutex_);
return table_.GetOccupancy();
}
size_t LRUCacheShard::GetTableAddressCount() const {
DMutexLock l(mutex_);
return table_.GetTableSize();
}
LRUCache::LRUCache(size_t capacity, size_t estimated_value_size,
int num_shard_bits, bool strict_capacity_limit,
CacheMetadataChargePolicy metadata_charge_policy)
: ShardedCache(capacity, num_shard_bits, strict_capacity_limit,
nullptr /*allocator*/) {
assert(estimated_value_size > 0 ||
metadata_charge_policy != kDontChargeCacheMetadata);
size_t per_shard = GetPerShardCapacity();
InitShards([=](LRUCacheShard* cs) {
new (cs) LRUCacheShard(per_shard, estimated_value_size,
strict_capacity_limit, metadata_charge_policy);
});
}
void* LRUCache::Value(Handle* handle) {
return reinterpret_cast<const LRUHandle*>(handle)->value;
}
size_t LRUCache::GetCharge(Handle* handle) const {
return reinterpret_cast<const LRUHandle*>(handle)->GetCharge(
GetShard(0).metadata_charge_policy_);
}
Cache::DeleterFn LRUCache::GetDeleter(Handle* handle) const {
auto h = reinterpret_cast<const LRUHandle*>(handle);
return h->deleter;
}
} // namespace fast_lru_cache
std::shared_ptr<Cache> NewFastLRUCache(
size_t capacity, size_t estimated_value_size, int num_shard_bits,
bool strict_capacity_limit,
CacheMetadataChargePolicy metadata_charge_policy) {
if (num_shard_bits >= 20) {
return nullptr; // The cache cannot be sharded into too many fine pieces.
}
if (num_shard_bits < 0) {
num_shard_bits = GetDefaultCacheShardBits(capacity);
}
return std::make_shared<fast_lru_cache::LRUCache>(
capacity, estimated_value_size, num_shard_bits, strict_capacity_limit,
metadata_charge_policy);
}
} // namespace ROCKSDB_NAMESPACE