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// 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 <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include "leveldb/cache.h"
#include "port/port.h"
#include "port/thread_annotations.h"
#include "util/hash.h"
#include "util/mutexlock.h"
namespace leveldb {
Cache::~Cache() {
}
namespace {
// LRU cache implementation
//
// Cache entries have an "in_cache" boolean indicating whether the cache has a
// reference on the entry. The only ways that this can become false without the
// entry being passed to its "deleter" are via Erase(), via Insert() when
// an element with a duplicate key is inserted, or on destruction of the cache.
//
// The cache keeps two linked lists of items in the cache. All items in the
// cache are in one list or the other, and never both. Items still referenced
// by clients but erased from the cache are in neither list. The lists are:
// - in-use: contains the items currently referenced by clients, in no
// particular order. (This list is used for invariant checking. If we
// removed the check, elements that would otherwise be on this list could be
// left as disconnected singleton lists.)
// - LRU: contains the items not currently referenced by clients, in LRU order
// Elements are moved between these lists by the Ref() and Unref() methods,
// when they detect an element in the cache acquiring or losing its only
// external reference.
// An entry is a variable length heap-allocated structure. Entries
// are kept in a circular doubly linked list ordered by access time.
struct LRUHandle {
void* value;
void (*deleter)(const Slice&, void* value);
LRUHandle* next_hash;
LRUHandle* next;
LRUHandle* prev;
size_t charge; // TODO(opt): Only allow uint32_t?
size_t key_length;
bool in_cache; // Whether entry is in the cache.
uint32_t refs; // References, including cache reference, if present.
uint32_t hash; // Hash of key(); used for fast sharding and comparisons
char key_data[1]; // Beginning of key
Slice key() const {
// next_ is only equal to this if the LRU handle is the list head of an
// empty list. List heads never have meaningful keys.
assert(next != this);
return Slice(key_data, key_length);
}
};
// We provide our own simple hash table since it removes a whole bunch
// of porting hacks and is also faster than some of the built-in hash
// table implementations in some of the compiler/runtime combinations
// we have tested. E.g., readrandom speeds up by ~5% over the g++
// 4.4.3's builtin hashtable.
class HandleTable {
public:
HandleTable() : length_(0), elems_(0), list_(nullptr) { Resize(); }
~HandleTable() { delete[] list_; }
LRUHandle* Lookup(const Slice& key, uint32_t hash) {
return *FindPointer(key, hash);
}
LRUHandle* Insert(LRUHandle* h) {
LRUHandle** ptr = FindPointer(h->key(), h->hash);
LRUHandle* old = *ptr;
h->next_hash = (old == nullptr ? nullptr : old->next_hash);
*ptr = h;
if (old == nullptr) {
++elems_;
if (elems_ > length_) {
// Since each cache entry is fairly large, we aim for a small
// average linked list length (<= 1).
Resize();
}
}
return old;
}
LRUHandle* Remove(const Slice& key, uint32_t hash) {
LRUHandle** ptr = FindPointer(key, hash);
LRUHandle* result = *ptr;
if (result != nullptr) {
*ptr = result->next_hash;
--elems_;
}
return result;
}
private:
// The table consists of an array of buckets where each bucket is
// a linked list of cache entries that hash into the bucket.
uint32_t length_;
uint32_t elems_;
LRUHandle** list_;
// Return a pointer to slot that points to a cache entry that
// matches key/hash. If there is no such cache entry, return a
// pointer to the trailing slot in the corresponding linked list.
LRUHandle** FindPointer(const Slice& key, uint32_t hash) {
LRUHandle** ptr = &list_[hash & (length_ - 1)];
while (*ptr != nullptr &&
((*ptr)->hash != hash || key != (*ptr)->key())) {
ptr = &(*ptr)->next_hash;
}
return ptr;
}
void Resize() {
uint32_t new_length = 4;
while (new_length < elems_) {
new_length *= 2;
}
LRUHandle** new_list = new LRUHandle*[new_length];
memset(new_list, 0, sizeof(new_list[0]) * new_length);
uint32_t count = 0;
for (uint32_t i = 0; i < length_; i++) {
LRUHandle* h = list_[i];
while (h != nullptr) {
LRUHandle* next = h->next_hash;
uint32_t hash = h->hash;
LRUHandle** ptr = &new_list[hash & (new_length - 1)];
h->next_hash = *ptr;
*ptr = h;
h = next;
count++;
}
}
assert(elems_ == count);
delete[] list_;
list_ = new_list;
length_ = new_length;
}
};
// A single shard of sharded cache.
class LRUCache {
public:
LRUCache();
~LRUCache();
// Separate from constructor so caller can easily make an array of LRUCache
void SetCapacity(size_t capacity) { capacity_ = capacity; }
// Like Cache methods, but with an extra "hash" parameter.
Cache::Handle* Insert(const Slice& key, uint32_t hash,
void* value, size_t charge,
void (*deleter)(const Slice& key, void* value));
Cache::Handle* Lookup(const Slice& key, uint32_t hash);
void Release(Cache::Handle* handle);
void Erase(const Slice& key, uint32_t hash);
void Prune();
size_t TotalCharge() const {
MutexLock l(&mutex_);
return usage_;
}
private:
void LRU_Remove(LRUHandle* e);
void LRU_Append(LRUHandle*list, LRUHandle* e);
void Ref(LRUHandle* e);
void Unref(LRUHandle* e);
bool FinishErase(LRUHandle* e) EXCLUSIVE_LOCKS_REQUIRED(mutex_);
// Initialized before use.
size_t capacity_;
// mutex_ protects the following state.
mutable port::Mutex mutex_;
size_t usage_ GUARDED_BY(mutex_);
// Dummy head of LRU list.
// lru.prev is newest entry, lru.next is oldest entry.
// Entries have refs==1 and in_cache==true.
LRUHandle lru_ GUARDED_BY(mutex_);
// Dummy head of in-use list.
// Entries are in use by clients, and have refs >= 2 and in_cache==true.
LRUHandle in_use_ GUARDED_BY(mutex_);
HandleTable table_ GUARDED_BY(mutex_);
};
LRUCache::LRUCache()
: usage_(0) {
// Make empty circular linked lists.
lru_.next = &lru_;
lru_.prev = &lru_;
in_use_.next = &in_use_;
in_use_.prev = &in_use_;
}
LRUCache::~LRUCache() {
assert(in_use_.next == &in_use_); // Error if caller has an unreleased handle
for (LRUHandle* e = lru_.next; e != &lru_; ) {
LRUHandle* next = e->next;
assert(e->in_cache);
e->in_cache = false;
assert(e->refs == 1); // Invariant of lru_ list.
Unref(e);
e = next;
}
}
void LRUCache::Ref(LRUHandle* e) {
if (e->refs == 1 && e->in_cache) { // If on lru_ list, move to in_use_ list.
LRU_Remove(e);
LRU_Append(&in_use_, e);
}
e->refs++;
}
void LRUCache::Unref(LRUHandle* e) {
assert(e->refs > 0);
e->refs--;
if (e->refs == 0) { // Deallocate.
assert(!e->in_cache);
(*e->deleter)(e->key(), e->value);
free(e);
} else if (e->in_cache && e->refs == 1) {
// No longer in use; move to lru_ list.
LRU_Remove(e);
LRU_Append(&lru_, e);
}
}
void LRUCache::LRU_Remove(LRUHandle* e) {
e->next->prev = e->prev;
e->prev->next = e->next;
}
void LRUCache::LRU_Append(LRUHandle* list, LRUHandle* e) {
// Make "e" newest entry by inserting just before *list
e->next = list;
e->prev = list->prev;
e->prev->next = e;
e->next->prev = e;
}
Cache::Handle* LRUCache::Lookup(const Slice& key, uint32_t hash) {
MutexLock l(&mutex_);
LRUHandle* e = table_.Lookup(key, hash);
if (e != nullptr) {
Ref(e);
}
return reinterpret_cast<Cache::Handle*>(e);
}
void LRUCache::Release(Cache::Handle* handle) {
MutexLock l(&mutex_);
Unref(reinterpret_cast<LRUHandle*>(handle));
}
Cache::Handle* LRUCache::Insert(
const Slice& key, uint32_t hash, void* value, size_t charge,
void (*deleter)(const Slice& key, void* value)) {
MutexLock l(&mutex_);
LRUHandle* e = reinterpret_cast<LRUHandle*>(
malloc(sizeof(LRUHandle)-1 + key.size()));
e->value = value;
e->deleter = deleter;
e->charge = charge;
e->key_length = key.size();
e->hash = hash;
e->in_cache = false;
e->refs = 1; // for the returned handle.
memcpy(e->key_data, key.data(), key.size());
if (capacity_ > 0) {
e->refs++; // for the cache's reference.
e->in_cache = true;
LRU_Append(&in_use_, e);
usage_ += charge;
FinishErase(table_.Insert(e));
} else { // don't cache. (capacity_==0 is supported and turns off caching.)
// next is read by key() in an assert, so it must be initialized
e->next = nullptr;
}
while (usage_ > capacity_ && lru_.next != &lru_) {
LRUHandle* old = lru_.next;
assert(old->refs == 1);
bool erased = FinishErase(table_.Remove(old->key(), old->hash));
if (!erased) { // to avoid unused variable when compiled NDEBUG
assert(erased);
}
}
return reinterpret_cast<Cache::Handle*>(e);
}
// If e != nullptr, finish removing *e from the cache; it has already been
// removed from the hash table. Return whether e != nullptr.
bool LRUCache::FinishErase(LRUHandle* e) {
if (e != nullptr) {
assert(e->in_cache);
LRU_Remove(e);
e->in_cache = false;
usage_ -= e->charge;
Unref(e);
}
return e != nullptr;
}
void LRUCache::Erase(const Slice& key, uint32_t hash) {
MutexLock l(&mutex_);
FinishErase(table_.Remove(key, hash));
}
void LRUCache::Prune() {
MutexLock l(&mutex_);
while (lru_.next != &lru_) {
LRUHandle* e = lru_.next;
assert(e->refs == 1);
bool erased = FinishErase(table_.Remove(e->key(), e->hash));
if (!erased) { // to avoid unused variable when compiled NDEBUG
assert(erased);
}
}
}
static const int kNumShardBits = 4;
static const int kNumShards = 1 << kNumShardBits;
class ShardedLRUCache : public Cache {
private:
LRUCache shard_[kNumShards];
port::Mutex id_mutex_;
uint64_t last_id_;
static inline uint32_t HashSlice(const Slice& s) {
return Hash(s.data(), s.size(), 0);
}
static uint32_t Shard(uint32_t hash) {
return hash >> (32 - kNumShardBits);
}
public:
explicit ShardedLRUCache(size_t capacity)
: last_id_(0) {
const size_t per_shard = (capacity + (kNumShards - 1)) / kNumShards;
for (int s = 0; s < kNumShards; s++) {
shard_[s].SetCapacity(per_shard);
}
}
virtual ~ShardedLRUCache() { }
virtual Handle* Insert(const Slice& key, void* value, size_t charge,
void (*deleter)(const Slice& key, void* value)) {
const uint32_t hash = HashSlice(key);
return shard_[Shard(hash)].Insert(key, hash, value, charge, deleter);
}
virtual Handle* Lookup(const Slice& key) {
const uint32_t hash = HashSlice(key);
return shard_[Shard(hash)].Lookup(key, hash);
}
virtual void Release(Handle* handle) {
LRUHandle* h = reinterpret_cast<LRUHandle*>(handle);
shard_[Shard(h->hash)].Release(handle);
}
virtual void Erase(const Slice& key) {
const uint32_t hash = HashSlice(key);
shard_[Shard(hash)].Erase(key, hash);
}
virtual void* Value(Handle* handle) {
return reinterpret_cast<LRUHandle*>(handle)->value;
}
virtual uint64_t NewId() {
MutexLock l(&id_mutex_);
return ++(last_id_);
}
virtual void Prune() {
for (int s = 0; s < kNumShards; s++) {
shard_[s].Prune();
}
}
virtual size_t TotalCharge() const {
size_t total = 0;
for (int s = 0; s < kNumShards; s++) {
total += shard_[s].TotalCharge();
}
return total;
}
};
} // end anonymous namespace
Cache* NewLRUCache(size_t capacity) {
return new ShardedLRUCache(capacity);
}
} // namespace leveldb