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328 lignes
8.5 KiB
328 lignes
8.5 KiB
// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include <assert.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include "leveldb/cache.h"
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#include "port/port.h"
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#include "util/hash.h"
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#include "util/mutexlock.h"
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namespace leveldb {
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Cache::~Cache() {
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}
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namespace {
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// LRU cache implementation
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// An entry is a variable length heap-allocated structure. Entries
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// are kept in a circular doubly linked list ordered by access time.
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struct LRUHandle {
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void* value;
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void (*deleter)(const Slice&, void* value);
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LRUHandle* next_hash;
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LRUHandle* next;
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LRUHandle* prev;
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size_t charge; // TODO(opt): Only allow uint32_t?
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size_t key_length;
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uint32_t refs;
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uint32_t hash; // Hash of key(); used for fast sharding and comparisons
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char key_data[1]; // Beginning of key
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Slice key() const {
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// For cheaper lookups, we allow a temporary Handle object
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// to store a pointer to a key in "value".
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if (next == this) {
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return *(reinterpret_cast<Slice*>(value));
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} else {
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return Slice(key_data, key_length);
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}
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}
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};
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// We provide our own simple hash table since it removes a whole bunch
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// of porting hacks and is also faster than some of the built-in hash
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// table implementations in some of the compiler/runtime combinations
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// we have tested. E.g., readrandom speeds up by ~5% over the g++
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// 4.4.3's builtin hashtable.
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class HandleTable {
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public:
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HandleTable() : length_(0), elems_(0), list_(NULL) { Resize(); }
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~HandleTable() { delete[] list_; }
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LRUHandle* Lookup(const Slice& key, uint32_t hash) {
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return *FindPointer(key, hash);
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}
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LRUHandle* Insert(LRUHandle* h) {
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LRUHandle** ptr = FindPointer(h->key(), h->hash);
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LRUHandle* old = *ptr;
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h->next_hash = (old == NULL ? NULL : old->next_hash);
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*ptr = h;
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if (old == NULL) {
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++elems_;
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if (elems_ > length_) {
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// Since each cache entry is fairly large, we aim for a small
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// average linked list length (<= 1).
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Resize();
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}
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}
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return old;
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}
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LRUHandle* Remove(const Slice& key, uint32_t hash) {
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LRUHandle** ptr = FindPointer(key, hash);
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LRUHandle* result = *ptr;
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if (result != NULL) {
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*ptr = result->next_hash;
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--elems_;
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}
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return result;
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}
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private:
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// The table consists of an array of buckets where each bucket is
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// a linked list of cache entries that hash into the bucket.
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uint32_t length_;
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uint32_t elems_;
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LRUHandle** list_;
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// Return a pointer to slot that points to a cache entry that
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// matches key/hash. If there is no such cache entry, return a
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// pointer to the trailing slot in the corresponding linked list.
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LRUHandle** FindPointer(const Slice& key, uint32_t hash) {
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LRUHandle** ptr = &list_[hash & (length_ - 1)];
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while (*ptr != NULL &&
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((*ptr)->hash != hash || key != (*ptr)->key())) {
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ptr = &(*ptr)->next_hash;
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}
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return ptr;
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}
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void Resize() {
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uint32_t new_length = 4;
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while (new_length < elems_) {
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new_length *= 2;
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}
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LRUHandle** new_list = new LRUHandle*[new_length];
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memset(new_list, 0, sizeof(new_list[0]) * new_length);
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uint32_t count = 0;
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for (uint32_t i = 0; i < length_; i++) {
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LRUHandle* h = list_[i];
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while (h != NULL) {
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LRUHandle* next = h->next_hash;
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Slice key = h->key();
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uint32_t hash = h->hash;
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LRUHandle** ptr = &new_list[hash & (new_length - 1)];
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h->next_hash = *ptr;
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*ptr = h;
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h = next;
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count++;
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}
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}
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assert(elems_ == count);
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delete[] list_;
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list_ = new_list;
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length_ = new_length;
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}
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};
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// A single shard of sharded cache.
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class LRUCache {
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public:
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LRUCache();
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~LRUCache();
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// Separate from constructor so caller can easily make an array of LRUCache
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void SetCapacity(size_t capacity) { capacity_ = capacity; }
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// Like Cache methods, but with an extra "hash" parameter.
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Cache::Handle* Insert(const Slice& key, uint32_t hash,
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void* value, size_t charge,
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void (*deleter)(const Slice& key, void* value));
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Cache::Handle* Lookup(const Slice& key, uint32_t hash);
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void Release(Cache::Handle* handle);
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void Erase(const Slice& key, uint32_t hash);
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private:
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void LRU_Remove(LRUHandle* e);
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void LRU_Append(LRUHandle* e);
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void Unref(LRUHandle* e);
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// Initialized before use.
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size_t capacity_;
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// mutex_ protects the following state.
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port::Mutex mutex_;
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size_t usage_;
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uint64_t last_id_;
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// Dummy head of LRU list.
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// lru.prev is newest entry, lru.next is oldest entry.
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LRUHandle lru_;
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HandleTable table_;
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};
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LRUCache::LRUCache()
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: usage_(0),
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last_id_(0) {
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// Make empty circular linked list
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lru_.next = &lru_;
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lru_.prev = &lru_;
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}
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LRUCache::~LRUCache() {
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for (LRUHandle* e = lru_.next; e != &lru_; ) {
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LRUHandle* next = e->next;
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assert(e->refs == 1); // Error if caller has an unreleased handle
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Unref(e);
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e = next;
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}
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}
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void LRUCache::Unref(LRUHandle* e) {
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assert(e->refs > 0);
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e->refs--;
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if (e->refs <= 0) {
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usage_ -= e->charge;
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(*e->deleter)(e->key(), e->value);
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free(e);
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}
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}
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void LRUCache::LRU_Remove(LRUHandle* e) {
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e->next->prev = e->prev;
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e->prev->next = e->next;
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}
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void LRUCache::LRU_Append(LRUHandle* e) {
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// Make "e" newest entry by inserting just before lru_
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e->next = &lru_;
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e->prev = lru_.prev;
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e->prev->next = e;
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e->next->prev = e;
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}
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Cache::Handle* LRUCache::Lookup(const Slice& key, uint32_t hash) {
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MutexLock l(&mutex_);
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LRUHandle* e = table_.Lookup(key, hash);
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if (e != NULL) {
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e->refs++;
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LRU_Remove(e);
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LRU_Append(e);
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}
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return reinterpret_cast<Cache::Handle*>(e);
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}
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void LRUCache::Release(Cache::Handle* handle) {
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MutexLock l(&mutex_);
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Unref(reinterpret_cast<LRUHandle*>(handle));
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}
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Cache::Handle* LRUCache::Insert(
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const Slice& key, uint32_t hash, void* value, size_t charge,
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void (*deleter)(const Slice& key, void* value)) {
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MutexLock l(&mutex_);
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LRUHandle* e = reinterpret_cast<LRUHandle*>(
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malloc(sizeof(LRUHandle)-1 + key.size()));
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e->value = value;
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e->deleter = deleter;
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e->charge = charge;
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e->key_length = key.size();
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e->hash = hash;
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e->refs = 2; // One from LRUCache, one for the returned handle
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memcpy(e->key_data, key.data(), key.size());
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LRU_Append(e);
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usage_ += charge;
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LRUHandle* old = table_.Insert(e);
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if (old != NULL) {
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LRU_Remove(old);
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Unref(old);
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}
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while (usage_ > capacity_ && lru_.next != &lru_) {
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LRUHandle* old = lru_.next;
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LRU_Remove(old);
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table_.Remove(old->key(), old->hash);
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Unref(old);
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}
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return reinterpret_cast<Cache::Handle*>(e);
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}
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void LRUCache::Erase(const Slice& key, uint32_t hash) {
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MutexLock l(&mutex_);
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LRUHandle* e = table_.Remove(key, hash);
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if (e != NULL) {
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LRU_Remove(e);
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Unref(e);
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}
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}
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static const int kNumShardBits = 4;
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static const int kNumShards = 1 << kNumShardBits;
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class ShardedLRUCache : public Cache {
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private:
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LRUCache shard_[kNumShards];
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port::Mutex id_mutex_;
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uint64_t last_id_;
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static inline uint32_t HashSlice(const Slice& s) {
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return Hash(s.data(), s.size(), 0);
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}
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static uint32_t Shard(uint32_t hash) {
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return hash >> (32 - kNumShardBits);
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}
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public:
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explicit ShardedLRUCache(size_t capacity)
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: last_id_(0) {
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const size_t per_shard = (capacity + (kNumShards - 1)) / kNumShards;
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for (int s = 0; s < kNumShards; s++) {
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shard_[s].SetCapacity(per_shard);
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}
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}
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virtual ~ShardedLRUCache() { }
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virtual Handle* Insert(const Slice& key, void* value, size_t charge,
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void (*deleter)(const Slice& key, void* value)) {
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const uint32_t hash = HashSlice(key);
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return shard_[Shard(hash)].Insert(key, hash, value, charge, deleter);
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}
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virtual Handle* Lookup(const Slice& key) {
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const uint32_t hash = HashSlice(key);
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return shard_[Shard(hash)].Lookup(key, hash);
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}
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virtual void Release(Handle* handle) {
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LRUHandle* h = reinterpret_cast<LRUHandle*>(handle);
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shard_[Shard(h->hash)].Release(handle);
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}
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virtual void Erase(const Slice& key) {
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const uint32_t hash = HashSlice(key);
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shard_[Shard(hash)].Erase(key, hash);
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}
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virtual void* Value(Handle* handle) {
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return reinterpret_cast<LRUHandle*>(handle)->value;
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}
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virtual uint64_t NewId() {
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MutexLock l(&id_mutex_);
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return ++(last_id_);
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}
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};
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} // end anonymous namespace
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Cache* NewLRUCache(size_t capacity) {
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return new ShardedLRUCache(capacity);
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}
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}
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