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401 righe
12 KiB
401 righe
12 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 "leveldb/cache.h"
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#include <cassert>
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#include <cstdio>
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#include <cstdlib>
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#include "port/port.h"
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#include "port/thread_annotations.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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namespace {
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// LRU cache implementation
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//
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// Cache entries have an "in_cache" boolean indicating whether the cache has a
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// reference on the entry. The only ways that this can become false without the
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// entry being passed to its "deleter" are via Erase(), via Insert() when
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// an element with a duplicate key is inserted, or on destruction of the cache.
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//
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// The cache keeps two linked lists of items in the cache. All items in the
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// cache are in one list or the other, and never both. Items still referenced
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// by clients but erased from the cache are in neither list. The lists are:
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// - in-use: contains the items currently referenced by clients, in no
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// particular order. (This list is used for invariant checking. If we
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// removed the check, elements that would otherwise be on this list could be
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// left as disconnected singleton lists.)
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// - LRU: contains the items not currently referenced by clients, in LRU order
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// Elements are moved between these lists by the Ref() and Unref() methods,
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// when they detect an element in the cache acquiring or losing its only
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// external reference.
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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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bool in_cache; // Whether entry is in the cache.
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uint32_t refs; // References, including cache reference, if present.
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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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// next is only equal to this if the LRU handle is the list head of an
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// empty list. List heads never have meaningful keys.
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assert(next != this);
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return Slice(key_data, key_length);
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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_(nullptr) { 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 == nullptr ? nullptr : old->next_hash);
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*ptr = h;
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if (old == nullptr) {
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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 != nullptr) {
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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 != nullptr && ((*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 != nullptr) {
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LRUHandle* next = h->next_hash;
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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, void* value,
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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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void Prune();
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size_t TotalCharge() const {
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MutexLock l(&mutex_);
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return usage_;
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}
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private:
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void LRU_Remove(LRUHandle* e);
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void LRU_Append(LRUHandle* list, LRUHandle* e);
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void Ref(LRUHandle* e);
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void Unref(LRUHandle* e);
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bool FinishErase(LRUHandle* e) EXCLUSIVE_LOCKS_REQUIRED(mutex_);
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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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mutable port::Mutex mutex_;
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size_t usage_ GUARDED_BY(mutex_);
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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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// Entries have refs==1 and in_cache==true.
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LRUHandle lru_ GUARDED_BY(mutex_);
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// Dummy head of in-use list.
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// Entries are in use by clients, and have refs >= 2 and in_cache==true.
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LRUHandle in_use_ GUARDED_BY(mutex_);
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HandleTable table_ GUARDED_BY(mutex_);
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};
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LRUCache::LRUCache() : capacity_(0), usage_(0) {
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// Make empty circular linked lists.
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lru_.next = &lru_;
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lru_.prev = &lru_;
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in_use_.next = &in_use_;
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in_use_.prev = &in_use_;
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}
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LRUCache::~LRUCache() {
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assert(in_use_.next == &in_use_); // Error if caller has an unreleased handle
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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->in_cache);
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e->in_cache = false;
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assert(e->refs == 1); // Invariant of lru_ list.
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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::Ref(LRUHandle* e) {
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if (e->refs == 1 && e->in_cache) { // If on lru_ list, move to in_use_ list.
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LRU_Remove(e);
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LRU_Append(&in_use_, e);
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}
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e->refs++;
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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) { // Deallocate.
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assert(!e->in_cache);
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(*e->deleter)(e->key(), e->value);
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free(e);
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} else if (e->in_cache && e->refs == 1) {
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// No longer in use; move to lru_ list.
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LRU_Remove(e);
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LRU_Append(&lru_, 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* list, LRUHandle* e) {
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// Make "e" newest entry by inserting just before *list
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e->next = list;
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e->prev = list->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 != nullptr) {
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Ref(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(const Slice& key, uint32_t hash, void* value,
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size_t charge,
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void (*deleter)(const Slice& key,
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void* value)) {
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MutexLock l(&mutex_);
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LRUHandle* e =
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reinterpret_cast<LRUHandle*>(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->in_cache = false;
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e->refs = 1; // for the returned handle.
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std::memcpy(e->key_data, key.data(), key.size());
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if (capacity_ > 0) {
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e->refs++; // for the cache's reference.
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e->in_cache = true;
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LRU_Append(&in_use_, e);
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usage_ += charge;
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FinishErase(table_.Insert(e));
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} else { // don't cache. (capacity_==0 is supported and turns off caching.)
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// next is read by key() in an assert, so it must be initialized
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e->next = nullptr;
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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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assert(old->refs == 1);
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bool erased = FinishErase(table_.Remove(old->key(), old->hash));
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if (!erased) { // to avoid unused variable when compiled NDEBUG
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assert(erased);
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}
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}
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return reinterpret_cast<Cache::Handle*>(e);
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}
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// If e != nullptr, finish removing *e from the cache; it has already been
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// removed from the hash table. Return whether e != nullptr.
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bool LRUCache::FinishErase(LRUHandle* e) {
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if (e != nullptr) {
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assert(e->in_cache);
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LRU_Remove(e);
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e->in_cache = false;
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usage_ -= e->charge;
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Unref(e);
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}
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return e != nullptr;
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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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FinishErase(table_.Remove(key, hash));
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}
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void LRUCache::Prune() {
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MutexLock l(&mutex_);
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while (lru_.next != &lru_) {
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LRUHandle* e = lru_.next;
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assert(e->refs == 1);
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bool erased = FinishErase(table_.Remove(e->key(), e->hash));
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if (!erased) { // to avoid unused variable when compiled NDEBUG
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assert(erased);
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}
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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) { return hash >> (32 - kNumShardBits); }
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public:
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explicit ShardedLRUCache(size_t capacity) : 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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~ShardedLRUCache() override {}
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Handle* Insert(const Slice& key, void* value, size_t charge,
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void (*deleter)(const Slice& key, void* value)) override {
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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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Handle* Lookup(const Slice& key) override {
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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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void Release(Handle* handle) override {
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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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void Erase(const Slice& key) override {
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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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void* Value(Handle* handle) override {
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return reinterpret_cast<LRUHandle*>(handle)->value;
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}
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uint64_t NewId() override {
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MutexLock l(&id_mutex_);
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return ++(last_id_);
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}
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void Prune() override {
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for (int s = 0; s < kNumShards; s++) {
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shard_[s].Prune();
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}
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}
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size_t TotalCharge() const override {
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size_t total = 0;
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for (int s = 0; s < kNumShards; s++) {
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total += shard_[s].TotalCharge();
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}
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return total;
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}
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};
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} // end anonymous namespace
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Cache* NewLRUCache(size_t capacity) { return new ShardedLRUCache(capacity); }
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} // namespace leveldb
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