72 lines
2.7 KiB
C
72 lines
2.7 KiB
C
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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//
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// 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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#pragma once
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#include <cassert>
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#include <cstdint>
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#include "port/port.h"
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#include "rocksdb/slice.h"
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namespace rocksdb {
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// An entry is a variable length heap-allocated structure.
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// Entries are referenced by cache and/or by any external entity.
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// The cache keeps all its entries in table. Some elements
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// are also stored on LRU list.
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//
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// LRUHandle can be in these states:
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// 1. Referenced externally AND in hash table.
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// In that case the entry is *not* in the LRU. (refs > 1 && in_cache == true)
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// 2. Not referenced externally and in hash table. In that case the entry is
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// in the LRU and can be freed. (refs == 1 && in_cache == true)
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// 3. Referenced externally and not in hash table. In that case the entry is
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// in not on LRU and not in table. (refs >= 1 && in_cache == false)
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//
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// All newly created LRUHandles are in state 1. If you call LRUCache::Release
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// on entry in state 1, it will go into state 2. To move from state 1 to
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// state 3, either call LRUCache::Erase or LRUCache::Insert with the same key.
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// To move from state 2 to state 1, use LRUCache::Lookup.
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// Before destruction, make sure that no handles are in state 1. This means
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// that any successful LRUCache::Lookup/LRUCache::Insert have a matching
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// RUCache::Release (to move into state 2) or LRUCache::Erase (for state 3)
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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; // a number of refs to this entry
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// cache itself is counted as 1
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bool in_cache; // true, if this entry is referenced by the hash table
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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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void Free() {
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assert((refs == 1 && in_cache) || (refs == 0 && !in_cache));
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(*deleter)(key(), value);
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delete[] reinterpret_cast<char*>(this);
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}
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};
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} // end namespace rocksdb
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