138 lines
5.5 KiB
C++
138 lines
5.5 KiB
C++
// Copyright (c) 2013, 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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// 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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//
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// A Cache is an interface that maps keys to values. It has internal
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// synchronization and may be safely accessed concurrently from
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// multiple threads. It may automatically evict entries to make room
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// for new entries. Values have a specified charge against the cache
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// capacity. For example, a cache where the values are variable
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// length strings, may use the length of the string as the charge for
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// the string.
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//
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// A builtin cache implementation with a least-recently-used eviction
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// policy is provided. Clients may use their own implementations if
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// they want something more sophisticated (like scan-resistance, a
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// custom eviction policy, variable cache sizing, etc.)
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#ifndef STORAGE_ROCKSDB_INCLUDE_CACHE_H_
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#define STORAGE_ROCKSDB_INCLUDE_CACHE_H_
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#include <memory>
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#include <stdint.h>
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#include "rocksdb/slice.h"
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namespace rocksdb {
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using std::shared_ptr;
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class Cache;
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// Create a new cache with a fixed size capacity. The cache is sharded
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// to 2^numShardBits shards, by hash of the key. The total capacity
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// is divided and evenly assigned to each shard. Inside each shard,
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// the eviction is done in two passes: first try to free spaces by
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// evicting entries that are among the most least used removeScanCountLimit
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// entries and do not have reference other than by the cache itself, in
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// the least-used order. If not enough space is freed, further free the
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// entries in least used order.
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//
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// The functions without parameter numShardBits and/or removeScanCountLimit
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// use default values. removeScanCountLimit's default value is 0, which
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// means a strict LRU order inside each shard.
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extern shared_ptr<Cache> NewLRUCache(size_t capacity);
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extern shared_ptr<Cache> NewLRUCache(size_t capacity, int numShardBits);
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extern shared_ptr<Cache> NewLRUCache(size_t capacity, int numShardBits,
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int removeScanCountLimit);
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class Cache {
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public:
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Cache() { }
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// Destroys all existing entries by calling the "deleter"
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// function that was passed to the constructor.
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virtual ~Cache();
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// Opaque handle to an entry stored in the cache.
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struct Handle { };
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// Insert a mapping from key->value into the cache and assign it
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// the specified charge against the total cache capacity.
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//
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// Returns a handle that corresponds to the mapping. The caller
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// must call this->Release(handle) when the returned mapping is no
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// longer needed.
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//
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// When the inserted entry is no longer needed, the key and
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// value will be passed to "deleter".
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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)) = 0;
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// If the cache has no mapping for "key", returns nullptr.
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//
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// Else return a handle that corresponds to the mapping. The caller
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// must call this->Release(handle) when the returned mapping is no
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// longer needed.
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virtual Handle* Lookup(const Slice& key) = 0;
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// Release a mapping returned by a previous Lookup().
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// REQUIRES: handle must not have been released yet.
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// REQUIRES: handle must have been returned by a method on *this.
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virtual void Release(Handle* handle) = 0;
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// Return the value encapsulated in a handle returned by a
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// successful Lookup().
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// REQUIRES: handle must not have been released yet.
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// REQUIRES: handle must have been returned by a method on *this.
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virtual void* Value(Handle* handle) = 0;
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// If the cache contains entry for key, erase it. Note that the
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// underlying entry will be kept around until all existing handles
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// to it have been released.
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virtual void Erase(const Slice& key) = 0;
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// Return a new numeric id. May be used by multiple clients who are
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// sharing the same cache to partition the key space. Typically the
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// client will allocate a new id at startup and prepend the id to
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// its cache keys.
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virtual uint64_t NewId() = 0;
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// returns the maximum configured capacity of the cache
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virtual size_t GetCapacity() const = 0;
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// returns the memory size for the entries residing in the cache.
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virtual size_t GetUsage() const = 0;
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// Call this on shutdown if you want to speed it up. Cache will disown
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// any underlying data and will not free it on delete. This call will leak
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// memory - call this only if you're shutting down the process.
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// Any attempts of using cache after this call will fail terribly.
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// Always delete the DB object before calling this method!
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virtual void DisownData() {
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// default implementation is noop
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};
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// Apply callback to all entries in the cache
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// If thread_safe is true, it will also lock the accesses. Otherwise, it will
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// access the cache without the lock held
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virtual void ApplyToAllCacheEntries(void (*callback)(void*, size_t),
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bool thread_safe) = 0;
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private:
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void LRU_Remove(Handle* e);
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void LRU_Append(Handle* e);
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void Unref(Handle* e);
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// No copying allowed
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Cache(const Cache&);
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void operator=(const Cache&);
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};
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} // namespace rocksdb
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#endif // STORAGE_ROCKSDB_UTIL_CACHE_H_
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