arangodb/arangod/Cache/PlainBucket.h

////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2017 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// Licensed under the Apache License, Version 2.0 (the "License");
/// you may not use this file except in compliance with the License.
/// You may obtain a copy of the License at
///
///     http://www.apache.org/licenses/LICENSE-2.0
///
/// Unless required by applicable law or agreed to in writing, software
/// distributed under the License is distributed on an "AS IS" BASIS,
/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
/// See the License for the specific language governing permissions and
/// limitations under the License.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Daniel H. Larkin
////////////////////////////////////////////////////////////////////////////////

#ifndef ARANGODB_CACHE_PLAIN_BUCKET_H
#define ARANGODB_CACHE_PLAIN_BUCKET_H

#include "Basics/Common.h"
#include "Cache/CachedValue.h"
#include "Cache/Common.h"
#include "Cache/State.h"

#include <stdint.h>
#include <atomic>

namespace arangodb {
namespace cache {

////////////////////////////////////////////////////////////////////////////////
/// @brief Bucket structure for PlainCache.
///
/// Contains only a State variable and five slots each for hashes and data
/// pointers. Most querying and manipulation can be handled via the exposed
/// methods. Bucket must be locked before doing anything else to ensure proper
/// synchronization. Data entries are carefully laid out to ensure the structure
/// fits in a single cacheline.
////////////////////////////////////////////////////////////////////////////////
struct PlainBucket {
  State _state;

  uint32_t _paddingExplicit; // fill 4-byte gap for alignment purposes

  // actual cached entries
  static constexpr size_t slotsData = 10;
  uint32_t _cachedHashes[slotsData];
  CachedValue* _cachedData[slotsData];

// padding, if necessary?
#ifdef TRI_PADDING_32
  uint32_t _padding[slotsData];
#endif

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Initialize an empty bucket.
  //////////////////////////////////////////////////////////////////////////////
  PlainBucket();

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Attempt to lock bucket (failing after maxTries attempts).
  //////////////////////////////////////////////////////////////////////////////
  bool lock(int64_t maxTries);

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Unlock the bucket. Requires bucket to be locked.
  //////////////////////////////////////////////////////////////////////////////
  void unlock();

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Checks whether the bucket is locked.
  //////////////////////////////////////////////////////////////////////////////
  bool isLocked() const;

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Checks whether the bucket has been migrated. Requires state to be
  /// locked.
  //////////////////////////////////////////////////////////////////////////////
  bool isMigrated() const;

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Checks whether bucket is full. Requires state to be locked.
  //////////////////////////////////////////////////////////////////////////////
  bool isFull() const;

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Looks up a given key and returns associated value. Requires state
  /// to be locked.
  ///
  /// Takes an input hash and key (specified by pointer and size), and searches
  /// the bucket for a matching entry. If a matching entry is found, it is
  /// returned. By default, a matching entry will be moved to the front of the
  /// bucket to allow basic LRU semantics. If no matching entry is found,
  /// nothing will be changed and a nullptr will be returned.
  //////////////////////////////////////////////////////////////////////////////
  CachedValue* find(uint32_t hash, void const* key, uint32_t keySize,
                    bool moveToFront = true);

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Inserts a given value. Requires state to be locked.
  ///
  /// Requires that the bucket is not full and does not already contain an item
  /// with the same key. If it is full, the item will not be inserted. If an
  /// item with the same key exists, this is not detected but it is likely to
  /// produce bugs later on down the line. When inserting, the item is put into
  /// the first empty slot, then moved to the front. If attempting to insert and
  /// the bucket is full, the user should evict an item and specify the
  /// optimizeForInsertion flag to be true.
  //////////////////////////////////////////////////////////////////////////////
  void insert(uint32_t hash, CachedValue* value);

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Removes an item with the given key if one exists. Requires state to
  /// be locked.
  ///
  /// Search for a matching key. If none exists, do nothing and return a
  /// nullptr. If one exists, remove it from the bucket and return the pointer
  /// to the value. Upon removal, the empty slot generated is moved to the back
  /// of the bucket (to remove the gap).
  //////////////////////////////////////////////////////////////////////////////
  CachedValue* remove(uint32_t hash, void const* key, uint32_t keySize);

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Searches for the best candidate in the bucket to evict. Requires
  /// state to be locked.
  ///
  /// Usually returns a pointer to least recently used freeable value. If the
  /// bucket contains no values or all have outstanding references, then it
  /// returns nullptr. In the case that ignoreRefCount is set to true, then it
  /// simply returns the least recently used value, regardless of freeability.
  //////////////////////////////////////////////////////////////////////////////
  CachedValue* evictionCandidate(bool ignoreRefCount = false) const;

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Evicts the given value from the bucket. Requires state to be
  /// locked.
  ///
  /// By default, it will move the empty slot to the back of the bucket. If
  /// preparing an empty slot for insertion, specify the second parameter to be
  /// true. This will move the empty slot to the front instead.
  //////////////////////////////////////////////////////////////////////////////
  void evict(CachedValue* value, bool optimizeForInsertion = false);

  //////////////////////////////////////////////////////////////////////////////
  /// @brief Reinitializes a bucket to be completely empty and unlocked.
  /// Requires state to be locked.
  //////////////////////////////////////////////////////////////////////////////
  void clear();

 private:
  void moveSlot(size_t slot, bool moveToFront);
};

// ensure that PlainBucket is exactly BUCKET_SIZE
static_assert(sizeof(PlainBucket) == BUCKET_SIZE,
              "Expected sizeof(PlainBucket) == BUCKET_SIZE.");

};  // end namespace cache
};  // end namespace arangodb

#endif