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/**
* Copyright (c) 2018 Inria
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met: redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer;
* redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution;
* neither the name of the copyright holders nor the names of its
* contributors may be used to endorse or promote products derived from
* this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*
* Authors: Daniel Carvalho
*/
/**
* @file
* Declaration of a Pseudo-Least Recently Used replacement policy.
* The victim is chosen using a tree of bit timestamps.
*
* A tree contains consists of leafs that represent the direction to take when
* searching for the LRU entry.
*
* Let's assume each tree contains 8 replacement data entries. For example, if
* these entries are named from A to H, and the tree's bits are:
* ____1____
* __0__ __1__
* _0_ _1_ _1_ _0_
* A B C D E F G H
*
* Then the current PLRU entry is given by the sequence:
* 1 (get right child) -> 1 (get right child) -> 0 (get left child) -> G
*
* When an entry is touched the bits of the parent nodes are iteratively
* updated to point away from it. Therefore, if entry B is touched, its parent,
* grandparents, etc would be updated, and we'd end up with the following tree:
* ____1____
* __1__ __1__
* _0_ _1_ _1_ _0_
* A B C D E F G H
*
* Explanation: The parent of B must point away from it, that is, to the left
* child, but it is already doing so, so it is left unmodified (0). Then the
* grandparent must point to the right subtree, as B belongs to its left
* subtree (0 becomes 1). Lastly, the root must point away from the
* grandparent, so it is left unmodified (0).
*
* For invalidations the process is similar to touches, but instead of pointing
* away, the bits point toward the entry.
*
* Consecutive calls to instantiateEntry() use the same tree up to numLeaves.
* When numLeaves replacement datas have been created, a new tree is generated,
* and the counter is reset.
*/
#ifndef __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
#define __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
#include <cstdint>
#include <memory>
#include <vector>
#include "mem/cache/replacement_policies/base.hh"
struct TreePLRURPParams;
class TreePLRURP : public BaseReplacementPolicy
{
private:
/**
* Instead of implementing the tree itself with pointers, it is implemented
* as an array of bits. The index of the node defines its position in the
* tree, and its parent. Index 0 represents the root, 1 is its left node,
* and 2 is its right node. Then, in a BFS fashion this is expanded to the
* following nodes (3 and 4 are respectively 1's left and right nodes, and
* 5 and 6 are 2's left and right nodes, and so on).
*
* i.e., the following trees are equivalent in this representation:
* ____1____
* __0__ __1__
* _0_ _1_ _1_ _0_
* A B C D E F G H
*
* 1 0 1 0 1 1 0
*
* Notice that the replacement data entries are not represented in the tree
* to avoid unnecessary storage costs.
*/
typedef std::vector<bool> PLRUTree;
/**
* Number of leaves that share a single replacement data.
*/
const uint64_t numLeaves;
/**
* Count of the number of sharers of a replacement data. It is used when
* instantiating entries to share a replacement data among many replaceable
* entries.
*/
uint64_t count;
/**
* Holds the latest temporary tree instance created by instantiateEntry().
*/
PLRUTree* treeInstance;
protected:
/**
* Tree-PLRU-specific implementation of replacement data. Each replacement
* data shares its tree with other entries.
*/
struct TreePLRUReplData : ReplacementData
{
/**
* Theoretical index of this replacement data in the tree. In practice,
* the corresponding node does not exist, as the tree stores only the
* nodes that are not leaves.
*/
const uint64_t index;
/**
* Shared tree pointer. A tree is shared between numLeaves nodes, so
* that accesses to a replacement data entry updates the PLRU bits of
* all other replacement data entries in its set.
*/
std::shared_ptr<PLRUTree> tree;
/**
* Default constructor. Invalidate data.
*
* @param index Index of the corresponding entry in the tree.
* @param tree The shared tree pointer.
*/
TreePLRUReplData(const uint64_t index, std::shared_ptr<PLRUTree> tree);
};
public:
/** Convenience typedef. */
typedef TreePLRURPParams Params;
/**
* Construct and initiliaze this replacement policy.
*/
TreePLRURP(const Params *p);
/**
* Destructor.
*/
~TreePLRURP() {}
/**
* Invalidate replacement data to set it as the next probable victim.
* Makes tree leaf of replacement data the LRU (tree bits point to it).
*
* @param replacement_data Replacement data to be invalidated.
*/
void invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
const override;
/**
* Touch an entry to update its replacement data.
* Makes tree leaf of replacement data the MRU.
*
* @param replacement_data Replacement data to be touched.
*/
void touch(const std::shared_ptr<ReplacementData>& replacement_data) const
override;
/**
* Reset replacement data. Used when an entry is inserted. Provides the
* same functionality as touch().
*
* @param replacement_data Replacement data to be reset.
*/
void reset(const std::shared_ptr<ReplacementData>& replacement_data) const
override;
/**
* Find replacement victim using TreePLRU bits. It is assumed that all
* candidates share the same replacement data tree.
*
* @param candidates Replacement candidates, selected by indexing policy.
* @return Replacement entry to be replaced.
*/
ReplaceableEntry* getVictim(const ReplacementCandidates& candidates) const
override;
/**
* Instantiate a replacement data entry. Consecutive calls to this
* function use the same tree up to numLeaves. When numLeaves replacement
* data have been created, a new tree is generated, and the counter is
* reset.
* Therefore, it is essential that entries that share the same replacement
* data call this function consecutively.
*
* @return A shared pointer to the new replacement data.
*/
std::shared_ptr<ReplacementData> instantiateEntry() override;
};
#endif // __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
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