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28 * Authors: Daniel Carvalho
33 * Declaration of a Pseudo-Least Recently Used replacement policy.
34 * The victim is chosen using a tree of bit timestamps.
36 * A tree contains consists of leafs that represent the direction to take when
37 * searching for the LRU entry.
39 * Let's assume each tree contains 8 replacement data entries. For example, if
40 * these entries are named from A to H, and the tree's bits are:
46 * Then the current PLRU entry is given by the sequence:
47 * 1 (get right child) -> 1 (get right child) -> 0 (get left child) -> G
49 * When an entry is touched the bits of the parent nodes are iteratively
50 * updated to point away from it. Therefore, if entry B is touched, its parent,
51 * grandparents, etc would be updated, and we'd end up with the following tree:
57 * Explanation: The parent of B must point away from it, that is, to the left
58 * child, but it is already doing so, so it is left unmodified (0). Then the
59 * grandparent must point to the right subtree, as B belongs to its left
60 * subtree (0 becomes 1). Lastly, the root must point away from the
61 * grandparent, so it is left unmodified (0).
63 * For invalidations the process is similar to touches, but instead of pointing
64 * away, the bits point toward the entry.
66 * Consecutive calls to instantiateEntry() use the same tree up to numLeaves.
67 * When numLeaves replacement datas have been created, a new tree is generated,
68 * and the counter is reset.
71 #ifndef __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
72 #define __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
78 #include "mem/cache/replacement_policies/base.hh"
80 struct TreePLRURPParams;
82 class TreePLRURP : public BaseReplacementPolicy
86 * Instead of implementing the tree itself with pointers, it is implemented
87 * as an array of bits. The index of the node defines its position in the
88 * tree, and its parent. Index 0 represents the root, 1 is its left node,
89 * and 2 is its right node. Then, in a BFS fashion this is expanded to the
90 * following nodes (3 and 4 are respectively 1's left and right nodes, and
91 * 5 and 6 are 2's left and right nodes, and so on).
93 * i.e., the following trees are equivalent in this representation:
101 * Notice that the replacement data entries are not represented in the tree
102 * to avoid unnecessary storage costs.
104 typedef std::vector<bool> PLRUTree;
107 * Number of leaves that share a single replacement data.
109 const uint64_t numLeaves;
112 * Count of the number of sharers of a replacement data. It is used when
113 * instantiating entries to share a replacement data among many replaceable
119 * Holds the latest temporary tree instance created by instantiateEntry().
121 PLRUTree* treeInstance;
125 * Tree-PLRU-specific implementation of replacement data. Each replacement
126 * data shares its tree with other entries.
128 struct TreePLRUReplData : ReplacementData
131 * Theoretical index of this replacement data in the tree. In practice,
132 * the corresponding node does not exist, as the tree stores only the
133 * nodes that are not leaves.
135 const uint64_t index;
138 * Shared tree pointer. A tree is shared between numLeaves nodes, so
139 * that accesses to a replacement data entry updates the PLRU bits of
140 * all other replacement data entries in its set.
142 std::shared_ptr<PLRUTree> tree;
145 * Default constructor. Invalidate data.
147 * @param index Index of the corresponding entry in the tree.
148 * @param tree The shared tree pointer.
150 TreePLRUReplData(const uint64_t index, std::shared_ptr<PLRUTree> tree);
154 /** Convenience typedef. */
155 typedef TreePLRURPParams Params;
158 * Construct and initiliaze this replacement policy.
160 TreePLRURP(const Params *p);
168 * Invalidate replacement data to set it as the next probable victim.
169 * Makes tree leaf of replacement data the LRU (tree bits point to it).
171 * @param replacement_data Replacement data to be invalidated.
173 void invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
177 * Touch an entry to update its replacement data.
178 * Makes tree leaf of replacement data the MRU.
180 * @param replacement_data Replacement data to be touched.
182 void touch(const std::shared_ptr<ReplacementData>& replacement_data) const
186 * Reset replacement data. Used when an entry is inserted. Provides the
187 * same functionality as touch().
189 * @param replacement_data Replacement data to be reset.
191 void reset(const std::shared_ptr<ReplacementData>& replacement_data) const
195 * Find replacement victim using TreePLRU bits. It is assumed that all
196 * candidates share the same replacement data tree.
198 * @param candidates Replacement candidates, selected by indexing policy.
199 * @return Replacement entry to be replaced.
201 ReplaceableEntry* getVictim(const ReplacementCandidates& candidates) const
205 * Instantiate a replacement data entry. Consecutive calls to this
206 * function use the same tree up to numLeaves. When numLeaves replacement
207 * data have been created, a new tree is generated, and the counter is
209 * Therefore, it is essential that entries that share the same replacement
210 * data call this function consecutively.
212 * @return A shared pointer to the new replacement data.
214 std::shared_ptr<ReplacementData> instantiateEntry() override;
217 #endif // __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__