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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__
74 #include "mem/cache/replacement_policies/base.hh"
76 struct TreePLRURPParams;
78 class TreePLRURP : public BaseReplacementPolicy
82 * Instead of implementing the tree itself with pointers, it is implemented
83 * as an array of bits. The index of the node defines its position in the
84 * tree, and its parent. Index 0 represents the root, 1 is its left node,
85 * and 2 is its right node. Then, in a BFS fashion this is expanded to the
86 * following nodes (3 and 4 are respectively 1's left and right nodes, and
87 * 5 and 6 are 2's left and right nodes, and so on).
89 * i.e., the following trees are equivalent in this representation:
97 * Notice that the replacement data entries are not represented in the tree
98 * to avoid unnecessary storage costs.
100 typedef std::vector<bool> PLRUTree;
103 * Number of leaves that share a single replacement data.
105 const uint64_t numLeaves;
108 * Count of the number of sharers of a replacement data. It is used when
109 * instantiating entries to share a replacement data among many replaceable
115 * Holds the latest temporary tree instance created by instantiateEntry().
117 PLRUTree* treeInstance;
121 * Tree-PLRU-specific implementation of replacement data. Each replacement
122 * data shares its tree with other entries.
124 struct TreePLRUReplData : ReplacementData
127 * Theoretical index of this replacement data in the tree. In practice,
128 * the corresponding node does not exist, as the tree stores only the
129 * nodes that are not leaves.
131 const uint64_t index;
134 * Shared tree pointer. A tree is shared between numLeaves nodes, so
135 * that accesses to a replacement data entry updates the PLRU bits of
136 * all other replacement data entries in its set.
138 std::shared_ptr<PLRUTree> tree;
141 * Default constructor. Invalidate data.
143 * @param index Index of the corresponding entry in the tree.
144 * @param tree The shared tree pointer.
146 TreePLRUReplData(const uint64_t index, std::shared_ptr<PLRUTree> tree);
150 /** Convenience typedef. */
151 typedef TreePLRURPParams Params;
154 * Construct and initiliaze this replacement policy.
156 TreePLRURP(const Params *p);
164 * Invalidate replacement data to set it as the next probable victim.
165 * Makes tree leaf of replacement data the LRU (tree bits point to it).
167 * @param replacement_data Replacement data to be invalidated.
169 void invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
173 * Touch an entry to update its replacement data.
174 * Makes tree leaf of replacement data the MRU.
176 * @param replacement_data Replacement data to be touched.
178 void touch(const std::shared_ptr<ReplacementData>& replacement_data) const
182 * Reset replacement data. Used when an entry is inserted. Provides the
183 * same functionality as touch().
185 * @param replacement_data Replacement data to be reset.
187 void reset(const std::shared_ptr<ReplacementData>& replacement_data) const
191 * Find replacement victim using TreePLRU bits. It is assumed that all
192 * candidates share the same replacement data tree.
194 * @param candidates Replacement candidates, selected by indexing policy.
195 * @return Replacement entry to be replaced.
197 ReplaceableEntry* getVictim(const ReplacementCandidates& candidates) const
201 * Instantiate a replacement data entry. Consecutive calls to this
202 * function use the same tree up to numLeaves. When numLeaves replacement
203 * data have been created, a new tree is generated, and the counter is
205 * Therefore, it is essential that entries that share the same replacement
206 * data call this function consecutively.
208 * @return A shared pointer to the new replacement data.
210 std::shared_ptr<ReplacementData> instantiateEntry() override;
213 #endif // __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__