tree_plru_rp.hh revision 13221:48bce2835200 
1/**
2 * Copyright (c) 2018 Inria
3 * All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions are
7 * met: redistributions of source code must retain the above copyright
8 * notice, this list of conditions and the following disclaimer;
9 * redistributions in binary form must reproduce the above copyright
10 * notice, this list of conditions and the following disclaimer in the
11 * documentation and/or other materials provided with the distribution;
12 * neither the name of the copyright holders nor the names of its
13 * contributors may be used to endorse or promote products derived from
14 * this software without specific prior written permission.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
17 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
18 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
19 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
20 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
26 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
27 *
28 * Authors: Daniel Carvalho
29 */
30
31/**
32 * @file
33 * Declaration of a Pseudo-Least Recently Used replacement policy.
34 * The victim is chosen using a tree of bit timestamps.
35 *
36 * A tree contains consists of leafs that represent the direction to take when
37 * searching for the LRU entry.
38 *
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:
41 *    ____1____
42 *  __0__   __1__
43 * _0_ _1_ _1_ _0_
44 * A B C D E F G H
45 *
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
48 *
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:
52 *    ____1____
53 *  __1__   __1__
54 * _0_ _1_ _1_ _0_
55 * A B C D E F G H
56 *
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).
62 *
63 * For invalidations the process is similar to touches, but instead of pointing
64 * away, the bits point toward the entry.
65 *
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.
69 */
70
71#ifndef __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
72#define __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
73
74#include "mem/cache/replacement_policies/base.hh"
75
76struct TreePLRURPParams;
77
78class TreePLRURP : public BaseReplacementPolicy
79{
80  private:
81    /**
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).
88     *
89     * i.e., the following trees are equivalent in this representation:
90     *    ____1____
91     *  __0__   __1__
92     * _0_ _1_ _1_ _0_
93     * A B C D E F G H
94     *
95     * 1 0 1 0 1 1 0
96     *
97     * Notice that the replacement data entries are not represented in the tree
98     * to avoid unnecessary storage costs.
99     */
100    typedef std::vector<bool> PLRUTree;
101
102    /**
103     * Number of leaves that share a single replacement data.
104     */
105    const uint64_t numLeaves;
106
107    /**
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
110     * entries.
111     */
112    uint64_t count;
113
114    /**
115     * Holds the latest temporary tree instance created by instantiateEntry().
116     */
117    PLRUTree* treeInstance;
118
119  protected:
120    /**
121     * Tree-PLRU-specific implementation of replacement data. Each replacement
122     * data shares its tree with other entries.
123     */
124    struct TreePLRUReplData : ReplacementData
125    {
126        /**
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.
130         */
131        const uint64_t index;
132
133        /**
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.
137         */
138        std::shared_ptr<PLRUTree> tree;
139
140        /**
141         * Default constructor. Invalidate data.
142         *
143         * @param index Index of the corresponding entry in the tree.
144         * @param tree The shared tree pointer.
145         */
146        TreePLRUReplData(const uint64_t index, std::shared_ptr<PLRUTree> tree);
147    };
148
149  public:
150    /** Convenience typedef. */
151    typedef TreePLRURPParams Params;
152
153    /**
154     * Construct and initiliaze this replacement policy.
155     */
156    TreePLRURP(const Params *p);
157
158    /**
159     * Destructor.
160     */
161    ~TreePLRURP() {}
162
163    /**
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).
166     *
167     * @param replacement_data Replacement data to be invalidated.
168     */
169    void invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
170                                                              const override;
171
172    /**
173     * Touch an entry to update its replacement data.
174     * Makes tree leaf of replacement data the MRU.
175     *
176     * @param replacement_data Replacement data to be touched.
177     */
178    void touch(const std::shared_ptr<ReplacementData>& replacement_data) const
179                                                                     override;
180
181    /**
182     * Reset replacement data. Used when an entry is inserted. Provides the
183     * same functionality as touch().
184     *
185     * @param replacement_data Replacement data to be reset.
186     */
187    void reset(const std::shared_ptr<ReplacementData>& replacement_data) const
188                                                                     override;
189
190    /**
191     * Find replacement victim using TreePLRU bits. It is assumed that all
192     * candidates share the same replacement data tree.
193     *
194     * @param candidates Replacement candidates, selected by indexing policy.
195     * @return Replacement entry to be replaced.
196     */
197    ReplaceableEntry* getVictim(const ReplacementCandidates& candidates) const
198                                                                     override;
199
200    /**
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
204     * reset.
205     * Therefore, it is essential that entries that share the same replacement
206     * data call this function consecutively.
207     *
208     * @return A shared pointer to the new replacement data.
209     */
210    std::shared_ptr<ReplacementData> instantiateEntry() override;
211};
212
213#endif // __MEM_CACHE_REPLACEMENT_POLICIES_TREE_PLRU_RP_HH__
214