PseudoLRUPolicy.hh revision 9505 
1/*
2 * Copyright (c) 2007 Mark D. Hill and David A. Wood
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
29#ifndef __MEM_RUBY_SYSTEM_PSEUDOLRUPOLICY_HH__
30#define __MEM_RUBY_SYSTEM_PSEUDOLRUPOLICY_HH__
31
32#include "mem/ruby/system/AbstractReplacementPolicy.hh"
33
34/**
35 * Implementation of tree-based pseudo-LRU replacement
36 *
37 * Works for any associativity between 1 and 128.
38 *
39 * Also implements associativities that are not a power of 2 by
40 * ignoring paths that lead to a larger index (i.e. truncating the
41 * tree).  Note that when this occurs, the algorithm becomes less
42 * fair, as it will favor indicies in the larger (by index) half of
43 * the associative set. This is most unfair when the nearest power of
44 * 2 is one below the associativy, and most fair when it is one above.
45 */
46
47class PseudoLRUPolicy : public AbstractReplacementPolicy
48{
49  public:
50    PseudoLRUPolicy(Index num_sets, Index assoc);
51    ~PseudoLRUPolicy();
52
53    void touch(Index set, Index way, Tick time);
54    Index getVictim(Index set) const;
55
56  private:
57    unsigned int m_effective_assoc;    /** nearest (to ceiling) power of 2 */
58    unsigned int m_num_levels;         /** number of levels in the tree */
59    uint64* m_trees;                   /** bit representation of the
60                                        * trees, one for each set */
61};
62
63inline
64PseudoLRUPolicy::PseudoLRUPolicy(Index num_sets, Index assoc)
65    : AbstractReplacementPolicy(num_sets, assoc)
66{
67    // associativity cannot exceed capacity of tree representation
68    assert(num_sets > 0 && assoc > 1 && assoc <= (Index) sizeof(uint64)*4);
69
70    m_trees = NULL;
71    m_num_levels = 0;
72
73    m_effective_assoc = 1;
74    while (m_effective_assoc < assoc) {
75        // effective associativity is ceiling power of 2
76        m_effective_assoc <<= 1;
77    }
78    assoc = m_effective_assoc;
79    while (true) {
80        assoc /= 2;
81        if(!assoc) break;
82        m_num_levels++;
83    }
84    assert(m_num_levels < sizeof(unsigned int)*4);
85    m_trees = new uint64[m_num_sets];
86    for (unsigned i = 0; i < m_num_sets; i++) {
87        m_trees[i] = 0;
88    }
89}
90
91inline
92PseudoLRUPolicy::~PseudoLRUPolicy()
93{
94    if (m_trees != NULL)
95        delete[] m_trees;
96}
97
98inline void
99PseudoLRUPolicy::touch(Index set, Index index, Tick time)
100{
101    assert(index >= 0 && index < m_assoc);
102    assert(set >= 0 && set < m_num_sets);
103
104    int tree_index = 0;
105    int node_val;
106    for (int i = m_num_levels - 1; i >= 0; i--) {
107        node_val = (index >> i)&1;
108        if (node_val)
109            m_trees[set] |= node_val << tree_index;
110        else
111            m_trees[set] &= ~(1 << tree_index);
112        tree_index = node_val ? (tree_index*2)+2 : (tree_index*2)+1;
113    }
114    m_last_ref_ptr[set][index] = time;
115}
116
117inline Index
118PseudoLRUPolicy::getVictim(Index set) const
119{
120    // assert(m_assoc != 0);
121    Index index = 0;
122
123    int tree_index = 0;
124    int node_val;
125    for (unsigned i = 0; i < m_num_levels; i++){
126        node_val = (m_trees[set] >> tree_index) & 1;
127        index += node_val ? 0 : (m_effective_assoc >> (i + 1));
128        tree_index = node_val ? (tree_index * 2) + 1 : (tree_index * 2) + 2;
129    }
130    assert(index >= 0 && index < m_effective_assoc);
131
132    /* return either the found index or the max possible index */
133    /* NOTE: this is not a fair replacement when assoc is not a power of 2 */
134    return (index > (m_assoc - 1)) ? m_assoc - 1 : index;
135}
136
137#endif // __MEM_RUBY_SYSTEM_PSEUDOLRUPOLICY_HH__
138