PseudoLRUPolicy.hh revision 6145 
1
2#ifndef PSEUDOLRUPOLICY_H
3#define PSEUDOLRUPOLICY_H
4
5#include "AbstractReplacementPolicy.hh"
6
7/**
8 * Implementation of tree-based pseudo-LRU replacement
9 *
10 * Works for any associativity between 1 and 128.
11 *
12 * Also implements associativities that are not a power of 2 by
13 * ignoring paths that lead to a larger index (i.e. truncating the
14 * tree).  Note that when this occurs, the algorithm becomes less
15 * fair, as it will favor indicies in the larger (by index) half of
16 * the associative set. This is most unfair when the nearest power of
17 * 2 is one below the associativy, and most fair when it is one above.
18 */
19
20class PseudoLRUPolicy : public AbstractReplacementPolicy {
21 public:
22
23  PseudoLRUPolicy(Index num_sets, Index assoc);
24  ~PseudoLRUPolicy();
25
26  void touch(Index set, Index way, Time time);
27  Index getVictim(Index set) const;
28
29 private:
30  unsigned int m_effective_assoc;    /** nearest (to ceiling) power of 2 */
31  unsigned int m_num_levels;         /** number of levels in the tree */
32  uint64* m_trees;                   /** bit representation of the trees, one for each set */
33};
34
35inline
36PseudoLRUPolicy::PseudoLRUPolicy(Index num_sets, Index assoc)
37  : AbstractReplacementPolicy(num_sets, assoc)
38{
39  int num_tree_nodes;
40
41  // associativity cannot exceed capacity of tree representation
42  assert(num_sets > 0 && assoc > 1 && assoc <= (Index) sizeof(uint64)*4);
43
44  m_trees = NULL;
45  m_num_levels = 0;
46
47  m_effective_assoc = 1;
48  while(m_effective_assoc < assoc){
49    m_effective_assoc <<= 1;  // effective associativity is ceiling power of 2
50  }
51  assoc = m_effective_assoc;
52  while(true){
53    assoc /= 2;
54    if(!assoc) break;
55    m_num_levels++;
56  }
57  assert(m_num_levels < sizeof(unsigned int)*4);
58  num_tree_nodes = ((int)pow(2, m_num_levels))-1;
59  m_trees = new uint64[m_num_sets];
60  for(unsigned int i=0; i< m_num_sets; i++){
61    m_trees[i] = 0;
62  }
63}
64
65inline
66PseudoLRUPolicy::~PseudoLRUPolicy()
67{
68  if(m_trees != NULL)
69    delete[] m_trees;
70}
71
72inline
73void PseudoLRUPolicy::touch(Index set, Index index, Time time){
74  assert(index >= 0 && index < m_assoc);
75  assert(set >= 0 && set < m_num_sets);
76
77  int tree_index = 0;
78  int node_val;
79  for(int i=m_num_levels -1; i>=0; i--){
80    node_val = (index >> i)&1;
81    if(node_val)
82      m_trees[set] |= node_val << tree_index;
83    else
84      m_trees[set] &= ~(1 << tree_index);
85    tree_index = node_val ? (tree_index*2)+2 : (tree_index*2)+1;
86  }
87  m_last_ref_ptr[set][index] = time;
88}
89
90inline
91Index PseudoLRUPolicy::getVictim(Index set) const {
92  //  assert(m_assoc != 0);
93
94  Index index = 0;
95
96  int tree_index = 0;
97  int node_val;
98  for(unsigned int i=0;i<m_num_levels;i++){
99    node_val = (m_trees[set]>>tree_index)&1;
100    index += node_val?0:(m_effective_assoc >> (i+1));
101    tree_index = node_val? (tree_index*2)+1 : (tree_index*2)+2;
102  }
103  assert(index >= 0 && index < m_effective_assoc);
104
105  /* return either the found index or the max possible index */
106  /* NOTE: this is not a fair replacement when assoc is not a power of 2 */
107  return (index > (m_assoc-1)) ? m_assoc-1:index;
108}
109
110#endif // PSEUDOLRUPOLICY_H
111