src/mem/ruby/structures/PseudoLRUPolicy.hh

   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_STRUCTURES_PSEUDOLRUPOLICY_HH__
  30 #define __MEM_RUBY_STRUCTURES_PSEUDOLRUPOLICY_HH__
  31
  32 #include "mem/ruby/structures/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
  47 class PseudoLRUPolicy : public AbstractReplacementPolicy
  48 {
  49   public:
  50     PseudoLRUPolicy(int64 num_sets, int64 assoc);
  51     ~PseudoLRUPolicy();
  52
  53     void touch(int64 set, int64 way, Tick time);
  54     int64 getVictim(int64 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
  63 inline
  64 PseudoLRUPolicy::PseudoLRUPolicy(int64 num_sets, int64 assoc)
  65     : AbstractReplacementPolicy(num_sets, assoc)
  66 {
  67     // associativity cannot exceed capacity of tree representation
  68     assert(num_sets > 0 && assoc > 1 && assoc <= 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
  91 inline
  92 PseudoLRUPolicy::~PseudoLRUPolicy()
  93 {
  94     if (m_trees != NULL)
  95         delete[] m_trees;
  96 }
  97
  98 inline void
  99 PseudoLRUPolicy::touch(int64 set, int64 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
 117 inline int64
 118 PseudoLRUPolicy::getVictim(int64 set) const
 119 {
 120     // assert(m_assoc != 0);
 121     int64 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_STRUCTURES_PSEUDOLRUPOLICY_HH__