src/mem/cache/replacement_policies/brrip_rp.hh

   1 /**
   2  * Copyright (c) 2018-2020 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
  29 /**
  30  * @file
  31  * Declaration of a Re-Reference Interval Prediction replacement policy.
  32  *
  33  * Not-Recently Used (NRU) is an approximation of LRU that uses a single bit
  34  * to determine if an entry is going to be re-referenced in the near or distant
  35  * future.
  36  *
  37  * Re-Reference Interval Prediction (RRIP) is an extension of NRU that uses a
  38  * re-reference prediction value to determine if entries are going to be re-
  39  * used in the near future or not.
  40  *
  41  * The higher the value of the RRPV, the more distant the entry is from its
  42  * next access.
  43  *
  44  * Bimodal Re-Reference Interval Prediction (BRRIP) is an extension of RRIP
  45  * that has a probability of not inserting entries as the LRU. This probability
  46  * is controlled by the bimodal throtle parameter (btp).
  47  *
  48  * From the original paper, this implementation of RRIP is also called
  49  * Static RRIP (SRRIP), as it always inserts entries with the same RRPV.
  50  */
  51
  52 #ifndef __MEM_CACHE_REPLACEMENT_POLICIES_BRRIP_RP_HH__
  53 #define __MEM_CACHE_REPLACEMENT_POLICIES_BRRIP_RP_HH__
  54
  55 #include "base/sat_counter.hh"
  56 #include "mem/cache/replacement_policies/base.hh"
  57
  58 struct BRRIPRPParams;
  59
  60 namespace ReplacementPolicy {
  61
  62 class BRRIP : public Base
  63 {
  64   protected:
  65     /** BRRIP-specific implementation of replacement data. */
  66     struct BRRIPReplData : ReplacementData
  67     {
  68         /**
  69          * Re-Reference Interval Prediction Value.
  70          * Some values have specific names (according to the paper):
  71          * 0 -> near-immediate re-rereference interval
  72          * max_RRPV-1 -> long re-rereference interval
  73          * max_RRPV -> distant re-rereference interval
  74          */
  75         SatCounter rrpv;
  76
  77         /** Whether the entry is valid. */
  78         bool valid;
  79
  80         /**
  81          * Default constructor. Invalidate data.
  82          */
  83         BRRIPReplData(const int num_bits)
  84             : rrpv(num_bits), valid(false)
  85         {
  86         }
  87     };
  88
  89     /**
  90      * Number of RRPV bits. An entry that saturates its RRPV has the longest
  91      * possible re-reference interval, that is, it is likely not to be used
  92      * in the near future, and is among the best eviction candidates.
  93      * A maximum RRPV of 1 implies in a NRU.
  94      */
  95     const unsigned numRRPVBits;
  96
  97     /**
  98      * The hit priority (HP) policy replaces entries that do not receive cache
  99      * hits over any cache entry that receives a hit, while the frequency
 100      * priority (FP) policy replaces infrequently re-referenced entries.
 101      */
 102     const bool hitPriority;
 103
 104     /**
 105      * Bimodal throtle parameter. Value in the range [0,100] used to decide
 106      * if a new entry is inserted with long or distant re-reference.
 107      */
 108     const unsigned btp;
 109
 110   public:
 111     typedef BRRIPRPParams Params;
 112     BRRIP(const Params &p);
 113     ~BRRIP() = default;
 114
 115     /**
 116      * Invalidate replacement data to set it as the next probable victim.
 117      * Set RRPV as the the most distant re-reference.
 118      *
 119      * @param replacement_data Replacement data to be invalidated.
 120      */
 121     void invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
 122                                                               const override;
 123
 124     /**
 125      * Touch an entry to update its replacement data.
 126      *
 127      * @param replacement_data Replacement data to be touched.
 128      */
 129     void touch(const std::shared_ptr<ReplacementData>& replacement_data) const
 130                                                                      override;
 131
 132     /**
 133      * Reset replacement data. Used when an entry is inserted.
 134      * Set RRPV according to the insertion policy used.
 135      *
 136      * @param replacement_data Replacement data to be reset.
 137      */
 138     void reset(const std::shared_ptr<ReplacementData>& replacement_data) const
 139                                                                      override;
 140
 141     /**
 142      * Find replacement victim using rrpv.
 143      *
 144      * @param cands Replacement candidates, selected by indexing policy.
 145      * @return Replacement entry to be replaced.
 146      */
 147     ReplaceableEntry* getVictim(const ReplacementCandidates& candidates) const
 148                                                                      override;
 149
 150     /**
 151      * Instantiate a replacement data entry.
 152      *
 153      * @return A shared pointer to the new replacement data.
 154      */
 155     std::shared_ptr<ReplacementData> instantiateEntry() override;
 156 };
 157
 158 } // namespace ReplacementPolicy
 159
 160 #endif // __MEM_CACHE_REPLACEMENT_POLICIES_BRRIP_RP_HH__