src/mem/cache/replacement_policies/second_chance_rp.hh

/**
 * Copyright (c) 2018 Inria
 * All rights reserved.
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 *
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/**
 * @file
 * Declaration of a Second-Chance replacement policy.
 * The victim is chosen using the timestamp. The oldest entry is chosen
 * to be evicted, if it hasn't been touched since its insertion. If it
 * has been touched, it is given a second chance and re-inserted at the
 * end of the queue.
 */

#ifndef __MEM_CACHE_REPLACEMENT_POLICIES_SECOND_CHANCE_RP_HH__
#define __MEM_CACHE_REPLACEMENT_POLICIES_SECOND_CHANCE_RP_HH__

#include "mem/cache/replacement_policies/base.hh"
#include "mem/cache/replacement_policies/fifo_rp.hh"

struct SecondChanceRPParams;

class SecondChanceRP : public FIFORP
{
  protected:
    /** Second-Chance-specific implementation of replacement data. */
    struct SecondChanceReplData : public FIFOReplData
    {
        /**
         * This is different from isTouched because isTouched accounts only
         * for insertion, while this bit is reset every new re-insertion.
         * @sa SecondChanceRP.
         */
        bool hasSecondChance;

        /**
         * Default constructor.
         */
        SecondChanceReplData() : FIFOReplData(), hasSecondChance(false) {}
    };

    /**
     * Use replacement data's second chance.
     *
     * @param replacement_data Entry that will use its second chance.
     */
    void useSecondChance(
        const std::shared_ptr<SecondChanceReplData>& replacement_data) const;

  public:
    /** Convenience typedef. */
    typedef SecondChanceRPParams Params;

    /**
     * Construct and initiliaze this replacement policy.
     */
    SecondChanceRP(const Params *p);

    /**
     * Destructor.
     */
    ~SecondChanceRP() {}

    /**
     * Invalidate replacement data to set it as the next probable victim.
     * Invalid entries do not have a second chance, and their last touch tick
     * is set as the oldest possible.
     *
     * @param replacement_data Replacement data to be invalidated.
     */
    void invalidate(const std::shared_ptr<ReplacementData>& replacement_data)
                                                              const override;

    /**
     * Touch an entry to update its re-insertion tick and second chance bit.
     *
     * @param replacement_data Replacement data to be touched.
     */
    void touch(const std::shared_ptr<ReplacementData>& replacement_data) const
                                                                     override;

    /**
     * Reset replacement data. Used when an entry is inserted or re-inserted
     * in the queue.
     * Sets its insertion tick and second chance bit.
     *
     * @param replacement_data Replacement data to be reset.
     */
    void reset(const std::shared_ptr<ReplacementData>& replacement_data) const
                                                                     override;

    /**
     * Find replacement victim using insertion timestamps and second chance
     * bit.
     *
     * @param cands Replacement candidates, selected by indexing policy.
     * @return Replacement entry to be replaced.
     */
    ReplaceableEntry* getVictim(const ReplacementCandidates& candidates) const
                                                                     override;

    /**
     * Instantiate a replacement data entry.
     *
     * @return A shared pointer to the new replacement data.
     */
    std::shared_ptr<ReplacementData> instantiateEntry() override;
};

#endif // __MEM_CACHE_REPLACEMENT_POLICIES_SECOND_CHANCE_RP_HH__