2006-06-23 19:41:55 +02:00
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/**
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* =========================================================================
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* File : cache_adt.h
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* Project : 0 A.D.
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* Description : Customizable cache data structure.
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*
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* @author Jan.Wassenberg@stud.uni-karlsruhe.de
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* =========================================================================
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*/
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/*
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* Copyright (c) 2005-2006 Jan Wassenberg
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*
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* Redistribution and/or modification are also permitted under the
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* terms of the GNU General Public License as published by the
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* Free Software Foundation (version 2 or later, at your option).
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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*/
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#ifndef CACHE_ADT_H__
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#define CACHE_ADT_H__
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#include <cfloat>
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#include <list>
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#include <map>
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2006-09-30 17:46:40 +02:00
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#include <queue>
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2006-06-23 19:41:55 +02:00
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/*
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Cache for items of variable size and value/"cost".
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underlying displacement algorithm is pluggable; default is "Landlord".
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template reference:
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Entry provides size, cost, credit and credit_density().
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rationale:
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- made a template instead of exposing Cache::Entry because
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that would drag a lot of stuff out of Cache.
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- calculates its own density since that entails a Divider functor,
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which requires storage inside Entry.
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Entries is a collection with iterator and begin()/end() and
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"static Entry& entry_from_it(iterator)".
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rationale:
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- STL map has pair<key, item> as its value_type, so this
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function would return it->second. however, we want to support
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other container types (where we'd just return *it).
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Manager is a template parameterized on typename Key and class Entry.
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its interface is as follows:
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// is the cache empty?
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bool empty() const;
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// add (key, entry) to cache.
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void add(Key key, const Entry& entry);
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// if the entry identified by <key> is not in cache, return false;
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// otherwise return true and pass back a pointer to it.
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bool find(Key key, const Entry** pentry) const;
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// remove an entry from cache, which is assumed to exist!
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// this makes sense because callers will typically first use find() to
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// return info about the entry; this also checks if present.
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void remove(Key key);
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// mark <entry> as just accessed for purpose of cache management.
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// it will tend to be kept in cache longer.
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void on_access(Entry& entry);
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// caller's intent is to remove the least valuable entry.
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// in implementing this, you have the latitude to "shake loose"
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// several entries (e.g. because their 'value' is equal).
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// they must all be push_back-ed into the list; Cache will dole
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// them out one at a time in FIFO order to callers.
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//
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// rationale:
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// - it is necessary for callers to receive a copy of the
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// Entry being evicted - e.g. file_cache owns its items and
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// they must be passed back to allocator when evicted.
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// - e.g. Landlord can potentially see several entries become
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// evictable in one call to remove_least_valuable. there are
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// several ways to deal with this:
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// 1) generator interface: we return one of { empty, nevermind,
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// removed, remove-and-call-again }. this greatly complicates
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// the call site.
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// 2) return immediately after finding an item to evict.
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// this changes cache behavior - entries stored at the
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// beginning would be charged more often (unfair).
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// resuming charging at the next entry doesn't work - this
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// would have to be flushed when adding, at which time there
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// is no provision for returning any items that may be evicted.
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// 3) return list of all entries "shaken loose". this incurs
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// frequent mem allocs, which can be alleviated via suballocator.
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// note: an intrusive linked-list doesn't make sense because
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// entries to be returned need to be copied anyway (they are
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// removed from the manager's storage).
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void remove_least_valuable(std::list<Entry>& entry_list)
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*/
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//
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// functors to calculate minimum credit density (MCD)
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//
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// MCD is required for the Landlord algorithm's evict logic.
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// [Young02] calls it '\delta'.
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// scan over all entries and return MCD.
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template<class Entries> float ll_calc_min_credit_density(const Entries& entries)
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{
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float min_credit_density = FLT_MAX;
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for(typename Entries::const_iterator it = entries.begin(); it != entries.end(); ++it)
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{
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const float credit_density = Entries::entry_from_it(it).credit_density();
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min_credit_density = fminf(min_credit_density, credit_density);
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}
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return min_credit_density;
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}
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// note: no warning is given that the MCD entry is being removed!
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// (reduces overhead in remove_least_valuable)
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// these functors must account for that themselves (e.g. by resetting
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// their state directly after returning MCD).
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// determine MCD by scanning over all entries.
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// tradeoff: O(N) time complexity, but all notify* calls are no-ops.
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template<class Entry, class Entries>
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class McdCalc_Naive
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{
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public:
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void notify_added(const Entry&) const {}
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void notify_decreased(const Entry&) const {}
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void notify_impending_increase_or_remove(const Entry&) const {}
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void notify_increased_or_removed(const Entry&) const {}
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float operator()(const Entries& entries) const
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{
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const float mcd = ll_calc_min_credit_density(entries);
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return mcd;
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}
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};
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// cache previous MCD and update it incrementally (when possible).
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// tradeoff: amortized O(1) time complexity, but notify* calls must
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// perform work whenever something in the cache changes.
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template<class Entry, class Entries>
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class McdCalc_Cached
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{
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public:
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McdCalc_Cached() : min_credit_density(FLT_MAX), min_valid(false) {}
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void notify_added(const Entry& entry)
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{
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// when adding a new item, the minimum credit density can only
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// decrease or remain the same; acting as if entry's credit had
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// been decreased covers both cases.
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notify_decreased(entry);
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}
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void notify_decreased(const Entry& entry)
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{
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min_credit_density = MIN(min_credit_density, entry.credit_density());
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}
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void notify_impending_increase_or_remove(const Entry& entry)
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{
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// remember if this entry had the smallest density
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is_min_entry = feq(min_credit_density, entry.credit_density());
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}
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void notify_increased_or_removed(const Entry& UNUSED(entry))
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{
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// .. it did and was increased or removed. we must invalidate
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// MCD and recalculate it next time.
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if(is_min_entry)
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{
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min_valid = false;
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min_credit_density = -1.0f;
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}
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}
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float operator()(const Entries& entries)
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{
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if(min_valid)
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{
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// the entry that has MCD will be removed anyway by caller;
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// we need to invalidate here because they don't call
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// notify_increased_or_removed.
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min_valid = false;
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return min_credit_density;
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}
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// this is somewhat counterintuitive. since we're calculating
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// MCD directly, why not mark our cached version of it valid
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// afterwards? reason is that our caller will remove the entry with
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// MCD, so it'll be invalidated anyway.
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// instead, our intent is to calculate MCD for the *next time*.
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const float ret = ll_calc_min_credit_density(entries);
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min_valid = true;
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min_credit_density = FLT_MAX;
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return ret;
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}
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private:
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float min_credit_density;
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bool min_valid;
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// temporary flag set by notify_impending_increase_or_remove
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bool is_min_entry;
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};
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//
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// Landlord cache management policy: see [Young02].
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//
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// in short, each entry has credit initially set to cost. when wanting to
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// remove an item, all are charged according to MCD and their size;
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// entries are evicted if their credit is exhausted. accessing an entry
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// restores "some" of its credit.
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template<typename Key, typename Entry, template<class Entry, class Entries> class McdCalc = McdCalc_Cached>
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class Landlord
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{
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public:
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bool empty() const
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{
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return map.empty();
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}
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void add(Key key, const Entry& entry)
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{
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// adapter for add_ (which returns an iterator)
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(void)add_(key, entry);
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}
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bool find(Key key, const Entry** pentry) const
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{
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MapCIt it = map.find(key);
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if(it == map.end())
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return false;
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*pentry = &it->second;
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return true;
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}
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void remove(Key key)
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{
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MapIt it = map.find(key);
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debug_assert(it != map.end());
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remove_(it);
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}
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void on_access(Entry& entry)
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{
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mcd_calc.notify_impending_increase_or_remove(entry);
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// Landlord algorithm calls for credit to be reset to anything
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// between its current value and the cost.
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const float gain = 0.75f; // restore most credit
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entry.credit = gain*entry.cost + (1.0f-gain)*entry.credit;
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mcd_calc.notify_increased_or_removed(entry);
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}
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void remove_least_valuable(std::list<Entry>& entry_list)
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{
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// we are required to evict at least one entry. one iteration
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// ought to suffice, due to definition of min_credit_density and
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// tolerance; however, we provide for repeating if necessary.
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again:
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// messing with this (e.g. raising if tiny) would result in
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// different evictions than Landlord_Lazy, which is unacceptable.
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// nor is doing so necessary: if mcd is tiny, so is credit.
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const float min_credit_density = mcd_calc(map);
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debug_assert(min_credit_density > 0.0f);
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for(MapIt it = map.begin(); it != map.end();) // no ++it
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{
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Entry& entry = it->second;
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charge(entry, min_credit_density);
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if(should_evict(entry))
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{
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entry_list.push_back(entry);
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// annoying: we have to increment <it> before erasing
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MapIt it_to_remove = it++;
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map.erase(it_to_remove);
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}
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else
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{
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mcd_calc.notify_decreased(entry);
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++it;
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}
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}
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if(entry_list.empty())
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goto again;
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}
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protected:
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// note: use hash_map instead of map for better locality
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// (relevant when iterating over all items in remove_least_valuable)
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class Map : public STL_HASH_MAP<Key, Entry>
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{
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public:
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static Entry& entry_from_it(typename Map::iterator it) { return it->second; }
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static const Entry& entry_from_it(typename Map::const_iterator it) { return it->second; }
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};
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typedef typename Map::iterator MapIt;
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typedef typename Map::const_iterator MapCIt;
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Map map;
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// add entry and return iterator pointing to it.
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MapIt add_(Key key, const Entry& entry)
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{
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typedef std::pair<MapIt, bool> PairIB;
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typename Map::value_type val = std::make_pair(key, entry);
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PairIB ret = map.insert(val);
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debug_assert(ret.second); // must not already be in map
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mcd_calc.notify_added(entry);
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return ret.first;
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}
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// remove entry (given by iterator) directly.
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void remove_(MapIt it)
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{
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const Entry& entry = it->second;
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mcd_calc.notify_impending_increase_or_remove(entry);
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mcd_calc.notify_increased_or_removed(entry);
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map.erase(it);
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}
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void charge(Entry& entry, float delta)
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{
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entry.credit -= delta * entry.size;
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// don't worry about entry.size being 0 - if so, cost
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// should also be 0, so credit will already be 0 anyway.
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}
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// for each entry, 'charge' it (i.e. reduce credit by) delta * its size.
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// delta is typically MCD (see above); however, several such updates
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// may be lumped together to save time. Landlord_Lazy does this.
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void charge_all(float delta)
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{
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for(MapIt it = map.begin(); it != map.end(); ++it)
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{
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Entry& entry = it->second;
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entry.credit -= delta * entry.size;
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if(!should_evict(entry))
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mcd_calc.notify_decreased(entry);
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}
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}
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// is entry's credit exhausted? if so, it should be evicted.
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bool should_evict(const Entry& entry)
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{
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// we need a bit of leeway because density calculations may not
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// be exact. choose value carefully: must not be high enough to
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// trigger false positives.
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return entry.credit < 0.0001f;
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}
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private:
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McdCalc<Entry, Map> mcd_calc;
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};
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// Cache manger policies. (these are partial specializations of Landlord,
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// adapting it to the template params required by Cache)
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template<class Key, class Entry> class Landlord_Naive : public Landlord<Key, Entry, McdCalc_Naive> {};
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template<class Key, class Entry> class Landlord_Cached: public Landlord<Key, Entry, McdCalc_Cached> {};
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// variant of Landlord that adds a priority queue to directly determine
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// which entry to evict. this allows lumping several charge operations
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// together and thus reduces iteration over all entries.
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// tradeoff: O(logN) removal (instead of N), but additional O(N) storage.
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template<typename Key, class Entry>
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class Landlord_Lazy : public Landlord_Naive<Key, Entry>
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{
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typedef typename Landlord_Naive<Key, Entry>::Map Map;
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typedef typename Landlord_Naive<Key, Entry>::MapIt MapIt;
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typedef typename Landlord_Naive<Key, Entry>::MapCIt MapCIt;
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public:
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Landlord_Lazy() { pending_delta = 0.0f; }
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void add(Key key, const Entry& entry)
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{
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// we must apply pending_delta now - otherwise, the existing delta
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// would later be applied to this newly added item (incorrect).
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commit_pending_delta();
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MapIt it = Parent::add_(key, entry);
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pri_q.push(it);
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}
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void remove(Key key)
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{
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Parent::remove(key);
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// reconstruct pri_q from current map. this is slow (N*logN) and
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// could definitely be done better, but we don't bother since
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// remove is a very rare operation (e.g. invalidating entries).
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while(!pri_q.empty())
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pri_q.pop();
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for(MapCIt it = this->map.begin(); it != this->map.end(); ++it)
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pri_q.push(it);
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}
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void on_access(Entry& entry)
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{
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Parent::on_access(entry);
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// entry's credit was changed. we now need to reshuffle the
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// pri queue to reflect this.
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pri_q.ensure_heap_order();
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}
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void remove_least_valuable(std::list<Entry>& entry_list)
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{
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MapIt least_valuable_it = pri_q.top(); pri_q.pop();
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Entry& entry = Map::entry_from_it(least_valuable_it);
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entry_list.push_back(entry);
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// add to pending_delta the MCD that would have resulted
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// if removing least_valuable_it normally.
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// first, calculate actual credit (i.e. apply pending_delta to
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// this entry); then add the resulting density to pending_delta.
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entry.credit -= pending_delta*entry.size;
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const float credit_density = entry.credit_density();
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debug_assert(credit_density > 0.0f);
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pending_delta += credit_density;
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Parent::remove_(least_valuable_it);
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}
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private:
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typedef Landlord_Naive<Key, Entry> Parent;
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// sort iterators by credit_density of the Entry they reference.
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struct CD_greater
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{
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bool operator()(MapIt it1, MapIt it2) const
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{
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return Map::entry_from_it(it1).credit_density() >
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Map::entry_from_it(it2).credit_density();
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}
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};
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// wrapper on top of priority_queue that allows 'heap re-sift'
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// (see on_access).
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// notes:
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// - greater comparator makes pri_q.top() the one with
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// LEAST credit_density, which is what we want.
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// - deriving from an STL container is a bit dirty, but we need this
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// to get at the underlying data (priority_queue interface is not
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// very capable).
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class PriQ: public std::priority_queue<MapIt, std::vector<MapIt>, CD_greater>
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{
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public:
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void ensure_heap_order()
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{
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// TODO: this is actually N*logN - ouch! that explains high
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// CPU cost in profile. this is called after only 1 item has
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// changed, so a logN "sift" operation ought to suffice.
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// that's not supported by the STL heap functions, so we'd
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// need a better implementation. pending..
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std::make_heap(this->c.begin(), this->c.end(), this->comp);
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}
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};
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PriQ pri_q;
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// delta values that have accumulated over several
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// remove_least_valuable() calls. applied during add().
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float pending_delta;
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void commit_pending_delta()
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{
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if(pending_delta > 0.0f)
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{
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this->charge_all(pending_delta);
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pending_delta = 0.0f;
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// we've changed entry credit, so the heap order *may* have been
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// violated; reorder the pri queue. (I don't think so,
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// due to definition of delta, but we'll play it safe)
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pri_q.ensure_heap_order();
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}
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}
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};
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//
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// functor that implements division of first arg by second
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//
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// this is used to calculate credit_density(); performance matters
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// because this is called for each entry during each remove operation.
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// floating-point division (fairly slow)
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class Divider_Naive
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{
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public:
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Divider_Naive() {} // needed for default CacheEntry ctor
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Divider_Naive(float UNUSED(x)) {}
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float operator()(float val, float divisor) const
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{
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return val / divisor;
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}
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};
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// caches reciprocal of divisor and multiplies by that.
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// tradeoff: only 4 clocks (instead of 20), but 4 bytes extra per entry.
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class Divider_Recip
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{
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float recip;
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public:
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Divider_Recip() {} // needed for default CacheEntry ctor
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Divider_Recip(float x) { recip = 1.0f / x; }
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float operator()(float val, float UNUSED(divisor)) const
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{
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return val * recip;
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}
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};
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// TODO: use SSE/3DNow RCP instruction? not yet, because not all systems
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// support it and overhead of detecting this support eats into any gains.
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// initial implementation for testing purposes; quite inefficient.
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template<typename Key, typename Entry>
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class LRU
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{
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public:
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bool empty() const
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{
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return lru.empty();
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}
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void add(Key key, const Entry& entry)
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{
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lru.push_back(KeyAndEntry(key, entry));
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}
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bool find(Key key, const Entry** pentry) const
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{
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CIt it = std::find_if(lru.begin(), lru.end(), KeyEq(key));
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if(it == lru.end())
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return false;
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*pentry = &it->entry;
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return true;
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}
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void remove(Key key)
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{
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std::remove_if(lru.begin(), lru.end(), KeyEq(key));
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}
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void on_access(Entry& entry)
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{
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for(It it = lru.begin(); it != lru.end(); ++it)
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{
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if(&entry == &it->entry)
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{
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add(it->key, it->entry);
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lru.erase(it);
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return;
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}
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}
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debug_warn("entry not found in list");
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}
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void remove_least_valuable(std::list<Entry>& entry_list)
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{
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entry_list.push_back(lru.front().entry);
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lru.pop_front();
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}
|
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private:
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|
struct KeyAndEntry
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|
{
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|
Key key;
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|
Entry entry;
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|
KeyAndEntry(Key key_, const Entry& entry_)
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: key(key_), entry(entry_) {}
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|
};
|
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|
|
class KeyEq
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{
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|
|
Key key;
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public:
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KeyEq(Key key_) : key(key_) {}
|
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|
|
bool operator()(const KeyAndEntry& ke) const
|
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|
|
{
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|
|
return ke.key == key;
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|
|
}
|
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|
|
};
|
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|
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|
|
typedef std::list<KeyAndEntry> List;
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|
|
typedef typename List::iterator It;
|
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|
|
typedef typename List::const_iterator CIt;
|
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|
|
List lru;
|
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|
|
};
|
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|
|
|
|
|
|
|
|
|
|
//
|
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|
|
// Cache
|
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|
//
|
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|
|
|
|
|
|
template
|
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|
|
<
|
|
|
|
typename Key, typename Item,
|
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|
|
// see documentation above for Manager's interface.
|
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|
|
template<typename Key, class Entry> class Manager = Landlord_Cached,
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|
|
class Divider = Divider_Naive
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|
|
>
|
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|
|
class Cache
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|
|
{
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|
|
public:
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|
|
Cache() : mgr() {}
|
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|
|
|
|
|
|
void add(Key key, Item item, size_t size, uint cost)
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|
|
{
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|
|
return mgr.add(key, Entry(item, size, cost));
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|
|
}
|
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|
|
|
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|
|
// remove the entry identified by <key>. expected usage is to check
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|
|
// if present and determine size via retrieve(), so no need for
|
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|
|
// error checking.
|
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|
|
// useful for invalidating single cache entries.
|
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|
|
void remove(Key key)
|
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|
|
{
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|
|
mgr.remove(key);
|
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|
|
}
|
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|
|
|
|
|
|
// if there is no entry for <key> in the cache, return false.
|
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|
|
// otherwise, return true and pass back item and (optionally) size.
|
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|
|
//
|
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|
|
// if refill_credit (default), the cache manager 'rewards' this entry,
|
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|
|
// tending to keep it in cache longer. this parameter is not used in
|
|
|
|
// normal operation - it's only for special cases where we need to
|
|
|
|
// make an end run around the cache accounting (e.g. for cache simulator).
|
|
|
|
bool retrieve(Key key, Item& item, size_t* psize = 0, bool refill_credit = true)
|
|
|
|
{
|
|
|
|
const Entry* entry;
|
|
|
|
if(!mgr.find(key, &entry))
|
|
|
|
return false;
|
|
|
|
|
|
|
|
item = entry->item;
|
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|
|
if(psize)
|
|
|
|
*psize = entry->size;
|
|
|
|
|
|
|
|
if(refill_credit)
|
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|
|
mgr.on_access((Entry&)*entry);
|
|
|
|
|
|
|
|
return true;
|
|
|
|
}
|
|
|
|
|
|
|
|
// toss out the least valuable entry. return false if cache is empty,
|
|
|
|
// otherwise true and (optionally) pass back its item and size.
|
|
|
|
bool remove_least_valuable(Item* pItem = 0, size_t* pSize = 0)
|
|
|
|
{
|
|
|
|
// as an artefact of the cache eviction policy, several entries
|
|
|
|
// may be "shaken loose" by one call to remove_least_valuable.
|
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|
|
// we cache them in a list to disburden callers (they always get
|
|
|
|
// exactly one).
|
|
|
|
if(entries_awaiting_eviction.empty())
|
|
|
|
{
|
|
|
|
if(empty())
|
|
|
|
return false;
|
|
|
|
|
|
|
|
mgr.remove_least_valuable(entries_awaiting_eviction);
|
|
|
|
debug_assert(!entries_awaiting_eviction.empty());
|
|
|
|
}
|
|
|
|
|
|
|
|
const Entry& entry = entries_awaiting_eviction.front();
|
|
|
|
if(pItem)
|
|
|
|
*pItem = entry.item;
|
|
|
|
if(pSize)
|
|
|
|
*pSize = entry.size;
|
|
|
|
entries_awaiting_eviction.pop_front();
|
|
|
|
|
|
|
|
return true;
|
|
|
|
}
|
|
|
|
|
|
|
|
bool empty() const
|
|
|
|
{
|
|
|
|
return mgr.empty();
|
|
|
|
}
|
|
|
|
|
|
|
|
private:
|
|
|
|
// this is applicable to all cache management policies and stores all
|
|
|
|
// required information. a Divider functor is used to implement
|
|
|
|
// division for credit_density.
|
|
|
|
template<class InnerDivider> struct CacheEntry
|
|
|
|
{
|
|
|
|
Item item;
|
|
|
|
size_t size;
|
|
|
|
uint cost;
|
|
|
|
float credit;
|
|
|
|
|
|
|
|
InnerDivider divider;
|
|
|
|
|
|
|
|
// needed for mgr.remove_least_valuable's entry_copy
|
|
|
|
CacheEntry() {}
|
|
|
|
|
|
|
|
CacheEntry(Item item_, size_t size_, uint cost_)
|
|
|
|
: item(item_), divider((float)size_)
|
|
|
|
{
|
|
|
|
size = size_;
|
|
|
|
cost = cost_;
|
|
|
|
credit = cost;
|
|
|
|
|
|
|
|
// else divider will fail
|
|
|
|
debug_assert(size != 0);
|
|
|
|
}
|
|
|
|
|
|
|
|
float credit_density() const
|
|
|
|
{
|
|
|
|
return divider(credit, (float)size);
|
|
|
|
}
|
|
|
|
};
|
|
|
|
typedef CacheEntry<Divider> Entry;
|
|
|
|
|
|
|
|
// see note in remove_least_valuable().
|
|
|
|
std::list<Entry> entries_awaiting_eviction;
|
|
|
|
|
|
|
|
Manager<Key, Entry> mgr;
|
|
|
|
};
|
|
|
|
|
|
|
|
#endif // #ifndef CACHE_ADT_H__
|