cache_adt.h

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