block_cache.h

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/***************************************************************************
 *  include/stxxl/bits/containers/hash_map/block_cache.h
 *
 *  Part of the STXXL. See http://stxxl.sourceforge.net
 *
 *  Copyright (C) 2007 Markus Westphal <[email protected]>
 *  Copyright (C) 2014 Timo Bingmann <[email protected]>
 *
 *  Distributed under the Boost Software License, Version 1.0.
 *  (See accompanying file LICENSE_1_0.txt or copy at
 *  http://www.boost.org/LICENSE_1_0.txt)
 **************************************************************************/

#ifndef STXXL_CONTAINERS_HASH_MAP_BLOCK_CACHE_HEADER
#define STXXL_CONTAINERS_HASH_MAP_BLOCK_CACHE_HEADER

#ifdef STXXL_BOOST_CONFIG
 #include <boost/config.hpp>
#endif

#include <stxxl/bits/noncopyable.h>
#include <stxxl/bits/compat/hash_map.h>
#include <stxxl/bits/mng/block_manager.h>
#include <stxxl/bits/containers/pager.h>

#include <vector>
#include <list>

STXXL_BEGIN_NAMESPACE

namespace hash_map {

//! Used inside block_cache for buffering write requests of cached blocks.
template <class BlockType>
class block_cache_write_buffer : private noncopyable
{
public:
    typedef BlockType block_type;
    typedef typename block_type::bid_type bid_type;

protected:
    std::vector<block_type*> blocks_;
    std::vector<request_ptr> reqs_;
    std::vector<unsigned_type> free_blocks_;
    std::list<unsigned_type> busy_blocks_; // TODO make that a circular-buffer

public:
    block_cache_write_buffer(unsigned_type size)
    {
        blocks_.reserve(size);
        free_blocks_.reserve(size);
        reqs_.resize(size);

        for (unsigned_type i = 0; i < size; i++) {
            blocks_.push_back(new block_type());
            free_blocks_.push_back(i);
        }
    }

    //! Writes the given block back to disk;
    //! callers have to exchange the passed block with the returned one!
    block_type * write(block_type* write_block, const bid_type& bid)
    {
        if (free_blocks_.empty()) {
            unsigned_type i_buffer = busy_blocks_.front();
            busy_blocks_.pop_front();

            if (reqs_[i_buffer].valid())
                reqs_[i_buffer]->wait();

            free_blocks_.push_back(i_buffer);
        }

        unsigned_type i_buffer = free_blocks_.back();
        free_blocks_.pop_back();
        block_type* buffer = blocks_[i_buffer];

        blocks_[i_buffer] = write_block;
        reqs_[i_buffer] = blocks_[i_buffer]->write(bid);
        busy_blocks_.push_back(i_buffer);

        return buffer;
    }

    void flush()
    {
        while (!busy_blocks_.empty()) {
            unsigned_type i_buffer = busy_blocks_.front();
            busy_blocks_.pop_front();
            if (reqs_[i_buffer].valid())
                reqs_[i_buffer]->wait();
        }
        busy_blocks_.clear();
        free_blocks_.clear();
        for (unsigned_type i = 0; i < blocks_.size(); i++)
            free_blocks_.push_back(i);
    }

    void swap(block_cache_write_buffer& obj)
    {
        std::swap(blocks_, obj.blocks_);
        std::swap(reqs_, obj.reqs_);
        std::swap(free_blocks_, obj.free_blocks_);
        std::swap(busy_blocks_, obj.busy_blocks_);
    }

    ~block_cache_write_buffer()
    {
        flush();
        for (unsigned_type i = 0; i < blocks_.size(); i++)
            delete blocks_[i];
    }
};

//! Cache of blocks contained in an external memory hash map. Uses the
//! stxxl::lru_pager as eviction algorithm.
template <class BlockType>
class block_cache : private noncopyable
{
public:
    typedef BlockType block_type;
    typedef typename block_type::bid_type bid_type;
    typedef typename block_type::value_type subblock_type;
    typedef typename subblock_type::bid_type subblock_bid_type;

protected:
    struct bid_eq
    {
        bool operator () (const bid_type& a, const bid_type& b) const
        {
            return (a.storage == b.storage && a.offset == b.offset);
        }
    };

    struct bid_hash
    {
        size_t operator () (const bid_type& bid) const
        {
            return longhash1(bid.offset + reinterpret_cast<uint64>(bid.storage));
        }
#ifdef STXXL_MSVC
        bool operator () (const bid_type& a, const bid_type& b) const
        {
            return (a.storage < b.storage) ||
                   (a.storage == b.storage && a.offset < b.offset);
        }
        enum
        {                                  // parameters for hash table
            bucket_size = 4,               // 0 < bucket_size
            min_buckets = 8                // min_buckets = 2 ^^ N, 0 < N
        };
#endif
    };

    typedef stxxl::lru_pager<> pager_type;
    typedef block_cache_write_buffer<block_type> write_buffer_type;

    typedef typename compat_hash_map<bid_type, unsigned_type,
                                     bid_hash>::result bid_map_type;

    enum { valid_all = block_type::size };

    write_buffer_type write_buffer_;

    //! cached blocks
    std::vector<block_type*> blocks_;
    //! bids of cached blocks
    std::vector<bid_type> bids_;
    std::vector<unsigned_type> retain_count_;

    //! true iff block has been altered while in cache
    std::vector<unsigned char> dirty_;

    //! valid_all or the actually loaded subblock's index
    std::vector<unsigned_type> valid_subblock_;

    //! free blocks as indices to blocks_-vector
    std::vector<unsigned_type> free_blocks_;
    std::vector<request_ptr> reqs_;

    bid_map_type bid_map_;
    pager_type pager_;

    /* statistics */
    int64 n_found;
    int64 n_not_found;
    int64 n_read;
    int64 n_written;
    int64 n_clean_forced;
    int64 n_wrong_subblock;

public:
    //! Construct a new block-cache.
    //! \param cache_size cache-size in number of blocks
    block_cache(unsigned_type cache_size)
        : write_buffer_(config::get_instance()->disks_number() * 2),
          blocks_(cache_size),
          bids_(cache_size),
          retain_count_(cache_size),
          dirty_(cache_size, false),
          valid_subblock_(cache_size),
          free_blocks_(cache_size),
          reqs_(cache_size),
          pager_(cache_size),
          n_found(0),
          n_not_found(0),
          n_read(0),
          n_written(0),
          n_clean_forced(0),
          n_wrong_subblock(0)
    {
        for (unsigned_type i = 0; i < cache_size; i++)
        {
            blocks_[i] = new block_type();
            free_blocks_[i] = i;
        }
    }

    //! Return cache-size
    unsigned_type size() const
    {
        return blocks_.size();
    }

    ~block_cache()
    {
        STXXL_VERBOSE1("hash_map::block_cache destructor addr=" << this);

        for (typename bid_map_type::const_iterator i = bid_map_.begin();
             i != bid_map_.end(); ++i)
        {
            const unsigned_type i_block = (*i).second;

            if (reqs_[i_block].valid())
                reqs_[i_block]->wait();

            if (dirty_[i_block]) {
                blocks_[i_block] =
                    write_buffer_.write(blocks_[i_block], bids_[i_block]);
            }
        }
        write_buffer_.flush();

        for (unsigned_type i = 0; i < size(); ++i)
            delete blocks_[i];
    }

protected:
    //! Force a block from the cache; write back to disk if dirty
    void kick_block()
    {
        unsigned_type i_block2kick;

        unsigned_type max_tries = size() + 1;
        unsigned_type i = 0;
        do
        {
            ++i;
            i_block2kick = pager_.kick();
            if (i == max_tries)
            {
                throw std::runtime_error(
                          "The block cache is too small,"
                          "no block can be kicked out (all blocks are retained)!"
                          );
            }
            pager_.hit(i_block2kick);
        } while (retain_count_[i_block2kick] > 0);

        if (valid_subblock_[i_block2kick] == valid_all &&
            reqs_[i_block2kick].valid())
        {
            reqs_[i_block2kick]->wait();
        }

        if (dirty_[i_block2kick])
        {
            blocks_[i_block2kick] =
                write_buffer_.write(blocks_[i_block2kick], bids_[i_block2kick]);
            ++n_written;
        }
        else
            ++n_clean_forced;

        bid_map_.erase(bids_[i_block2kick]);
        free_blocks_.push_back(i_block2kick);
    }

public:
    //! Retain a block in cache. Blocks, that are retained by at least one
    //! client, won't get kicked. Make sure to release all retained blocks
    //! again.
    //!
    //! \param bid block, whose retain-count is to be increased
    //! \return true if block was cached, false otherwise
    bool retain_block(const bid_type& bid)
    {
        typename bid_map_type::const_iterator it = bid_map_.find(bid);
        if (it == bid_map_.end())
            return false;

        unsigned_type i_block = (*it).second;
        retain_count_[i_block]++;
        return true;
    }

    //! Release a block (decrement retain-count). If the retain-count reaches
    //! 0, a block may be kicked again.
    //!
    //! \param bid block, whose retain-count is to be decremented
    //! \return true if operation was successfull (block cached and
    //!         retain-count > 0), false otherwise
    bool release_block(const bid_type& bid)
    {
        typename bid_map_type::const_iterator it = bid_map_.find(bid);
        if (it == bid_map_.end())
            return false;

        unsigned_type i_block = (*it).second;
        if (retain_count_[i_block] == 0)
            return false;

        retain_count_[i_block]--;
        return true;
    }

    //! Set given block's dirty-flag. Note: If the given block was only
    //! partially loaded, it will be completely reloaded.
    //!
    //! \return true if block cached, false otherwise
    bool make_dirty(const bid_type& bid)
    {
        typename bid_map_type::const_iterator it = bid_map_.find(bid);
        if (it == bid_map_.end())
            return false;

        unsigned_type i_block = (*it).second;

        // only complete blocks can be marked as dirty
        if (valid_subblock_[i_block] != valid_all)
        {
            reqs_[i_block] = blocks_[i_block]->read(bid);
            valid_subblock_[i_block] = valid_all;
        }

        if (reqs_[i_block].valid()) {
            if (reqs_[i_block]->poll() == false)
                reqs_[i_block]->wait();
        }

        dirty_[i_block] = true;
        return true;
    }

    //! Retrieve a subblock from the cache. If not yet cached, only the
    //! subblock will be loaded.
    //!
    //! \param bid block, to which the requested subblock belongs
    //! \param i_subblock index of requested subblock
    //! \return pointer to subblock
    subblock_type * get_subblock(const bid_type& bid, unsigned_type i_subblock)
    {
        block_type* block;
        unsigned_type i_block;
        n_read++;

        // block (partly) cached?
        typename bid_map_type::const_iterator it = bid_map_.find(bid);
        if (it != bid_map_.end())
        {
            i_block = (*it).second;
            block = blocks_[i_block];

            // complete block or wanted subblock is in the cache
            if (valid_subblock_[i_block] == valid_all ||
                valid_subblock_[i_block] == i_subblock)
            {
                ++n_found;

                if (valid_subblock_[i_block] == valid_all &&
                    reqs_[i_block].valid())
                {
                    // request not yet completed?
                    if (reqs_[i_block]->poll() == false)
                        reqs_[i_block]->wait();
                }

                return &((*block)[i_subblock]);
            }

            // wrong subblock in cache
            else
            {
                ++n_not_found;
                ++n_wrong_subblock;
                // actually loading the subblock will be done below

                // note: if a client still holds a reference to the "old"
                // subblock, it will find its data to be still valid.
            }
        }
        // block not cached
        else
        {
            n_not_found++;

            if (free_blocks_.empty())
                kick_block();

            i_block = free_blocks_.back(), free_blocks_.pop_back();
            block = blocks_[i_block];

            bid_map_[bid] = i_block;
            bids_[i_block] = bid;
            dirty_[i_block] = false;
            retain_count_[i_block] = 0;
        }

        // now actually load the wanted subblock and store it within *block
        subblock_bid_type subblock_bid(
            bid.storage, bid.offset + i_subblock * subblock_type::raw_size
            );
        request_ptr req = ((*block)[i_subblock]).read(subblock_bid);
        req->wait();

        valid_subblock_[i_block] = i_subblock;
        pager_.hit(i_block);

        return &((*block)[i_subblock]);
    }

    //! Load a block in advance.
    //! \param bid Identifier of the block to load
    void prefetch_block(const bid_type& bid)
    {
        unsigned_type i_block;

        // cached
        typename bid_map_type::const_iterator it = bid_map_.find(bid);
        if (it != bid_map_.end())
        {
            i_block = (*it).second;

            // complete block cached; we can finish here
            if (valid_subblock_[i_block] == valid_all) {
                pager_.hit(i_block);
                return;
            }

            // only a single subblock is cached; we have to load the
            // complete block (see below)
        }
        // not even a subblock cached
        else {
            if (free_blocks_.empty())
                kick_block();

            i_block = free_blocks_.back(), free_blocks_.pop_back();

            bid_map_[bid] = i_block;
            bids_[i_block] = bid;
            retain_count_[i_block] = 0;
            dirty_[i_block] = false;
        }

        // now actually load the block
        reqs_[i_block] = blocks_[i_block]->read(bid);
        valid_subblock_[i_block] = valid_all;
        pager_.hit(i_block);
    }

    //! Write all dirty blocks back to disk
    void flush()
    {
        for (typename bid_map_type::const_iterator i = bid_map_.begin();
             i != bid_map_.end(); ++i)
        {
            const unsigned_type i_block = (*i).second;
            if (dirty_[i_block])
            {
                blocks_[i_block] =
                    write_buffer_.write(blocks_[i_block], bids_[i_block]);

                dirty_[i_block] = false;
            }
        }
        write_buffer_.flush();
    }

    //! Empty cache; don't write back dirty blocks
    void clear()
    {
        free_blocks_.clear();
        for (unsigned_type i = 0; i < size(); i++)
        {
            if (reqs_[i].valid()) {
                reqs_[i]->cancel();
                reqs_[i]->wait();
            }

            free_blocks_.push_back(i);
        }
        bid_map_.clear();
    }

    //! Print statistics: Number of hits/misses, blocks forced from cache or
    //! written back.
    void print_statistics(std::ostream& o = std::cout) const
    {
        o << "Blocks found                      : " << n_found << " (" << 100. * double(n_found) / double(n_read) << "%)" << std::endl;
        o << "Blocks not found                  : " << n_not_found << std::endl;
        o << "Blocks read                       : " << n_read << std::endl;
        o << "Blocks written                    : " << n_written << std::endl;
        o << "Clean blocks forced from the cache: " << n_clean_forced << std::endl;
        o << "Wrong subblock cached             : " << n_wrong_subblock << std::endl;
    }

    //! Reset all counters to zero
    void reset_statistics()
    {
        n_found = 0;
        n_not_found = 0;
        n_read = 0;
        n_written = 0;
        n_clean_forced = 0;
        n_wrong_subblock = 0;
    }

    //! Exchange contents of two caches
    //! \param obj cache to swap contents with
    void swap(block_cache& obj)
    {
        write_buffer_.swap(obj.write_buffer_);
        std::swap(blocks_, obj.blocks_);
        std::swap(bids_, obj.bids_);
        std::swap(retain_count_, obj.retain_count_);

        std::swap(dirty_, obj.dirty_);
        std::swap(valid_subblock_, obj.valid_subblock_);

        std::swap(free_blocks_, obj.free_blocks_);
        std::swap(reqs_, obj.reqs_);

        std::swap(bid_map_, obj.bid_map_);
        std::swap(pager_, obj.pager_);

        std::swap(n_found, obj.n_found);
        std::swap(n_not_found, obj.n_found);
        std::swap(n_read, obj.n_read);
        std::swap(n_written, obj.n_written);
        std::swap(n_clean_forced, obj.n_clean_forced);
        std::swap(n_wrong_subblock, obj.n_wrong_subblock);
    }

#if 0   // for debugging, requires data items to be ostream-able.

    //! Show currently cached blocks
    void dump_cache(std::ostream& os) const
    {
        for (size_t i = 0; i < blocks_.size(); i++)
        {
            bid_type bid = bids_[i];
            if (bid_map_.count(bid) == 0) {
                os << "Block " << i << ": empty\n";
                continue;
            }

            os << "Block " << i << ": bid=" << bids_[i]
               << " dirty=" << dirty_[i]
               << " retain_count=" << retain_count_[i]
               << " valid_subblock=" << valid_subblock_[i] << "\n";

            for (size_t k = 0; k < block_type::size; k++) {
                os << "  Subbblock " << k << ": ";
                if (valid_subblock_[i] != valid_all && valid_subblock_[i] != k)
                {
                    os << "not valid\n";
                    continue;
                }
                for (size_t l = 0; l < block_type::value_type::size; l++) {
                    os << "(" << (*blocks_[i])[k][l].first
                       << ", " << (*blocks_[i])[k][l].second << ") ";
                }
                os << std::endl;
            }
        }
    }
#endif
};

} // namespace hash_map

STXXL_END_NAMESPACE

namespace std {

template <class BlockType>
void swap(stxxl::hash_map::block_cache_write_buffer<BlockType>& a,
          stxxl::hash_map::block_cache_write_buffer<BlockType>& b)
{
    a.swap(b);
}

template <class HashMap>
void swap(stxxl::hash_map::block_cache<HashMap>& a,
          stxxl::hash_map::block_cache<HashMap>& b)
{
    a.swap(b);
}

} // namespace std

#endif // !STXXL_CONTAINERS_HASH_MAP_BLOCK_CACHE_HEADER