rocksdb/memtable/hash_cuckoo_rep.cc

//  Copyright (c) 2011-present, Facebook, Inc.  All rights reserved.
//  This source code is licensed under both the GPLv2 (found in the
//  COPYING file in the root directory) and Apache 2.0 License
//  (found in the LICENSE.Apache file in the root directory).
//

#ifndef ROCKSDB_LITE
#include "memtable/hash_cuckoo_rep.h"

#include <algorithm>
#include <atomic>
#include <limits>
#include <memory>
#include <queue>
#include <string>
#include <vector>

#include "db/memtable.h"
#include "memtable/skiplist.h"
#include "memtable/stl_wrappers.h"
#include "port/port.h"
#include "rocksdb/memtablerep.h"
#include "util/murmurhash.h"

namespace rocksdb {
namespace {

// the default maximum size of the cuckoo path searching queue
static const int kCuckooPathMaxSearchSteps = 100;

struct CuckooStep {
  static const int kNullStep = -1;
  // the bucket id in the cuckoo array.
  int bucket_id_;
  // index of cuckoo-step array that points to its previous step,
  // -1 if it the beginning step.
  int prev_step_id_;
  // the depth of the current step.
  unsigned int depth_;

  CuckooStep() : bucket_id_(-1), prev_step_id_(kNullStep), depth_(1) {}

  CuckooStep(CuckooStep&& o) = default;

  CuckooStep& operator=(CuckooStep&& rhs) {
    bucket_id_ = std::move(rhs.bucket_id_);
    prev_step_id_ = std::move(rhs.prev_step_id_);
    depth_ = std::move(rhs.depth_);
    return *this;
  }

  CuckooStep(const CuckooStep&) = delete;
  CuckooStep& operator=(const CuckooStep&) = delete;

  CuckooStep(int bucket_id, int prev_step_id, int depth)
      : bucket_id_(bucket_id), prev_step_id_(prev_step_id), depth_(depth) {}
};

class HashCuckooRep : public MemTableRep {
 public:
  explicit HashCuckooRep(const MemTableRep::KeyComparator& compare,
                         Allocator* allocator, const size_t bucket_count,
                         const unsigned int hash_func_count,
                         const size_t approximate_entry_size)
      : MemTableRep(allocator),
        compare_(compare),
        allocator_(allocator),
        bucket_count_(bucket_count),
        approximate_entry_size_(approximate_entry_size),
        cuckoo_path_max_depth_(kDefaultCuckooPathMaxDepth),
        occupied_count_(0),
        hash_function_count_(hash_func_count),
        backup_table_(nullptr) {
    char* mem = reinterpret_cast<char*>(
        allocator_->Allocate(sizeof(std::atomic<const char*>) * bucket_count_));
    cuckoo_array_ = new (mem) std::atomic<char*>[bucket_count_];
    for (unsigned int bid = 0; bid < bucket_count_; ++bid) {
      cuckoo_array_[bid].store(nullptr, std::memory_order_relaxed);
    }

    cuckoo_path_ = reinterpret_cast<int*>(
        allocator_->Allocate(sizeof(int) * (cuckoo_path_max_depth_ + 1)));
    is_nearly_full_ = false;
  }

  // return false, indicating HashCuckooRep does not support merge operator.
  virtual bool IsMergeOperatorSupported() const override { return false; }

  // return false, indicating HashCuckooRep does not support snapshot.
  virtual bool IsSnapshotSupported() const override { return false; }

  // Returns true iff an entry that compares equal to key is in the collection.
  virtual bool Contains(const char* internal_key) const override;

  virtual ~HashCuckooRep() override {}

  // Insert the specified key (internal_key) into the mem-table.  Assertion
  // fails if
  // the current mem-table already contains the specified key.
  virtual void Insert(KeyHandle handle) override;

  // This function returns bucket_count_ * approximate_entry_size_ when any
  // of the followings happen to disallow further write operations:
  // 1. when the fullness reaches kMaxFullnes.
  // 2. when the backup_table_ is used.
  //
  // otherwise, this function will always return 0.
  virtual size_t ApproximateMemoryUsage() override {
    if (is_nearly_full_) {
      return bucket_count_ * approximate_entry_size_;
    }
    return 0;
  }

  virtual void Get(const LookupKey& k, void* callback_args,
                   bool (*callback_func)(void* arg,
                                         const char* entry)) override;

  class Iterator : public MemTableRep::Iterator {
    std::shared_ptr<std::vector<const char*>> bucket_;
    std::vector<const char*>::const_iterator mutable cit_;
    const KeyComparator& compare_;
    std::string tmp_;  // For passing to EncodeKey
    bool mutable sorted_;
    void DoSort() const;

   public:
    explicit Iterator(std::shared_ptr<std::vector<const char*>> bucket,
                      const KeyComparator& compare);

    // Initialize an iterator over the specified collection.
    // The returned iterator is not valid.
    // explicit Iterator(const MemTableRep* collection);
    virtual ~Iterator() override{};

    // Returns true iff the iterator is positioned at a valid node.
    virtual bool Valid() const override;

    // Returns the key at the current position.
    // REQUIRES: Valid()
    virtual const char* key() const override;

    // Advances to the next position.
    // REQUIRES: Valid()
    virtual void Next() override;

    // Advances to the previous position.
    // REQUIRES: Valid()
    virtual void Prev() override;

    // Advance to the first entry with a key >= target
    virtual void Seek(const Slice& user_key, const char* memtable_key) override;

    // Retreat to the last entry with a key <= target
    virtual void SeekForPrev(const Slice& user_key,
                             const char* memtable_key) override;

    // Position at the first entry in collection.
    // Final state of iterator is Valid() iff collection is not empty.
    virtual void SeekToFirst() override;

    // Position at the last entry in collection.
    // Final state of iterator is Valid() iff collection is not empty.
    virtual void SeekToLast() override;
  };

  struct CuckooStepBuffer {
    CuckooStepBuffer() : write_index_(0), read_index_(0) {}
    ~CuckooStepBuffer() {}

    int write_index_;
    int read_index_;
    CuckooStep steps_[kCuckooPathMaxSearchSteps];

    CuckooStep& NextWriteBuffer() { return steps_[write_index_++]; }

    inline const CuckooStep& ReadNext() { return steps_[read_index_++]; }

    inline bool HasNewWrite() { return write_index_ > read_index_; }

    inline void reset() {
      write_index_ = 0;
      read_index_ = 0;
    }

    inline bool IsFull() { return write_index_ >= kCuckooPathMaxSearchSteps; }

    // returns the number of steps that has been read
    inline int ReadCount() { return read_index_; }

    // returns the number of steps that has been written to the buffer.
    inline int WriteCount() { return write_index_; }
  };

 private:
  const MemTableRep::KeyComparator& compare_;
  // the pointer to Allocator to allocate memory, immutable after construction.
  Allocator* const allocator_;
  // the number of hash bucket in the hash table.
  const size_t bucket_count_;
  // approximate size of each entry
  const size_t approximate_entry_size_;
  // the maxinum depth of the cuckoo path.
  const unsigned int cuckoo_path_max_depth_;
  // the current number of entries in cuckoo_array_ which has been occupied.
  size_t occupied_count_;
  // the current number of hash functions used in the cuckoo hash.
  unsigned int hash_function_count_;
  // the backup MemTableRep to handle the case where cuckoo hash cannot find
  // a vacant bucket for inserting the key of a put request.
  std::shared_ptr<MemTableRep> backup_table_;
  // the array to store pointers, pointing to the actual data.
  std::atomic<char*>* cuckoo_array_;
  // a buffer to store cuckoo path
  int* cuckoo_path_;
  // a boolean flag indicating whether the fullness of bucket array
  // reaches the point to make the current memtable immutable.
  bool is_nearly_full_;

  // the default maximum depth of the cuckoo path.
  static const unsigned int kDefaultCuckooPathMaxDepth = 10;

  CuckooStepBuffer step_buffer_;

  // returns the bucket id assogied to the input slice based on the
  unsigned int GetHash(const Slice& slice, const int hash_func_id) const {
    // the seeds used in the Murmur hash to produce different hash functions.
    static const int kMurmurHashSeeds[HashCuckooRepFactory::kMaxHashCount] = {
        545609244,  1769731426, 763324157,  13099088,   592422103,
        1899789565, 248369300,  1984183468, 1613664382, 1491157517};
    return static_cast<unsigned int>(
        MurmurHash(slice.data(), static_cast<int>(slice.size()),
                   kMurmurHashSeeds[hash_func_id]) %
        bucket_count_);
  }

  // A cuckoo path is a sequence of bucket ids, where each id points to a
  // location of cuckoo_array_.  This path describes the displacement sequence
  // of entries in order to store the desired data specified by the input user
  // key.  The path starts from one of the locations associated with the
  // specified user key and ends at a vacant space in the cuckoo array. This
  // function will update the cuckoo_path.
  //
  // @return true if it found a cuckoo path.
  bool FindCuckooPath(const char* internal_key, const Slice& user_key,
                      int* cuckoo_path, size_t* cuckoo_path_length,
                      int initial_hash_id = 0);

  // Perform quick insert by checking whether there is a vacant bucket in one
  // of the possible locations of the input key.  If so, then the function will
  // return true and the key will be stored in that vacant bucket.
  //
  // This function is a helper function of FindCuckooPath that discovers the
  // first possible steps of a cuckoo path.  It begins by first computing
  // the possible locations of the input keys (and stores them in bucket_ids.)
  // Then, if one of its possible locations is vacant, then the input key will
  // be stored in that vacant space and the function will return true.
  // Otherwise, the function will return false indicating a complete search
  // of cuckoo-path is needed.
  bool QuickInsert(const char* internal_key, const Slice& user_key,
                   int bucket_ids[], const int initial_hash_id);

  // Returns the pointer to the internal iterator to the buckets where buckets
  // are sorted according to the user specified KeyComparator.  Note that
  // any insert after this function call may affect the sorted nature of
  // the returned iterator.
  virtual MemTableRep::Iterator* GetIterator(Arena* arena) override {
    std::vector<const char*> compact_buckets;
    for (unsigned int bid = 0; bid < bucket_count_; ++bid) {
      const char* bucket = cuckoo_array_[bid].load(std::memory_order_relaxed);
      if (bucket != nullptr) {
        compact_buckets.push_back(bucket);
      }
    }
    MemTableRep* backup_table = backup_table_.get();
    if (backup_table != nullptr) {
      std::unique_ptr<MemTableRep::Iterator> iter(backup_table->GetIterator());
      for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
        compact_buckets.push_back(iter->key());
      }
    }
    if (arena == nullptr) {
      return new Iterator(
          std::shared_ptr<std::vector<const char*>>(
              new std::vector<const char*>(std::move(compact_buckets))),
          compare_);
    } else {
      auto mem = arena->AllocateAligned(sizeof(Iterator));
      return new (mem) Iterator(
          std::shared_ptr<std::vector<const char*>>(
              new std::vector<const char*>(std::move(compact_buckets))),
          compare_);
    }
  }
};

void HashCuckooRep::Get(const LookupKey& key, void* callback_args,
                        bool (*callback_func)(void* arg, const char* entry)) {
  Slice user_key = key.user_key();
  for (unsigned int hid = 0; hid < hash_function_count_; ++hid) {
    const char* bucket =
        cuckoo_array_[GetHash(user_key, hid)].load(std::memory_order_acquire);
    if (bucket != nullptr) {
      Slice bucket_user_key = UserKey(bucket);
      if (user_key == bucket_user_key) {
        callback_func(callback_args, bucket);
        break;
      }
    } else {
      // as Put() always stores at the vacant bucket located by the
      // hash function with the smallest possible id, when we first
      // find a vacant bucket in Get(), that means a miss.
      break;
    }
  }
  MemTableRep* backup_table = backup_table_.get();
  if (backup_table != nullptr) {
    backup_table->Get(key, callback_args, callback_func);
  }
}

void HashCuckooRep::Insert(KeyHandle handle) {
  static const float kMaxFullness = 0.90f;

  auto* key = static_cast<char*>(handle);
  int initial_hash_id = 0;
  size_t cuckoo_path_length = 0;
  auto user_key = UserKey(key);
  // find cuckoo path
  if (FindCuckooPath(key, user_key, cuckoo_path_, &cuckoo_path_length,
                     initial_hash_id) == false) {
    // if true, then we can't find a vacant bucket for this key even we
    // have used up all the hash functions.  Then use a backup memtable to
    // store such key, which will further make this mem-table become
    // immutable.
    if (backup_table_.get() == nullptr) {
      VectorRepFactory factory(10);
      backup_table_.reset(
          factory.CreateMemTableRep(compare_, allocator_, nullptr, nullptr));
      is_nearly_full_ = true;
    }
    backup_table_->Insert(key);
    return;
  }
  // when reaching this point, means the insert can be done successfully.
  occupied_count_++;
  if (occupied_count_ >= bucket_count_ * kMaxFullness) {
    is_nearly_full_ = true;
  }

  // perform kickout process if the length of cuckoo path > 1.
  if (cuckoo_path_length == 0) return;

  // the cuckoo path stores the kickout path in reverse order.
  // so the kickout or displacement is actually performed
  // in reverse order, which avoids false-negatives on read
  // by moving each key involved in the cuckoo path to the new
  // location before replacing it.
  for (size_t i = 1; i < cuckoo_path_length; ++i) {
    int kicked_out_bid = cuckoo_path_[i - 1];
    int current_bid = cuckoo_path_[i];
    // since we only allow one writer at a time, it is safe to do relaxed read.
    cuckoo_array_[kicked_out_bid]
        .store(cuckoo_array_[current_bid].load(std::memory_order_relaxed),
               std::memory_order_release);
  }
  int insert_key_bid = cuckoo_path_[cuckoo_path_length - 1];
  cuckoo_array_[insert_key_bid].store(key, std::memory_order_release);
}

bool HashCuckooRep::Contains(const char* internal_key) const {
  auto user_key = UserKey(internal_key);
  for (unsigned int hid = 0; hid < hash_function_count_; ++hid) {
    const char* stored_key =
        cuckoo_array_[GetHash(user_key, hid)].load(std::memory_order_acquire);
    if (stored_key != nullptr) {
      if (compare_(internal_key, stored_key) == 0) {
        return true;
      }
    }
  }
  return false;
}

bool HashCuckooRep::QuickInsert(const char* internal_key, const Slice& user_key,
                                int bucket_ids[], const int initial_hash_id) {
  int cuckoo_bucket_id = -1;

  // Below does the followings:
  // 0. Calculate all possible locations of the input key.
  // 1. Check if there is a bucket having same user_key as the input does.
  // 2. If there exists such bucket, then replace this bucket by the newly
  //    insert data and return.  This step also performs duplication check.
  // 3. If no such bucket exists but exists a vacant bucket, then insert the
  //    input data into it.
  // 4. If step 1 to 3 all fail, then return false.
  for (unsigned int hid = initial_hash_id; hid < hash_function_count_; ++hid) {
    bucket_ids[hid] = GetHash(user_key, hid);
    // since only one PUT is allowed at a time, and this is part of the PUT
    // operation, so we can safely perform relaxed load.
    const char* stored_key =
        cuckoo_array_[bucket_ids[hid]].load(std::memory_order_relaxed);
    if (stored_key == nullptr) {
      if (cuckoo_bucket_id == -1) {
        cuckoo_bucket_id = bucket_ids[hid];
      }
    } else {
      const auto bucket_user_key = UserKey(stored_key);
      if (bucket_user_key.compare(user_key) == 0) {
        cuckoo_bucket_id = bucket_ids[hid];
        break;
      }
    }
  }

  if (cuckoo_bucket_id != -1) {
    cuckoo_array_[cuckoo_bucket_id].store(const_cast<char*>(internal_key),
                                          std::memory_order_release);
    return true;
  }

  return false;
}

// Perform pre-check and find the shortest cuckoo path.  A cuckoo path
// is a displacement sequence for inserting the specified input key.
//
// @return true if it successfully found a vacant space or cuckoo-path.
//     If the return value is true but the length of cuckoo_path is zero,
//     then it indicates that a vacant bucket or an bucket with matched user
//     key with the input is found, and a quick insertion is done.
bool HashCuckooRep::FindCuckooPath(const char* internal_key,
                                   const Slice& user_key, int* cuckoo_path,
                                   size_t* cuckoo_path_length,
                                   const int initial_hash_id) {
  int bucket_ids[HashCuckooRepFactory::kMaxHashCount];
  *cuckoo_path_length = 0;

  if (QuickInsert(internal_key, user_key, bucket_ids, initial_hash_id)) {
    return true;
  }
  // If this step is reached, then it means:
  // 1. no vacant bucket in any of the possible locations of the input key.
  // 2. none of the possible locations of the input key has the same user
  //    key as the input `internal_key`.

  // the front and back indices for the step_queue_
  step_buffer_.reset();

  for (unsigned int hid = initial_hash_id; hid < hash_function_count_; ++hid) {
    /// CuckooStep& current_step = step_queue_[front_pos++];
    CuckooStep& current_step = step_buffer_.NextWriteBuffer();
    current_step.bucket_id_ = bucket_ids[hid];
    current_step.prev_step_id_ = CuckooStep::kNullStep;
    current_step.depth_ = 1;
  }

  while (step_buffer_.HasNewWrite()) {
    int step_id = step_buffer_.read_index_;
    const CuckooStep& step = step_buffer_.ReadNext();
    // Since it's a BFS process, then the first step with its depth deeper
    // than the maximum allowed depth indicates all the remaining steps
    // in the step buffer queue will all exceed the maximum depth.
    // Return false immediately indicating we can't find a vacant bucket
    // for the input key before the maximum allowed depth.
    if (step.depth_ >= cuckoo_path_max_depth_) {
      return false;
    }
    // again, we can perform no barrier load safely here as the current
    // thread is the only writer.
    Slice bucket_user_key =
        UserKey(cuckoo_array_[step.bucket_id_].load(std::memory_order_relaxed));
    if (step.prev_step_id_ != CuckooStep::kNullStep) {
      if (bucket_user_key == user_key) {
        // then there is a loop in the current path, stop discovering this path.
        continue;
      }
    }
    // if the current bucket stores at its nth location, then we only consider
    // its mth location where m > n.  This property makes sure that all reads
    // will not miss if we do have data associated to the query key.
    //
    // The n and m in the above statement is the start_hid and hid in the code.
    unsigned int start_hid = hash_function_count_;
    for (unsigned int hid = 0; hid < hash_function_count_; ++hid) {
      bucket_ids[hid] = GetHash(bucket_user_key, hid);
      if (step.bucket_id_ == bucket_ids[hid]) {
        start_hid = hid;
      }
    }
    // must found a bucket which is its current "home".
    assert(start_hid != hash_function_count_);

    // explore all possible next steps from the current step.
    for (unsigned int hid = start_hid + 1; hid < hash_function_count_; ++hid) {
      CuckooStep& next_step = step_buffer_.NextWriteBuffer();
      next_step.bucket_id_ = bucket_ids[hid];
      next_step.prev_step_id_ = step_id;
      next_step.depth_ = step.depth_ + 1;
      // once a vacant bucket is found, trace back all its previous steps
      // to generate a cuckoo path.
      if (cuckoo_array_[next_step.bucket_id_].load(std::memory_order_relaxed) ==
          nullptr) {
        // store the last step in the cuckoo path.  Note that cuckoo_path
        // stores steps in reverse order.  This allows us to move keys along
        // the cuckoo path by storing each key to the new place first before
        // removing it from the old place.  This property ensures reads will
        // not missed due to moving keys along the cuckoo path.
        cuckoo_path[(*cuckoo_path_length)++] = next_step.bucket_id_;
        int depth;
        for (depth = step.depth_; depth > 0 && step_id != CuckooStep::kNullStep;
             depth--) {
          const CuckooStep& prev_step = step_buffer_.steps_[step_id];
          cuckoo_path[(*cuckoo_path_length)++] = prev_step.bucket_id_;
          step_id = prev_step.prev_step_id_;
        }
        assert(depth == 0 && step_id == CuckooStep::kNullStep);
        return true;
      }
      if (step_buffer_.IsFull()) {
        // if true, then it reaches maxinum number of cuckoo search steps.
        return false;
      }
    }
  }

  // tried all possible paths but still not unable to find a cuckoo path
  // which path leads to a vacant bucket.
  return false;
}

HashCuckooRep::Iterator::Iterator(
    std::shared_ptr<std::vector<const char*>> bucket,
    const KeyComparator& compare)
    : bucket_(bucket),
      cit_(bucket_->end()),
      compare_(compare),
      sorted_(false) {}

void HashCuckooRep::Iterator::DoSort() const {
  if (!sorted_) {
    std::sort(bucket_->begin(), bucket_->end(),
              stl_wrappers::Compare(compare_));
    cit_ = bucket_->begin();
    sorted_ = true;
  }
}

// Returns true iff the iterator is positioned at a valid node.
bool HashCuckooRep::Iterator::Valid() const {
  DoSort();
  return cit_ != bucket_->end();
}

// Returns the key at the current position.
// REQUIRES: Valid()
const char* HashCuckooRep::Iterator::key() const {
  assert(Valid());
  return *cit_;
}

// Advances to the next position.
// REQUIRES: Valid()
void HashCuckooRep::Iterator::Next() {
  assert(Valid());
  if (cit_ == bucket_->end()) {
    return;
  }
  ++cit_;
}

// Advances to the previous position.
// REQUIRES: Valid()
void HashCuckooRep::Iterator::Prev() {
  assert(Valid());
  if (cit_ == bucket_->begin()) {
    // If you try to go back from the first element, the iterator should be
    // invalidated. So we set it to past-the-end. This means that you can
    // treat the container circularly.
    cit_ = bucket_->end();
  } else {
    --cit_;
  }
}

// Advance to the first entry with a key >= target
void HashCuckooRep::Iterator::Seek(const Slice& user_key,
                                   const char* memtable_key) {
  DoSort();
  // Do binary search to find first value not less than the target
  const char* encoded_key =
      (memtable_key != nullptr) ? memtable_key : EncodeKey(&tmp_, user_key);
  cit_ = std::equal_range(bucket_->begin(), bucket_->end(), encoded_key,
                          [this](const char* a, const char* b) {
                            return compare_(a, b) < 0;
                          }).first;
}

// Retreat to the last entry with a key <= target
void HashCuckooRep::Iterator::SeekForPrev(const Slice& user_key,
                                          const char* memtable_key) {
  assert(false);
}

// Position at the first entry in collection.
// Final state of iterator is Valid() iff collection is not empty.
void HashCuckooRep::Iterator::SeekToFirst() {
  DoSort();
  cit_ = bucket_->begin();
}

// Position at the last entry in collection.
// Final state of iterator is Valid() iff collection is not empty.
void HashCuckooRep::Iterator::SeekToLast() {
  DoSort();
  cit_ = bucket_->end();
  if (bucket_->size() != 0) {
    --cit_;
  }
}

}  // anom namespace

MemTableRep* HashCuckooRepFactory::CreateMemTableRep(
    const MemTableRep::KeyComparator& compare, Allocator* allocator,
    const SliceTransform* transform, Logger* logger) {
  // The estimated average fullness.  The write performance of any close hash
  // degrades as the fullness of the mem-table increases.  Setting kFullness
  // to a value around 0.7 can better avoid write performance degradation while
  // keeping efficient memory usage.
  static const float kFullness = 0.7f;
  size_t pointer_size = sizeof(std::atomic<const char*>);
  assert(write_buffer_size_ >= (average_data_size_ + pointer_size));
  size_t bucket_count =
    static_cast<size_t>(
      (write_buffer_size_ / (average_data_size_ + pointer_size)) / kFullness +
      1);
  unsigned int hash_function_count = hash_function_count_;
  if (hash_function_count < 2) {
    hash_function_count = 2;
  }
  if (hash_function_count > kMaxHashCount) {
    hash_function_count = kMaxHashCount;
  }
  return new HashCuckooRep(compare, allocator, bucket_count,
                           hash_function_count,
                           static_cast<size_t>(
                             (average_data_size_ + pointer_size) / kFullness)
                           );
}

MemTableRepFactory* NewHashCuckooRepFactory(size_t write_buffer_size,
                                            size_t average_data_size,
                                            unsigned int hash_function_count) {
  return new HashCuckooRepFactory(write_buffer_size, average_data_size,
                                  hash_function_count);
}

}  // namespace rocksdb
#endif  // ROCKSDB_LITE