前言
该实验要求实现一个基于线性探测法的哈希表，但是与直接放在内存中的哈希表不同的是，该实验假设哈希表非常大，无法整个放入内存中，因此需要将哈希表进行分割，将多个键值对放在一个 Page 中，然后搭配上一个实验实现的 Buffer Pool Manager 一起食用。哈希表的大致结构如下图所示：
下面介绍如何实现一个线程安全的哈希表。
代码实现
Page 布局
从上图可以看出，多个键值对被放在 Page 里面，作为 Page 的数据存在磁盘中。为了更好地组织和管理这些键值对，实验任务一要求我们实现两个类：HashTableHeaderPage 和 HashTableBlockPage，HashTableHeaderPage 保存着 block index 到 page id 的映射关系以及其他哈希表元数据，每个哈希表只有一个 HashTableHeaderPage，而 HashTableBlockPage 可以有多个。
Hash Table Header Page
HashTableHeaderPage 有以下几个字段：
| 字段 | 大小 | 描述 |
|---|---|---|
lsn_ |
4 bytes | Log sequence number (Used in Project 4) |
size_ |
4 bytes | Number of Key & Value pairs the hash table can hold |
page_id_ |
4 bytes | Self Page Id |
next_ind_ |
4 bytes | The next index to add a new entry to block_page_ids_ |
block_page_ids_ |
4080 bytes | Array of block page_id_t |
这些字段总共 4096 字节，正好是一个 Page 的大小，在 src/include/common/config.h 中可以修改 PAGE_SIZE 的大小。该类的实现代码如下：
namespace bustub { page_id_t HashTableHeaderPage::GetBlockPageId(size_t index) { assert(index < next_ind_); return block_page_ids_[index]; } page_id_t HashTableHeaderPage::GetPageId() const { return page_id_; } void HashTableHeaderPage::SetPageId(bustub::page_id_t page_id) { page_id_ = page_id; } lsn_t HashTableHeaderPage::GetLSN() const { return lsn_; } void HashTableHeaderPage::SetLSN(lsn_t lsn) { lsn_ = lsn; } void HashTableHeaderPage::AddBlockPageId(page_id_t page_id) { block_page_ids_[next_ind_++] = page_id; } size_t HashTableHeaderPage::NumBlocks() { return next_ind_; } void HashTableHeaderPage::SetSize(size_t size) { size_ = size; } size_t HashTableHeaderPage::GetSize() const { return size_; } } // namespace bustub Hash Table Block Page
HashTableBlockPage 包含多个 slot，用于保存键值对，所以该类定义了查询、插入和删除键值对的函数。为了跟踪每个 slot 的使用情况，该类包含以下三个数据成员：
occupied_: 第 i 位置 1 表示 Page 的第 i 个 slot 上存储了键值对或者之前存了键值对但之后被删除了（起到墓碑的作用）readable_: 第 i 位置 1 表示 Page 的第 i 个 slot 上存储了键值对array_: 用于保存键值对的数组
每个键值对的大小为 sizeof(std::pair<KeyType, ValueType>) 字节（记为 PS），每个键值对对应两个 bit（occupied 和 readable）即 1/4 个字节，所以一个 Page 最多能保存 BLOCK_ARRAY_SIZE = PAGE_SIZE / (PS + 1/4) 个键值对，即每个 Page 有 BLOCK_ARRAY_SIZE 个 slot。
由于 occupied_ 和 readable_ 被定义为 char 数组，所以需要两个辅助函数 GetBit 和 SetBit 来访问第 i 位的比特。
namespace bustub { /** * Store indexed key and and value together within block page. Supports * non-unique keys. * * Block page format (keys are stored in order): * ---------------------------------------------------------------- * | KEY(1) + VALUE(1) | KEY(2) + VALUE(2) | ... | KEY(n) + VALUE(n) * ---------------------------------------------------------------- * * Here '+' means concatenation. * */ template <typename KeyType, typename ValueType, typename KeyComparator> class HashTableBlockPage { public: // Delete all constructor / destructor to ensure memory safety HashTableBlockPage() = delete; KeyType KeyAt(slot_offset_t bucket_ind) const; ValueType ValueAt(slot_offset_t bucket_ind) const; bool Insert(slot_offset_t bucket_ind, const KeyType &key, const ValueType &value); void Remove(slot_offset_t bucket_ind); bool IsOccupied(slot_offset_t bucket_ind) const; bool IsReadable(slot_offset_t bucket_ind) const; private: bool GetBit(const std::atomic_char *array, slot_offset_t bucket_ind) const; void SetBit(std::atomic_char *array, slot_offset_t bucket_ind, bool value); std::atomic_char occupied_[(BLOCK_ARRAY_SIZE - 1) / 8 + 1]; // 0 if tombstone/brand new (never occupied), 1 otherwise. std::atomic_char readable_[(BLOCK_ARRAY_SIZE - 1) / 8 + 1]; MappingType array_[0]; }; } // namespace bustub 实现代码如下：
namespace bustub { template <typename KeyType, typename ValueType, typename KeyComparator> KeyType HASH_TABLE_BLOCK_TYPE::KeyAt(slot_offset_t bucket_ind) const { return array_[bucket_ind].first; } template <typename KeyType, typename ValueType, typename KeyComparator> ValueType HASH_TABLE_BLOCK_TYPE::ValueAt(slot_offset_t bucket_ind) const { return array_[bucket_ind].second; } template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_BLOCK_TYPE::Insert(slot_offset_t bucket_ind, const KeyType &key, const ValueType &value) { if (IsReadable(bucket_ind)) { return false; } array_[bucket_ind] = {key, value}; SetBit(readable_, bucket_ind, true); SetBit(occupied_, bucket_ind, true); return true; } template <typename KeyType, typename ValueType, typename KeyComparator> void HASH_TABLE_BLOCK_TYPE::Remove(slot_offset_t bucket_ind) { SetBit(readable_, bucket_ind, false); } template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_BLOCK_TYPE::IsOccupied(slot_offset_t bucket_ind) const { return GetBit(occupied_, bucket_ind); } template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_BLOCK_TYPE::IsReadable(slot_offset_t bucket_ind) const { return GetBit(readable_, bucket_ind); } template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_BLOCK_TYPE::GetBit(const std::atomic_char *array, slot_offset_t bucket_ind) const { return array[bucket_ind / 8] & (1 << bucket_ind % 8); } template <typename KeyType, typename ValueType, typename KeyComparator> void HASH_TABLE_BLOCK_TYPE::SetBit(std::atomic_char *array, slot_offset_t bucket_ind, bool value) { if (value) { array[bucket_ind / 8] |= (1 << bucket_ind % 8); } else { array[bucket_ind / 8] &= ~(1 << bucket_ind % 8); } } // DO NOT REMOVE ANYTHING BELOW THIS LINE template class HashTableBlockPage<int, int, IntComparator>; template class HashTableBlockPage<GenericKey<4>, RID, GenericComparator<4>>; template class HashTableBlockPage<GenericKey<8>, RID, GenericComparator<8>>; template class HashTableBlockPage<GenericKey<16>, RID, GenericComparator<16>>; template class HashTableBlockPage<GenericKey<32>, RID, GenericComparator<32>>; template class HashTableBlockPage<GenericKey<64>, RID, GenericComparator<64>>; } // namespace bustub 哈希表
声明
实验要求我们实现哈希表的插入、查找、删除和调整大小的的操作，对应的类声明如下，为了完成这些操作，我们多定义了几个私有的辅助函数和成员变量：
namespace bustub { #define HASH_TABLE_TYPE LinearProbeHashTable<KeyType, ValueType, KeyComparator> template <typename KeyType, typename ValueType, typename KeyComparator> class LinearProbeHashTable : public HashTable<KeyType, ValueType, KeyComparator> { public: explicit LinearProbeHashTable(const std::string &name, BufferPoolManager *buffer_pool_manager, const KeyComparator &comparator, size_t num_buckets, HashFunction<KeyType> hash_fn); bool Insert(Transaction *transaction, const KeyType &key, const ValueType &value) override; bool Remove(Transaction *transaction, const KeyType &key, const ValueType &value) override; bool GetValue(Transaction *transaction, const KeyType &key, std::vector<ValueType> *result) override; void Resize(size_t initial_size); size_t GetSize(); private: using slot_index_t = size_t; using block_index_t = size_t; enum class LockType { READ = 0, WRITE = 1 }; /** * initialize header page and allocate block pages for it * @param page the hash table header page */ void InitHeaderPage(HashTableHeaderPage *page); /** * get index according to key * @param key the key to be hashed * @return a tuple contains slot index, block page index and bucket index */ std::tuple<slot_index_t, block_index_t, slot_offset_t> GetIndex(const KeyType &key); /** * linear probe step forward * @param bucket_index the bucket index * @param block_index the hash table block page index * @param header_page hash table header page * @param raw_block_page raw hash table block page * @param block_page hash table block page * @param lock_type lock type of block page */ void StepForward(slot_offset_t &bucket_index, block_index_t &block_index, Page *&raw_block_page, HASH_TABLE_BLOCK_TYPE *&block_page, LockType lockType); bool InsertImpl(Transaction *transaction, const KeyType &key, const ValueType &value); inline bool IsMatch(HASH_TABLE_BLOCK_TYPE *block_page, slot_offset_t bucket_index, const KeyType &key, const ValueType &value) { return !comparator_(key, block_page->KeyAt(bucket_index)) && value == block_page->ValueAt(bucket_index); } inline HashTableHeaderPage *HeaderPageCast(Page *page) { return reinterpret_cast<HashTableHeaderPage *>(page->GetData()); } inline HASH_TABLE_BLOCK_TYPE *BlockPageCast(Page *page) { return reinterpret_cast<HASH_TABLE_BLOCK_TYPE *>(page->GetData()); } /** * get the slot number of hash table block page * @param block_index the index of hash table block page * @return the slot number of block page */ inline size_t GetBlockArraySize(block_index_t block_index){ return block_index < num_pages_ - 1 ? BLOCK_ARRAY_SIZE : last_block_array_size_; } // member variable page_id_t header_page_id_; BufferPoolManager *buffer_pool_manager_; KeyComparator comparator_; std::vector<page_id_t> page_ids_; size_t num_buckets_; size_t num_pages_; size_t last_block_array_size_; // Readers includes inserts and removes, writer is only resize ReaderWriterLatch table_latch_; // Hash function HashFunction<KeyType> hash_fn_; }; } // namespace bustub 构造函数
在构造函数中负责根据用户指定的 num_buckets （也就是 slot 的数量）分配 Page，最后一个 Page 的 slot 数量可能少于前面的 Page。这里还将每个 HashTableBlockPage 对应的 page_id 保存到 page_ids_ 成员里面了，这样之后就不需要仅仅为了知道某个 HashTableBlockPage 的 page_id 而去找 BufferPoolManager 索要 HashTableHeaderPage。
template <typename KeyType, typename ValueType, typename KeyComparator> HASH_TABLE_TYPE::LinearProbeHashTable(const std::string &name, BufferPoolManager *buffer_pool_manager, const KeyComparator &comparator, size_t num_buckets, HashFunction<KeyType> hash_fn) : buffer_pool_manager_(buffer_pool_manager), comparator_(comparator), num_buckets_(num_buckets), num_pages_((num_buckets - 1) / BLOCK_ARRAY_SIZE + 1), last_block_array_size_(num_buckets - (num_pages_ - 1) * BLOCK_ARRAY_SIZE), hash_fn_(std::move(hash_fn)) { auto page = buffer_pool_manager->NewPage(&header_page_id_); page->WLatch(); InitHeaderPage(HeaderPageCast(page)); page->WUnlatch(); buffer_pool_manager_->UnpinPage(header_page_id_, true); } template <typename KeyType, typename ValueType, typename KeyComparator> void HASH_TABLE_TYPE::InitHeaderPage(HashTableHeaderPage *header_page) { header_page->SetPageId(header_page_id_); header_page->SetSize(num_buckets_); page_ids_.clear(); for (size_t i = 0; i < num_pages_; ++i) { page_id_t page_id; buffer_pool_manager_->NewPage(&page_id); buffer_pool_manager_->UnpinPage(page_id, false); header_page->AddBlockPageId(page_id); page_ids_.push_back(page_id); } } 查找
哈希表使用线性探测法进行键值对的查找，由于实验要求哈希表支持插入同键不同值的键值对，所以在线性探测过程中需要将所有相同键的值插入 result 向量中：
template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_TYPE::GetValue(Transaction *transaction, const KeyType &key, std::vector<ValueType> *result) { table_latch_.RLock(); // get slot index, block page index and bucket index according to key auto [slot_index, block_index, bucket_index] = GetIndex(key); // get block page that contains the key auto raw_block_page = buffer_pool_manager_->FetchPage(page_ids_[block_index]); raw_block_page->RLatch(); auto block_page = BlockPageCast(raw_block_page); // linear probe while (block_page->IsOccupied(bucket_index)) { // find the correct position if (block_page->IsReadable(bucket_index) && !comparator_(key, block_page->KeyAt(bucket_index))) { result->push_back(block_page->ValueAt(bucket_index)); } StepForward(bucket_index, block_index, raw_block_page, block_page, LockType::READ); // break loop if we have returned to original position if (block_index * BLOCK_ARRAY_SIZE + bucket_index == slot_index) { break; } } // unlock raw_block_page->RUnlatch(); buffer_pool_manager_->UnpinPage(raw_block_page->GetPageId(), false); table_latch_.RUnlock(); return result->size() > 0; } GetIndex 函数根据 key 计算出对应的 slot_index、block_index 和 bucket_index（就是 slot offset），结合上图就能理解该函数的工作原理了：
template <typename KeyType, typename ValueType, typename KeyComparator> auto HASH_TABLE_TYPE::GetIndex(const KeyType &key) -> std::tuple<slot_index_t, block_index_t, slot_offset_t> { slot_index_t slot_index = hash_fn_.GetHash(key) % num_buckets_; block_index_t block_index = slot_index / BLOCK_ARRAY_SIZE; slot_offset_t bucket_index = slot_index % BLOCK_ARRAY_SIZE; return {slot_index, block_index, bucket_index}; } 在线性探测过程中，我们可能从从一个 HashTableBlockPage 跳到下一个，这时候需要更新 bucket_index 和 block_index：
template <typename KeyType, typename ValueType, typename KeyComparator> void HASH_TABLE_TYPE::StepForward(slot_offset_t &bucket_index, block_index_t &block_index, Page *&raw_block_page, HASH_TABLE_BLOCK_TYPE *&block_page, LockType lockType) { if (++bucket_index != GetBlockArraySize(block_index)) { return; } // move to next block page if (lockType == LockType::READ) { raw_block_page->RUnlatch(); } else { raw_block_page->WUnlatch(); } buffer_pool_manager_->UnpinPage(page_ids_[block_index], false); // update index bucket_index = 0; block_index = (block_index + 1) % num_pages_; // update page raw_block_page = buffer_pool_manager_->FetchPage(page_ids_[block_index]); if (lockType == LockType::READ) { raw_block_page->RLatch(); } else { raw_block_page->WLatch(); } block_page = BlockPageCast(raw_block_page); } 插入
实验要求哈希表不允许插入已经存在的键值对，同时插入过程中如果回到了最初的位置，说明没有可用的 slot 用于插入键值对，这时需要将哈希表的大小翻倍。由于 Resize 的函数也要用到插入操作，如果直接调用 Insert 会出现死锁，所以这里使用 InsertImpl 来实现插入：
template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_TYPE::Insert(Transaction *transaction, const KeyType &key, const ValueType &value) { table_latch_.RLock(); auto success = InsertImpl(transaction, key, value); table_latch_.RUnlock(); return success; } template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_TYPE::InsertImpl(Transaction *transaction, const KeyType &key, const ValueType &value) { // get slot index, block page index and bucket index according to key auto [slot_index, block_index, bucket_index] = GetIndex(key); // get block page that contains the key auto raw_block_page = buffer_pool_manager_->FetchPage(page_ids_[block_index]); raw_block_page->WLatch(); auto block_page = BlockPageCast(raw_block_page); bool success = true; while (!block_page->Insert(bucket_index, key, value)) { // return false if (key, value) pair already exists if (block_page->IsReadable(bucket_index) && IsMatch(block_page, bucket_index, key, value)) { success = false; break; } StepForward(bucket_index, block_index, raw_block_page, block_page, LockType::WRITE); // resize hash table if we have returned to original position if (block_index * BLOCK_ARRAY_SIZE + bucket_index == slot_index) { raw_block_page->WUnlatch(); buffer_pool_manager_->UnpinPage(raw_block_page->GetPageId(), false); Resize(num_pages_); std::tie(slot_index, block_index, bucket_index) = GetIndex(key); raw_block_page = buffer_pool_manager_->FetchPage(page_ids_[block_index]); raw_block_page->WLatch(); block_page = BlockPageCast(raw_block_page); } } raw_block_page->WUnlatch(); buffer_pool_manager_->UnpinPage(raw_block_page->GetPageId(), success); return success; } 调整大小
由于实验假设哈希表很大，所以我们不能将原本的键值对全部保存到内存中，然后调整 HashTableHeaderPage 的大小，复用 HashTableBlockPage 并创建新的 Page，再把键值对重新插入。而是应该直接创建新的 HashTableHeaderPage 和 HashTableBlockPage ，并删除旧的哈希表页：
template <typename KeyType, typename ValueType, typename KeyComparator> void HASH_TABLE_TYPE::Resize(size_t initial_size) { table_latch_.WLock(); num_buckets_ = 2 * initial_size; num_pages_ = (num_buckets_ - 1) / BLOCK_ARRAY_SIZE + 1; last_block_array_size_ = num_buckets_ - (num_pages_ - 1) * BLOCK_ARRAY_SIZE; // save the old header page id auto old_header_page_id = header_page_id_; std::vector<page_id_t> old_page_ids(page_ids_); // get the new header page auto raw_header_page = buffer_pool_manager_->NewPage(&header_page_id_); raw_header_page->WLatch(); InitHeaderPage(HeaderPageCast(raw_header_page)); // move (key, value) pairs to new space for (size_t block_index = 0; block_index < num_pages_; ++block_index) { auto old_page_id = old_page_ids[block_index]; auto raw_block_page = buffer_pool_manager_->FetchPage(old_page_id); raw_block_page->RLatch(); auto block_page = BlockPageCast(raw_block_page); // move (key, value) pair from each readable slot for (slot_offset_t bucket_index = 0; bucket_index < GetBlockArraySize(block_index); ++bucket_index) { if (block_page->IsReadable(bucket_index)) { InsertImpl(nullptr, block_page->KeyAt(bucket_index), block_page->ValueAt(bucket_index)); } } // delete old page raw_block_page->RUnlatch(); buffer_pool_manager_->UnpinPage(old_page_id, false); buffer_pool_manager_->DeletePage(old_page_id); } raw_header_page->WUnlatch(); buffer_pool_manager_->UnpinPage(header_page_id_, false); buffer_pool_manager_->DeletePage(old_header_page_id); table_latch_.WUnlock(); } 删除
删除操作和查找操作很像，不过是将找到的 slot 标上墓碑罢了：
template <typename KeyType, typename ValueType, typename KeyComparator> bool HASH_TABLE_TYPE::Remove(Transaction *transaction, const KeyType &key, const ValueType &value) { table_latch_.RLock(); // get slot index, block page index and bucket index according to key auto [slot_index, block_index, bucket_index] = GetIndex(key); // get block page that contains the key auto raw_block_page = buffer_pool_manager_->FetchPage(page_ids_[block_index]); raw_block_page->WLatch(); auto block_page = BlockPageCast(raw_block_page); bool success = false; while (block_page->IsOccupied(bucket_index)) { // remove the (key, value) pair if find the matched readable one if (IsMatch(block_page, bucket_index, key, value)) { if (block_page->IsReadable(bucket_index)) { block_page->Remove(bucket_index); success = true; } else { success = false; } break; } // step forward StepForward(bucket_index, block_index, raw_block_page, block_page, LockType::WRITE); // break loop if we have returned to original position if (block_index * BLOCK_ARRAY_SIZE + bucket_index == slot_index) { break; } } raw_block_page->WUnlatch(); buffer_pool_manager_->UnpinPage(raw_block_page->GetPageId(), success); table_latch_.RUnlock(); return success; } 获取大小
最后是获取哈希表的大小操作，直接返回 num_buckets_ 就行了：
template <typename KeyType, typename ValueType, typename KeyComparator> size_t HASH_TABLE_TYPE::GetSize() { return num_buckets_; } 测试
对哈希表的测试结果如下，6 个测试全部通过了：
总结
该实验主要考察对线性探测哈希表、缓冲池管理器和读写锁的理解，难度相比上一个实验略有提升，但是理解了哈希表的结构图之后应该就不难完成该实验了，以上~~
