DataBlockHashIndex: Standalone Implementation with Unit Test (#4139)
Summary: The first step of the `DataBlockHashIndex` implementation. A string based hash table is implemented and unit-tested. `DataBlockHashIndexBuilder`: `Add()` takes pairs of `<key, restart_index>`, and formats it into a string when `Finish()` is called. `DataBlockHashIndex`: initialized by the formatted string, and can interpret it as a hash table. Lookup for a key is supported by iterator operation. Pull Request resolved: https://github.com/facebook/rocksdb/pull/4139 Reviewed By: sagar0 Differential Revision: D8866764 Pulled By: fgwu fbshipit-source-id: 7f015f0098632c65979a22898a50424384730b10main
Fenggang Wu
7 years ago
committed by
Facebook Github Bot
parent
ea212e5316
commit
8805ec2f49
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#include <string> |
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#include <vector> |
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#include "rocksdb/slice.h" |
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#include "table/data_block_hash_index.h" |
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#include "util/coding.h" |
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#include "util/hash.h" |
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namespace rocksdb { |
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const uint32_t kSeed = 2018; |
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const uint32_t kSeed_tag = 214; /* second hash seed */ |
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inline uint16_t HashToBucket(const Slice& s, uint16_t num_buckets) { |
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return static_cast<uint16_t>( |
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rocksdb::Hash(s.data(), s.size(), kSeed) % num_buckets); |
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} |
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void DataBlockHashIndexBuilder::Add(const Slice& key, |
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const uint16_t& restart_index) { |
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uint16_t idx = HashToBucket(key, num_buckets_); |
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/* push a TAG to avoid false postive */ |
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/* the TAG is the hash function value of another seed */ |
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uint16_t tag = static_cast<uint16_t>( |
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rocksdb::Hash(key.data(), key.size(), kSeed_tag)); |
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buckets_[idx].push_back(tag); |
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buckets_[idx].push_back(restart_index); |
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estimate_ += 2 * sizeof(uint16_t); |
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} |
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void DataBlockHashIndexBuilder::Finish(std::string& buffer) { |
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// offset is the byte offset within the buffer
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std::vector<uint16_t> bucket_offsets(num_buckets_, 0); |
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uint16_t map_start = static_cast<uint16_t>(buffer.size()); |
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// write each bucket to the string
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for (uint16_t i = 0; i < num_buckets_; i++) { |
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// remember the start offset of the buckets in bucket_offsets
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bucket_offsets[i] = static_cast<uint16_t>(buffer.size()); |
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for (uint16_t elem : buckets_[i]) { |
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// the elem is alternative "TAG" and "offset"
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PutFixed16(&buffer, elem); |
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} |
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} |
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// write the bucket_offsets
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for (uint16_t i = 0; i < num_buckets_; i++) { |
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PutFixed16(&buffer, bucket_offsets[i]); |
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} |
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// write NUM_BUCK
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PutFixed16(&buffer, num_buckets_); |
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// write MAP_START
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PutFixed16(&buffer, map_start); |
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// Because we use uint16_t address, we only support block less than 64KB
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assert(buffer.size() < (1 << 16)); |
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} |
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void DataBlockHashIndexBuilder::Reset() { |
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// buckets_.clear();
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std::fill(buckets_.begin(), buckets_.end(), std::vector<uint16_t>()); |
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estimate_ = 0; |
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} |
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DataBlockHashIndex::DataBlockHashIndex(Slice block_content) { |
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assert(block_content.size() >= |
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2 * sizeof(uint16_t)); // NUM_BUCK and MAP_START
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data_ = block_content.data(); |
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size_ = static_cast<uint16_t>(block_content.size()); |
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map_start_ = data_ + DecodeFixed16(data_ + size_ - sizeof(uint16_t)); |
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assert(map_start_ < data_ + size_); |
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num_buckets_ = DecodeFixed16(data_ + size_ - 2 * sizeof(uint16_t)); |
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assert(num_buckets_ > 0); |
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assert(size_ >= sizeof(uint16_t) * (2 + num_buckets_)); |
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bucket_table_ = data_ + size_ - sizeof(uint16_t) * (2 + num_buckets_); |
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assert(map_start_ < bucket_table_); |
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} |
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DataBlockHashIndexIterator* DataBlockHashIndex::NewIterator( |
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const Slice& key) const { |
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uint16_t idx = HashToBucket(key, num_buckets_); |
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uint16_t bucket_off = DecodeFixed16(bucket_table_ + idx * sizeof(uint16_t)); |
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const char* limit; |
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if (idx < num_buckets_ - 1) { |
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// limited by the start offset of the next bucket
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limit = data_ + DecodeFixed16(bucket_table_ + (idx + 1) * sizeof(uint16_t)); |
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} else { |
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// limited by the location of the NUM_BUCK
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limit = data_ + (size_ - 2 * sizeof(uint16_t)); |
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} |
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uint16_t tag = (uint16_t)rocksdb::Hash(key.data(), key.size(), kSeed_tag); |
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return new DataBlockHashIndexIterator(data_ + bucket_off, limit, tag); |
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} |
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bool DataBlockHashIndexIterator::Valid() { |
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return current_ < end_; |
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} |
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void DataBlockHashIndexIterator::Next() { |
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for (current_ += 2 * sizeof(uint16_t); current_ < end_; |
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current_ += 2 * sizeof(uint16_t)) { |
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// stop at a offset that match the tag, i.e. a possible match
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uint16_t tag_found = DecodeFixed16(current_); |
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if (tag_found == tag_) { |
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break; |
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} |
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} |
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} |
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uint16_t DataBlockHashIndexIterator::Value() { |
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return DecodeFixed16(current_ + sizeof(uint16_t)); |
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} |
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} // namespace rocksdb
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@ -0,0 +1,138 @@ |
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#pragma once |
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#include <string> |
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#include <vector> |
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namespace rocksdb { |
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// This is an experimental feature aiming to reduce the CPU utilization of
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// point-lookup within a data-block. It is not used in per-table index-blocks.
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// It supports Get(), but not Seek() or Scan(). If the key does not exist,
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// the iterator is set to invalid.
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//
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// A serialized hash index is appended to the data-block. The new block data
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// format is as follows:
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//
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// DATA_BLOCK: [RI RI RI ... RI RI_IDX HASH_IDX FOOTER]
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//
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// RI: Restart Interval (the same as the default data-block format)
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// RI_IDX: Restart Interval index (the same as the default data-block format)
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// HASH_IDX: The new data-block hash index feature.
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// FOOTER: A 32bit block footer, which is the NUM_RESTARTS with the MSB as
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// the flag indicating if this hash index is in use. Note that
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// given a data block < 32KB, the MSB is never used. So we can
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// borrow the MSB as the hash index flag. Besides, this format is
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// compatible with the legacy data-blocks < 32KB, as the MSB is 0.
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//
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// If we zoom in the HASH_IDX, the format of the data-block hash index is as
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// follows:
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//
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// HASH_IDX: [B B B ... B IDX NUM_BUCK MAP_START]
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//
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// B: B = bucket, an array of pairs <TAG, restart index>.
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// TAG is the second hash value of the string. It is used to flag a
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// matching entry among different keys that are hashed to the same
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// bucket. A similar tagging idea is used in [Lim et. al, SOSP'11].
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// However we have a differnet hash design that is not based on cuckoo
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// hashing as Lim's paper is.
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// We do not have to store the length of individual buckets, as they
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// are delimited by the next bucket offset.
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// IDX: Array of offsets of the index hash bucket (relative to MAP_START)
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// NUM_BUCK: Number of buckets, which is the length of the IDX array.
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// MAP_START: the starting offset of the data-block hash index.
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//
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// Each bucket B has the following structure:
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// [TAG RESTART_INDEX][TAG RESTART_INDEX]...[TAG RESTART_INDEX]
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// where TAG is the hash value of the second hash funtion.
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//
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// pairs of <key, restart index> are inserted to the hash index. Queries will
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// first lookup this hash index to find the restart index, then go to the
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// corresponding restart interval to search linearly for the key.
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//
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// For a point-lookup for a key K:
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//
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// Hash1()
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// 1) K ===========> bucket_id
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//
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// 2) Look up this bucket_id in the IDX table to find the offset of the bucket
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//
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// Hash2()
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// 3) K ============> TAG
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// 3) examine the first field (which is TAG) of each entry within this bucket,
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// skip those without a matching TAG.
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// 4) for the entries matching the TAG, get the restart interval index from the
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// second field.
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//
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// (following step are implemented in block.cc)
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// 5) lookup the restart index table (refer to the traditional block format),
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// use the restart interval index to find the offset of the restart interval.
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// 6) linearly search the restart interval for the key.
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//
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class DataBlockHashIndexBuilder { |
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public: |
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explicit DataBlockHashIndexBuilder(uint16_t n) |
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: num_buckets_(n), |
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buckets_(n), |
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estimate_((n + 2) * |
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sizeof(uint16_t) /* n buckets, 2 num at the end */) {} |
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void Add(const Slice& key, const uint16_t& restart_index); |
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void Finish(std::string& buffer); |
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void Reset(); |
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inline size_t EstimateSize() { return estimate_; } |
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private: |
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uint16_t num_buckets_; |
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std::vector<std::vector<uint16_t>> buckets_; |
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size_t estimate_; |
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}; |
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class DataBlockHashIndexIterator; |
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class DataBlockHashIndex { |
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public: |
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explicit DataBlockHashIndex(Slice block_content); |
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inline uint16_t DataBlockHashMapStart() const { |
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return static_cast<uint16_t>(map_start_ - data_); |
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} |
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DataBlockHashIndexIterator* NewIterator(const Slice& key) const; |
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private: |
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const char *data_; |
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// To make the serialized hash index compact and to save the space overhead,
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// here all the data fields persisted in the block are in uint16 format.
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// We find that a uint16 is large enough to index every offset of a 64KiB
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// block.
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// So in other words, DataBlockHashIndex does not support block size equal
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// or greater then 64KiB.
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uint16_t size_; |
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uint16_t num_buckets_; |
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const char *map_start_; // start of the map
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const char *bucket_table_; // start offset of the bucket index table
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}; |
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class DataBlockHashIndexIterator { |
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public: |
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DataBlockHashIndexIterator(const char* start, const char* end, |
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const uint16_t tag) |
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: end_(end), tag_(tag) { |
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current_ = start - 2 * sizeof(uint16_t); |
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Next(); |
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} |
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bool Valid(); |
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void Next(); |
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uint16_t Value(); |
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private: |
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const char* end_; // the end of the bucket
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const uint16_t tag_; // the fingerprint (2nd hash value) of the searching key
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const char* current_; |
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}; |
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} // namespace rocksdb
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@ -0,0 +1,177 @@ |
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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#include <cstdlib> |
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#include <string> |
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#include <unordered_map> |
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#include "rocksdb/slice.h" |
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#include "table/data_block_hash_index.h" |
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#include "util/testharness.h" |
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#include "util/testutil.h" |
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namespace rocksdb { |
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bool SearchForOffset(DataBlockHashIndex& index, const Slice& key, |
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uint16_t& restart_point) { |
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std::unique_ptr<DataBlockHashIndexIterator> iter; |
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iter.reset(index.NewIterator(key)); |
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for (; iter->Valid(); iter->Next()) { |
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if (iter->Value() == restart_point) { |
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return true; |
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} |
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} |
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return false; |
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} |
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TEST(DataBlockHashIndex, DataBlockHashTestSmall) { |
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// bucket_num = 5, #keys = 2. 40% utilization
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DataBlockHashIndexBuilder builder(5); |
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for (uint16_t i = 0; i < 2; i++) { |
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std::string key("key" + std::to_string(i)); |
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uint16_t restart_point = i; |
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builder.Add(key, restart_point); |
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} |
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size_t estimated_size = builder.EstimateSize(); |
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std::string buffer("fake"), buffer2; |
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size_t original_size = buffer.size(); |
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estimated_size += original_size; |
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builder.Finish(buffer); |
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ASSERT_EQ(buffer.size(), estimated_size); |
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buffer2 = buffer; // test for the correctness of relative offset
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Slice s(buffer2); |
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DataBlockHashIndex index(s); |
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// the additional hash map should start at the end of the buffer
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ASSERT_EQ(original_size, index.DataBlockHashMapStart()); |
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for (uint16_t i = 0; i < 2; i++) { |
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std::string key("key" + std::to_string(i)); |
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uint16_t restart_point = i; |
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ASSERT_TRUE(SearchForOffset(index, key, restart_point)); |
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} |
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} |
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TEST(DataBlockHashIndex, DataBlockHashTest) { |
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// bucket_num = 200, #keys = 100. 50% utilization
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DataBlockHashIndexBuilder builder(200); |
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for (uint16_t i = 0; i < 100; i++) { |
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std::string key("key" + std::to_string(i)); |
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uint16_t restart_point = i; |
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builder.Add(key, restart_point); |
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} |
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size_t estimated_size = builder.EstimateSize(); |
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std::string buffer("fake content"), buffer2; |
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size_t original_size = buffer.size(); |
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estimated_size += original_size; |
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builder.Finish(buffer); |
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ASSERT_EQ(buffer.size(), estimated_size); |
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buffer2 = buffer; // test for the correctness of relative offset
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Slice s(buffer2); |
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DataBlockHashIndex index(s); |
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// the additional hash map should start at the end of the buffer
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ASSERT_EQ(original_size, index.DataBlockHashMapStart()); |
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for (uint16_t i = 0; i < 100; i++) { |
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std::string key("key" + std::to_string(i)); |
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uint16_t restart_point = i; |
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ASSERT_TRUE(SearchForOffset(index, key, restart_point)); |
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} |
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} |
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TEST(DataBlockHashIndex, DataBlockHashTestCollision) { |
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// bucket_num = 2. There will be intense hash collisions
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DataBlockHashIndexBuilder builder(2); |
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for (uint16_t i = 0; i < 100; i++) { |
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std::string key("key" + std::to_string(i)); |
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uint16_t restart_point = i; |
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builder.Add(key, restart_point); |
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} |
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size_t estimated_size = builder.EstimateSize(); |
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std::string buffer("some other fake content to take up space"), buffer2; |
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size_t original_size = buffer.size(); |
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estimated_size += original_size; |
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builder.Finish(buffer); |
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ASSERT_EQ(buffer.size(), estimated_size); |
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buffer2 = buffer; // test for the correctness of relative offset
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Slice s(buffer2); |
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DataBlockHashIndex index(s); |
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// the additional hash map should start at the end of the buffer
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ASSERT_EQ(original_size, index.DataBlockHashMapStart()); |
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for (uint16_t i = 0; i < 100; i++) { |
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std::string key("key" + std::to_string(i)); |
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uint16_t restart_point = i; |
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ASSERT_TRUE(SearchForOffset(index, key, restart_point)); |
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} |
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} |
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TEST(DataBlockHashIndex, DataBlockHashTestLarge) { |
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DataBlockHashIndexBuilder builder(1000); |
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std::unordered_map<std::string, uint16_t> m; |
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for (uint16_t i = 0; i < 10000; i++) { |
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if (i % 2) { |
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continue; // leave half of the keys out
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} |
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std::string key = "key" + std::to_string(i); |
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uint16_t restart_point = i; |
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builder.Add(key, restart_point); |
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m[key] = restart_point; |
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} |
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size_t estimated_size = builder.EstimateSize(); |
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std::string buffer("filling stuff"), buffer2; |
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size_t original_size = buffer.size(); |
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estimated_size += original_size; |
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builder.Finish(buffer); |
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ASSERT_EQ(buffer.size(), estimated_size); |
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buffer2 = buffer; // test for the correctness of relative offset
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Slice s(buffer2); |
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DataBlockHashIndex index(s); |
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// the additional hash map should start at the end of the buffer
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ASSERT_EQ(original_size, index.DataBlockHashMapStart()); |
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for (uint16_t i = 0; i < 100; i++) { |
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std::string key = "key" + std::to_string(i); |
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uint16_t restart_point = i; |
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if (m.count(key)) { |
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ASSERT_TRUE(m[key] == restart_point); |
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ASSERT_TRUE(SearchForOffset(index, key, restart_point)); |
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} else { |
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// we allow false positve, so don't test the nonexisting keys.
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// when false positive happens, the search will continue to the
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// restart intervals to see if the key really exist.
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} |
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} |
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} |
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} // namespace rocksdb
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int main(int argc, char** argv) { |
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::testing::InitGoogleTest(&argc, argv); |
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return RUN_ALL_TESTS(); |
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} |
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