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9d9d2965cb
Summary:
= Major Changes =
* Add a new mem-table representation, HashCuckooRep, which is based cuckoo hash.
Cuckoo hash uses multiple hash functions. This allows each key to have multiple
possible locations in the mem-table.
- Put: When insert a key, it will try to find whether one of its possible
locations is vacant and store the key. If none of its possible
locations are available, then it will kick out a victim key and
store at that location. The kicked-out victim key will then be
stored at a vacant space of its possible locations or kick-out
another victim. In this diff, the kick-out path (known as
cuckoo-path) is found using BFS, which guarantees to be the shortest.
- Get: Simply tries all possible locations of a key --- this guarantees
worst-case constant time complexity.
- Time complexity: O(1) for Get, and average O(1) for Put if the
fullness of the mem-table is below 80%.
- Default using two hash functions, the number of hash functions used
by the cuckoo-hash may dynamically increase if it fails to find a
short-enough kick-out path.
- Currently, HashCuckooRep does not support iteration and snapshots,
as our current main purpose of this is to optimize point access.
= Minor Changes =
* Add IsSnapshotSupported() to DB to indicate whether the current DB
supports snapshots. If it returns false, then DB::GetSnapshot() will
always return nullptr.
Test Plan:
Run existing tests. Will develop a test specifically for cuckoo hash in
the next diff.
Reviewers: sdong, haobo
Reviewed By: sdong
CC: leveldb, dhruba, igor
Differential Revision: https://reviews.facebook.net/D16155
43 lines
1.5 KiB
C++
43 lines
1.5 KiB
C++
// Copyright (c) 2013, Facebook, Inc. All rights reserved.
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// This source code is licensed under the BSD-style license found in the
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// LICENSE file in the root directory of this source tree. An additional grant
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// of patent rights can be found in the PATENTS file in the same directory.
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#ifndef ROCKSDB_LITE
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#pragma once
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#include "rocksdb/slice_transform.h"
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#include "rocksdb/memtablerep.h"
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namespace rocksdb {
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class HashCuckooRepFactory : public MemTableRepFactory {
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public:
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// maxinum number of hash functions used in the cuckoo hash.
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static const int kMaxHashCount = 10;
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explicit HashCuckooRepFactory(size_t write_buffer_size,
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size_t average_data_size,
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unsigned int hash_function_count)
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: write_buffer_size_(write_buffer_size),
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average_data_size_(average_data_size),
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hash_function_count_(hash_function_count) {}
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virtual ~HashCuckooRepFactory() {}
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virtual MemTableRep* CreateMemTableRep(
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const MemTableRep::KeyComparator& compare, Arena* arena,
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const SliceTransform* transform) override;
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virtual const char* Name() const override { return "HashCuckooRepFactory"; }
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private:
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size_t write_buffer_size_;
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size_t average_data_size_;
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const unsigned int hash_function_count_;
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};
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} // namespace rocksdb
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#endif // ROCKSDB_LITE
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