Summary: Currently, deadlock cycles are held in std::unordered_map. The problem with it is that it allocates/deallocates memory on every insertion/deletion. This limits throughput since we're doing this expensive operation while holding a global mutex. Fix this by using a vector which caches memory instead. Running the deadlock stress test, this change increased throughput from 39k txns/s -> 49k txns/s. The effect is more noticeable in MyRocks. Closes https://github.com/facebook/rocksdb/pull/1545 Differential Revision: D4205662 Pulled By: lth fbshipit-source-id: ff990e4main
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved. This
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// source code is licensed under the BSD-style license found in the LICENSE
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// file in the root directory of this source tree. An additional grant of
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// patent rights can be found in the PATENTS file in the same directory.
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//
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#pragma once |
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#include <algorithm> |
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#include <array> |
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#include <utility> |
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#include "util/autovector.h" |
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namespace rocksdb { |
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// This is similar to std::unordered_map, except that it tries to avoid
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// allocating or deallocating memory as much as possible. With
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// std::unordered_map, an allocation/deallocation is made for every insertion
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// or deletion because of the requirement that iterators remain valid even
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// with insertions or deletions. This means that the hash chains will be
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// implemented as linked lists.
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//
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// This implementation uses autovector as hash chains insteads.
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//
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template <typename K, typename V, size_t size = 128> |
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class HashMap { |
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std::array<autovector<std::pair<K, V>, 1>, size> table_; |
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public: |
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bool Contains(K key) { |
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auto& bucket = table_[key % size]; |
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auto it = std::find_if( |
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bucket.begin(), bucket.end(), |
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[key](const std::pair<K, V>& p) { return p.first == key; }); |
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return it != bucket.end(); |
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} |
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void Insert(K key, V value) { |
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auto& bucket = table_[key % size]; |
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bucket.push_back({key, value}); |
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} |
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void Delete(K key) { |
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auto& bucket = table_[key % size]; |
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auto it = std::find_if( |
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bucket.begin(), bucket.end(), |
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[key](const std::pair<K, V>& p) { return p.first == key; }); |
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if (it != bucket.end()) { |
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auto last = bucket.end() - 1; |
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if (it != last) { |
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*it = *last; |
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} |
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bucket.pop_back(); |
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} |
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} |
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V& Get(K key) { |
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auto& bucket = table_[key % size]; |
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auto it = std::find_if( |
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bucket.begin(), bucket.end(), |
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[key](const std::pair<K, V>& p) { return p.first == key; }); |
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return it->second; |
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} |
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}; |
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} // namespace rocksdb
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