mirror of https://github.com/facebook/rocksdb.git
84 lines
2.4 KiB
C++
84 lines
2.4 KiB
C++
// Copyright (c) 2017-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 <cstddef>
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#include <thread>
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#include <utility>
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#include <vector>
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#include "port/likely.h"
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#include "port/port.h"
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#include "util/random.h"
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namespace rocksdb {
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// An array of core-local values. Ideally the value type, T, is cache aligned to
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// prevent false sharing.
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template <typename T>
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class CoreLocalArray {
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public:
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CoreLocalArray();
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size_t Size() const;
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// returns pointer to the element corresponding to the core that the thread
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// currently runs on.
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T* Access() const;
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// same as above, but also returns the core index, which the client can cache
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// to reduce how often core ID needs to be retrieved. Only do this if some
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// inaccuracy is tolerable, as the thread may migrate to a different core.
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std::pair<T*, size_t> AccessElementAndIndex() const;
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// returns pointer to element for the specified core index. This can be used,
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// e.g., for aggregation, or if the client caches core index.
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T* AccessAtCore(size_t core_idx) const;
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private:
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std::unique_ptr<T[]> data_;
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int size_shift_;
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};
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template <typename T>
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CoreLocalArray<T>::CoreLocalArray() {
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int num_cpus = static_cast<int>(std::thread::hardware_concurrency());
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// find a power of two >= num_cpus and >= 8
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size_shift_ = 3;
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while (1 << size_shift_ < num_cpus) {
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++size_shift_;
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}
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data_.reset(new T[static_cast<size_t>(1) << size_shift_]);
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}
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template <typename T>
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size_t CoreLocalArray<T>::Size() const {
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return static_cast<size_t>(1) << size_shift_;
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}
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template <typename T>
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T* CoreLocalArray<T>::Access() const {
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return AccessElementAndIndex().first;
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}
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template <typename T>
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std::pair<T*, size_t> CoreLocalArray<T>::AccessElementAndIndex() const {
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int cpuid = port::PhysicalCoreID();
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size_t core_idx;
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if (UNLIKELY(cpuid < 0)) {
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// cpu id unavailable, just pick randomly
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core_idx = Random::GetTLSInstance()->Uniform(1 << size_shift_);
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} else {
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core_idx = static_cast<size_t>(cpuid & ((1 << size_shift_) - 1));
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}
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return {AccessAtCore(core_idx), core_idx};
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}
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template <typename T>
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T* CoreLocalArray<T>::AccessAtCore(size_t core_idx) const {
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assert(core_idx < static_cast<size_t>(1) << size_shift_);
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return &data_[core_idx];
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}
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} // namespace rocksdb
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