ceph/src/rocksdb/util/core_local.h

   1 //  Copyright (c) 2017-present, Facebook, Inc.  All rights reserved.
   2 //  This source code is licensed under both the GPLv2 (found in the
   3 //  COPYING file in the root directory) and Apache 2.0 License
   4 //  (found in the LICENSE.Apache file in the root directory).
   5
   6 #pragma once
   7
   8 #include <cstddef>
   9 #include <thread>
  10 #include <utility>
  11 #include <vector>
  12
  13 #include "port/likely.h"
  14 #include "port/port.h"
  15 #include "util/random.h"
  16
  17 namespace rocksdb {
  18
  19 // An array of core-local values. Ideally the value type, T, is cache aligned to
  20 // prevent false sharing.
  21 template <typename T>
  22 class CoreLocalArray {
  23  public:
  24   CoreLocalArray();
  25
  26   size_t Size() const;
  27   // returns pointer to the element corresponding to the core that the thread
  28   // currently runs on.
  29   T* Access() const;
  30   // same as above, but also returns the core index, which the client can cache
  31   // to reduce how often core ID needs to be retrieved. Only do this if some
  32   // inaccuracy is tolerable, as the thread may migrate to a different core.
  33   std::pair<T*, size_t> AccessElementAndIndex() const;
  34   // returns pointer to element for the specified core index. This can be used,
  35   // e.g., for aggregation, or if the client caches core index.
  36   T* AccessAtCore(size_t core_idx) const;
  37
  38  private:
  39   std::unique_ptr<T[]> data_;
  40   int size_shift_;
  41 };
  42
  43 template <typename T>
  44 CoreLocalArray<T>::CoreLocalArray() {
  45   int num_cpus = static_cast<int>(std::thread::hardware_concurrency());
  46   // find a power of two >= num_cpus and >= 8
  47   size_shift_ = 3;
  48   while (1 << size_shift_ < num_cpus) {
  49     ++size_shift_;
  50   }
  51   data_.reset(new T[static_cast<size_t>(1) << size_shift_]);
  52 }
  53
  54 template <typename T>
  55 size_t CoreLocalArray<T>::Size() const {
  56   return static_cast<size_t>(1) << size_shift_;
  57 }
  58
  59 template <typename T>
  60 T* CoreLocalArray<T>::Access() const {
  61   return AccessElementAndIndex().first;
  62 }
  63
  64 template <typename T>
  65 std::pair<T*, size_t> CoreLocalArray<T>::AccessElementAndIndex() const {
  66   int cpuid = port::PhysicalCoreID();
  67   size_t core_idx;
  68   if (UNLIKELY(cpuid < 0)) {
  69     // cpu id unavailable, just pick randomly
  70     core_idx = Random::GetTLSInstance()->Uniform(1 << size_shift_);
  71   } else {
  72     core_idx = static_cast<size_t>(cpuid & ((1 << size_shift_) - 1));
  73   }
  74   return {AccessAtCore(core_idx), core_idx};
  75 }
  76
  77 template <typename T>
  78 T* CoreLocalArray<T>::AccessAtCore(size_t core_idx) const {
  79   assert(core_idx < static_cast<size_t>(1) << size_shift_);
  80   return &data_[core_idx];
  81 }
  82
  83 }  // namespace rocksdb