Introduction

Scattered container store each type's data member sequentially for all objects. That is, the first data member of all objects is stored contiguosly in memory, then the second data member, and so on as shown in the following figure:

This improves cache line utilization when iterating sequentially over a container without accessing all object's data members. They also allow asynchronous processing of object's data member without false sharing.

The following containers are available:

• scattered::vector<T> (analogous to std::vector<T>).

Scattered is a Boost Software License'd header only C++1y library and is tested with Boost 1.54 (1.55 not supported yet, see issue tracker) and trunk clang/libc++. It depends on Boost.MPL and Boost.Fusion.

Example: scattered::vector<T>

#include <algorithm>
#include <boost/range/algorithm.hpp>
#include <boost/fusion/adapted/struct/adapt_assoc_struct.hpp>
#include "scattered/vector.hpp"

/// T is a struct; k contains keys to access the struct elements:
struct T {
  float x; double y; int i; bool b;
  struct k { struct x {}; struct y {}; struct i {}; struct b {}; };
};

/// This adapts the class as an associative fusion sequence
BOOST_FUSION_ADAPT_ASSOC_STRUCT(
    T, (float, x, T::k::x)(double, y, T::k::y)
       (int  , i, T::k::i)(bool  , b, T::k::b))

int main() {
  using scattered::get;
  using k = T::k;

  scattered::vector<T> vec(10);

  /// To modify the elements of the vector in place
  /// the keys are used on a reference proxy:
  int count = 0;
  for (auto i : vec) {
    get<k::x>(i) = static_cast<float>(count);
    get<k::y>(i) = static_cast<double>(count);
    get<k::i>(i) = count;
    get<k::b>(i) = count % 2 == 0;
    ++count;
  }

  /// Cache lines contain only "y" data-members:
  for (auto i : vec) { get<k::y>(i) += get<k::y>(i); }

  /// Boost and STL algorithms work out of the box
  boost::stable_sort(vec, [](auto i, auto j) {
    return get<k::x>(i) > get<k::x>(j);
  });

  /// Reference proxy implicitly converts to T
  boost::transform(vec, std::begin(vec), [](T i) { i.y *= i.y; return i; });
  vec.push_back(T{4.0, 3.0, 2, false});
}

Getting started

• ./configure.sh is used to provide libc++'s path and select the compilation mode (debug/release/sanitizers are provided, see ./configure,sh -h).
• make compiles the tests, ctest launches all tests.
• make docs builds the documentation.
• make update-format reformats the code with clang-format.
• make bench runs all benchmarks.

Caveats

Scattered containers use proxy reference types and, as a consequence, have the following caveats similar to those of std::vector<bool>:

// i's type = scattered::vector<T>::reference, not scattered::vector<T>::value_type
auto  i = *scatteredVector.begin();

// j's type = scattered::vector<T>::reference&
auto& j = *scatteredVector.begin();

// this modifies scatteredVector:
i = T{};

// to get a value you need to use value_type (or T)
T value = *scatteredVector.begin();

// This fails because T& cannot bind to scattered::vector<T>::reference&
T& ref = *catteredVector.begin();  // Compilation Error

Main idea behind implementation

Scattered relies on BOOST_FUSION_ADAPT_ASSOC_X to adapt classes as associative Boost.Fusion sequences. The scattered::get<key>(reference_proxy) function returns a reference to the object's data member associated with the key.

Benchmarks

The aim of the library is to maximize memory bandwidth usage for algorithms that iterate sequentially over the container. The following benchmarks are located in the benchmarks/ directory and can be run with make bench.

Todo:

See the roadmaps page in the issue list.

• Finish: scattered::vector will be provided.
• Other containers: scattered::flat_set and scattered::unordered_map.

Acknowledgments

The Scattered library resulted from efforts to improve the cache performance of numerical fluid mechanics codes at the Institute of Aerodynamics, Aachen. I'd like to thank Georg Geiser for introducing me to What Every Programmer Should Know About Memory. Furthermore, I want to thank the guests of the LoungeC++ for their discussions, company, and help. In particular, to Evgeny Panasyuk who motivated me to write this library.