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SoA-and-AoSoA-with-Cabana.md

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Array of Structures and Structure of Arrays with Cabana

Cabana is a MPI+Kokkos performance portable library for particle-based simulations. It provides particle data structures, algorithms, and utilities to enable simulations on a variety of platforms including many-core architectures and GPUs.

This use case describes the SoA and AoSoA classes provided by Cabana. Their goal is to provide a vectorization-friendly collections of contiguous elements that exhibit good alignment.

Structure of Arrays (SoA)

Cabana::SoA

Defined in header <Cabana_SoA.hpp>

template <typename DataTypes, int VectorLength>
struct SoA;

Cabana::SoA is a struct template that provides a way to store a fixed-size collection of heterogeneous arrays whose size is the specified vector length.

Conceptually Cabana::SoA<Cabana::MemberTypes<float[3], char>, 8> is equivalent to std::tuple<float[3][8], char[8]>.

The data structure keeps a separate, homogeneous data array for each particle field, each having the same number of elements. The motivation is easier vectorization for the compiler.

Template parameters

DataTypes : The types of the elements that the SoA stores as fixed size arrays. It is required to be a specialization of Cabana::MemberTypes which is defined as

template <typename... Types>
MemberTypes<Types...>;

VectorLength : The number of elements stored in contiguous memory locations for each data type.

Non-member functions

Cabana::get : accesses the specified element of the SoA.

Array of Structures of Arrays (AoSoA)

Cabana::AoSoA

Defined in header <Cabana_AoSoA.hpp>

template <class DataTypes, class DeviceType,
          int VectorLength = Impl::PerformanceTraits<
              typename DeviceType::execution_space>::vector_length,
          class MemoryTraits = Kokkos::MemoryManaged>
class AoSoA;

Template parameters

DataTypes : The types of the elements stored in the underlying Cabana::SoAs.

DeviceType : The Kokkos device type that carries the information about where to execute code and where to allocate storage.

VectorLength : The vector length for the structure of arrays (optional).

MemoryTraits : The Kokkos memory traits that tells who controls memory allocation and deallocation (optional).

Non-member functions

slice : accesses the particle data fields.