AoS and SoA

In computing, an array of structures (AoS), structure of arrays (SoA) or array of structures of arrays (AoSoA) are contrasting ways to arrange a sequence of records in memory, with regard to interleaving, and are of interest in SIMD and SIMT programming.

Structure of arrays

Structure of arrays (SoA) is a layout separating elements of a record (or 'struct' in the C programming language) into one parallel array per field. The motivation is easier manipulation with packed SIMD instructions in most instruction set architectures, since a single SIMD register can load homogeneous data, possibly transferred by a wide internal datapath (e.g. 128-bit). If only a specific part of the record is needed, only those parts need to be iterated over, allowing more data to fit onto a single cache line. The downside is requiring more cache ways when traversing data, and inefficient indexed addressing.

For example, to store N points in 3D space using a structure of arrays:

Array of structures

Array of structures (AoS) is the opposite (and more conventional) layout, in which data for different fields is interleaved.

This is often more intuitive, and supported directly by most programming languages.

For example, to store N points in 3D space using an array of structures:

Array of structures of arrays

Array of structures of arrays (AoSoA) or tiled array of structs is a hybrid approach between the previous layouts, in which data for different fields is interleaved using tiles or blocks with size equal to the SIMD vector size. This is often less intuitive, but can achieve the memory throughput of the SoA approach, while being more friendly to the cache locality and load port architectures of modern processors.