Array of Structs are always faster than Structs of arrays?

Question

I was wondering if the data layout Structs of Arrays (SoA) is always faster than an Array of Structs (AoS) or Array of Pointers (AoP) for problems with inputs that only fits in RAM programmed in C/JAVA.

Some days ago I was improving the performance of a Molecular Dynamic algorithm (in C), summarizing in this algorithm it is calculated the force interaction among particles based on their force and position.

Original the particles were represented by a struct containing 9 different doubles, 3 for particles forces (Fx,Fy,Fz) , 3 for positions and 3 for velocity. The algorithm had an array containing pointers to all the particles (AoP). I decided to change the layout from AoP to SoA to improve the cache use.

So, now I have a Struct with 9 array where each array stores forces, velocity and positions (x,y,z) of each particle. Each particle is accessed by it own array index.

I had a gain in performance (for an input that only fits in RAM) of about 1.9x, so I was wondering if typically changing from AoP or AoS to SoA it will always performance better, and if not in which types of algorithms this do not occurs.

Answer 1

Much depends of how useful all fields are. If you have a data structure where using one fields means you are likely to use all of them, then an array of struct is more efficient as it keeps together all the things you are likely to need.

Say you have time series data where you only need a small selection of the possible fields you have. You might have all sorts of data about an event or point in time, but you only need say 3-5 of them. In this case a structure of arrays is more efficient because a) you don't need to cache the fields you don't use b) you often access values in order i.e. caching a field, its next value and its next is useful.

For this reason, time-series information is often stored as a collection of columns.