Question

I got some objects with certain values, for example:
(1)

struct massPoint {
    double pos;
    double vel;
    double acc;
} objects[LOTS];

or the same in arrays:

(2)

double pos[LOTS];
double vel[LOTS];
double acc[LOTS];

First question: Is it right if i call (1) padded data and (2) serial data?

Second question: If i do some operations which would only affect vel and acc and no pos, and i have LOTS of them, would (2) be preferable since it would be better in terms of caching performance because the pos[] dont have to be cached this way and in (1) it has to? Or do i not get the concept at all?

Answer 1

No idea for your first question

For your second question there is no general answer this depends on your architecture and of your usage pattern.

• if you really have random (=
  unpredictable) access and each double
  makes up a cacheline and your data is correctly aligned both would be
  equivalent in terms of caching.
• your second method is clearly
  superior on modern architectures if
  you have streaming access to the
  data, that is for which the compiler
  / runtime / hardware can easily
  predict the future access and that have enough hardware registers for the all the pointers and the data
• your first method could be superior
  in cases you have only few registers,
  since for the second the compiler
  might need to keep track of your
  current index in the three different arrays

so in summary it may depend on a lot of factors, but a tendencies that the second method would be preferable under many circumstances