row vs column major

Question

1. Is Row-Major or Column-Major order faster? Why do you think that is. Please explain using execution counts.
2. In what scenario would you use Row-Major ordering when traversing a 2D array? In what scenario would you use Column-Major ordering?

Answer 1

I am assuming a common CPU architecture with a cache.

The situation is different on a GPU, for example.

Take the typical example of matrix multiplication.

It would be best to have one matrix in row major and the other in column major order.

The reason is that when fetching one element, you want to get a cache line containing

subsequent elements.

I hope this should answer question 2.

I do not know how to answer question 1, given that neither is faster in general.