What is meaning of locality of data structure?

Question

I was reading following article,

What Every Programmer Should Know About Compiler Optimizations

There are other important optimizations that are currently beyond the capabilities of any compiler—for example, replacing an inefficient algorithm with an efficient one, or changing the layout of a data structure to improve its locality.

Does that mean if I change sequence (layout) of data members in class, it can affect performance?

So,

class One
{
int data0;
abstract-data-type data1;
};

Differes in performance from,

class One
{
abstract-data-type data0;
int data1;
};

If this is true, what is rule of thumb while defining classes or data structure?

Answer 1

Locality in this sense is speaking mostly to cache locality. Writing data structures and algorithms to operate mostly out of cache makes the algorithm run as fast as it possibly can. Cache locality is one of the reasons quick sort is quick.

For a data structure, you want to keep the parts of your data structure that refer to each other relatively close to each other, to avoid flushing out useful cache lines.

Also, you can rearrange your data structure so that the compiler will use the minimum amount of memory required to hold all the members and still efficiently access them. This helps make sure your data structure consumes the minimum number of cache lines.

_{A single cache line on a current x86-64 architecture (core i7) is 64 bytes.}