Can some explain the performance behavior of the following memory allocating C program?

Question

On my machine Time A and Time B swap depending on whether A is defined or not (which changes the order in which the two callocs are called).

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Answer 1

You should also test using malloc instead of calloc. One thing that calloc does is to fill the allocated memory with zeros.

I believe in your case that when you calloc arr1 last and then assign to it, it is already faulted into cache memory, since it was the last one allocated and zero-filled. When you calloc arr1 first and arr2 second, then the zero-fill of arr2 pushes arr1 out of cache.