C: memcpy speed on dynamically allocated arrays

Question

I need help with the performance of the following code. It does a memcpy on two dynamically allocated arrays of arbitrary size:

int main()
{
  double *a, *b;
  u         


        

Answer 1

It's probably lazy page allocation, Linux only mapping the pages on first access. IIRC each page in a new block in Linux is a copy-on-write of a blank page, and your allocations are big enough to demand new blocks.

If you want to work around it, you could write one byte or word, at 4k intervals. That might get the virtual addresses mapped to RAM slightly faster than writing the whole of each page.

I wouldn't expect (most efficient workaround to force the lazy memory mapping to happen) plus (copy) to be significantly faster than just (copy) without the initialization of b, though. So unless there's a specific reason why you care about the performance just of the copy, not of the whole operation, then it's fairly futile. It's "pay now or pay later", Linux pays later, and you're only measuring the time for later.