Memcpy takes the same time as memset

Question

I want to measure memory bandwidth using memcpy. I modified the code from this answer:why vectorizing the loop does not have performance improvement which used

Answer 1

The point is that malloc and calloc on most platforms don't allocate memory; they allocate address space.

malloc etc work by:

• if the request can be fulfilled by the freelist, carve a chunk out of it
  • in case of calloc: the equivalent ofmemset(ptr, 0, size) is issued
• if not: ask the OS to extend the address space.

For systems with demand paging (COW) (an MMU could help here), the second options winds downto:

• create enough page table entries for the request, and fill them with a (COW) reference to /dev/zero
• add these PTEs to the address space of the process

This will consume no physical memory, except only for the Page Tables.

• Once the new memory is referenced for read, the read will come from /dev/zero. The /dev/zero device is a very special device, in this case mapped to every page of the new memory.
• but, if the new page is written, the COW logic kicks in (via a page fault):
  • physical memory is allocated
  • the /dev/zero page is copied to the new page
  • the new page is detached from the mother page
  • and the calling process can finally do the update which started all this