In malloc, why use brk at all? Why not just use mmap?

Question

Typical implementations of malloc use brk/sbrk as the primary means of claiming memory from the OS. However, they also use mmap

Answer 1

The obvious advantage is that you can grow the last allocation in place, which is something you can't do with mmap(2) (mremap(2) is a Linux extension, not portable).

For naive (and not-so-naive) programs which are using realloc(3) eg. to append to a string, this translates in a 1 or 2 orders of magnitude speed boost ;-)