For an expanding array in a 64 bit address space, reserving a big region and mmaping it in as you go is usually the top performing solution by a wide margin. At least on Linux, it is faster to speculatively mmap ahead with MAP_POPULATE rather than relying on page faults, too.

And, if you find you didn't reserve enough address space, Linux has mremap() which can grow the reserved region. Or map the region to two places at once (the original place and a new, larger place).