Zeroing on exit would be more secure, but significantly slower -- you want to exit quickly, so you can potentially start a replacement program, which would be expected to, at least sometimes, take time to allocate the same amount of memory. If it does allocate the whole amount immediately, it's not necessarily any slower in total time between zeroing at exit or on mapping; but it there's enough time for the pages to get zeroed in the background, that reduces the amount of time waiting for the kernel to do things.
It seems the jury is out on the benefits from a performance perspective (DragonflyBSD took out background zeroing, saying they were unable to observe a performance difference, so simpler code is better)
