NetBSD implements overcommitted swap - many processes malloc() (or mmap() which that really becomes in the current implementation) far more memory than they're ever going to actually use. It is only when some real physical memory is required (rather than simply a marker "zero filled page might be required here") that the system actually allocates any real resources. Similarly pages mapped from a file only need swap space if they're altered - otherwise the file serves as the backing store for it.
Once upon a time there was a method to turn overcommitted swap off, and require actual allocations (of RAM or swap) to be made for all reserved (virtual) memory. I used to enable that all the time - but I haven't seen any mention of it in ages, and the mechanism might no longer still exist. kre
