Allocator (currently defined only for Posix and Windows) using $(LINK2 https://en.wikipedia.org/wiki/Mmap, mmap) and $(LUCKY munmap) directly (or their Windows equivalents). There is no additional structure: each call to allocate(s) issues a call to mmap(null, s, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0), and each call to deallocate(b) issues munmap(b.ptr, b.length). So MmapAllocator is usually intended for allocating large chunks to be managed by fine-granular allocators.
The one shared instance.
Alignment is page-size and hardcoded to 4096 (even though on certain systems it could be larger).