Comment by vlovich123
16 hours ago
I think we can agree that Mitchell knows what he’s doing and isn’t playing around with mmap just because. It’s probably quite important to ensure a low memory footprint. But mmap in rust is not extra risky in some weird mystical way. It’s just a normal FFI function to get a pointer back and you can trivially build safe abstractions around it to ensure the lifetime of a slice doesn’t exceed the lifetime of the underlying map. It’s rust 101 and there’s nothing weird here that can cause the unsafe bits here to be extra dangerous (in general unsafe rust can be difficult to get right with certain constructs, but it doesn’t apply here).
I actually don't think I agree about mmap. Reading around it seems as though Mitchell had clever ideas for abusing mmap and those didn't work out. Now he's got mmap for the pages and it works so why replace it, but that does not mean you need mmap to deliver this performance and in fact I'd be extremely surprised if that were true as somebody who spent about a decade of his life mostly writing close-to-metal database code in C using mmap...
If you want a whole lot of bytes and you ask your allocator, do you know what almost any popular general purpose allocator will do on a vaguely decent modern Unix? Call mmap to get them for you. So at most you're cutting out a few CPU instructions worth of middle man.