Yeah that's using over 5 gigabytes, not 400 megabytes. Your process monitor is inaccurate; the memory used doesn't "count" because it's disk backed and the kernel is free to discard memory pages if it really needs the memory because it can always load it back from disk. But every time it does that you need to wait for the slow disk to read it back in again.
It is strange that it does that given that there's plenty of free memory available in the system (it has 256Gb of RAM and wasn't running anything else).
Not really, it's just a question of accounting. mmap is functionally the same as disk cache. As long as you've got the RAM, it'll run from RAM. If you really want, you can force llama.cpp not to use mmap and explicitly load everything into RAM, but there's not really any performance reason to do that - if the kernel keeps dropping your pages, you're under memory pressure anyway and "locking" that memory will probably end up either thrashing or invoking the OOM killer.
