People are discovering mmap, it has been used for a very long time especially in databases.
Also just to show how buggy mmap is, it's disabled in SQLite by default for example:
https://www.sqlite.org/mmap.html
Even now the windows version is buggy, but the great thing is that after it's fixed in llama.cpp, the open source community can just copy the solution.
I don't know of many interactive utilities that were known for fast startup time _because_ of the use of mmap, but now we have one, which means many more are coming :)
- depending on concurrent accesses mmap can just sigbus on you (e.g. if the mapped file is being truncated)
- mmap simply does not work with virtual filesystems, and will blow up on large files on 32b systems (windows also has further limitations on mmaps)
- depending on workload mmap may not be faster than regular reads and memory buffers, ripgrep will mmap when working on just a few files, but will use normal buffers for large file counts, because when you start reusing buffer you amortise allocation costs which you can't amortise when creating and destroying mappings
- not only that but mmap/munmap are also globally blocking on the process, so in multithreaded processes it's very bad to map/unmap a lot, you can stall your own application
- the semantics of mmap on crash are also somewhat risky, in that the OS will try very hard to sync, but that may not be desirable if the application crashes in the middle of a write
Also, I'm disappointed that VM languages don't turn SIGBUS accessing a memory mapped region into a VM exception. They could, and it wouldn't even be that hard.
For embedded databases that operate on shared file somewhere just mmap()ing that file allows you to use part of the same file as shared memory segment and for POSIX IPC and thus synchronization between otherwise unrelated processes that access the same data. One big caveat of this approach is that it depends on optional parts of POSIX that are realistically usable on Linux and Solaris and maybe some obscure systems (you can probably do the same thing on Windows NT), notably this does not work on essentially anything BSD-derived.