Comment by the-tomster
10 hours ago
> And how do you size the arena?
Two options: you either say "eh 20MB per request ought to be big enough" and resize it later if you need to. Or you take time to figure out reasonable limits in terms of whatever domain you're dealing with, and return descriptive errors when those limits are exceeded. Maybe you're saying your service supports up to N HTTP headers and M query parameters in the URL and so on. For almost anything there's a limit you can place that's reasonable. Having no limits at all just means the limit is when you run out of memory / disk / whatever.
If you like RAII and C++ just use that though, Zig is an alternative not an upgrade, imo.
Can arenas be made to use no actual memory unless it is being used, so the virtual memory subsystem only allocates pages when written to?
Yup! In C you can mmap out a big anonymous region and then it will only take up actual resident space for pages you use. I'm not sure if Zig exposes this in the stdlib somewhere or if you'd just do it through libc.
If you're doing something like per-request or per-frame arenas you can clear the arena without having to reallocate the whole mapping. And depending how fancy you want to get you can mark the pages unused when you clear the arena so that the OS can reclaim them if needed.