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points by the-tomster 2 days ago | 0 comments
> 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.

[–] grg0 1 day ago
The problem is not finding limits, although that does take decent engineering for anything non-trivial. The problem is that a hard limit assumes your requests have a relatively uniform distribution of memory requirements. Otherwise, you are wasting memory. Or you have N limits instead, which just complicates the problem.

Resize: realloc invalidates pointers. If you're saying allocate more chunks and link them, that is technically no longer an arena?

Requesting virtual memory and committing lazily seems a good trade-off except on Windows, which does not allow to over-commit. Somehow I don't find that very satisfying because it relies on the OS and on the hardware to provide virtual memory support. But maybe it's just what it is.

[–] palata 1 day ago
> Two options: you either say "eh 20MB per request ought to be big enough"

"Memory is cheap" and all that? :-)

[–] MrBuddyCasino 1 day ago
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?
[–] the-tomster 1 day ago
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.