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Comment by akira2501 | Hacker News Reader

> How do other mmap/madvise options influence this (for instance, MADV_SEQUENTIAL, MADV_WILLNEED, MADV_POPULATE, MADV_POPULATE_READ, mlock)? (Hypothesis: these options will make it more likely that data is pre-cached and thus fall into fast path more often, but without a guarantee.)

That probably should have been the first thing to try. Too mad the mmap2 crate does not expose this.

Also looking at the mmap2 crate, it chooses some rather opinionated defaults depending on which function you actually call, and it makes accessing things like HUGEPAGE maps somewhat difficult.. and for whatever reason includes the MMAP_STACK flag when you call through this path.

I feel like a lot of rust authors put faith in crates that, upon inspection, are generally poorly designed and do not expose the underlying interface properly. It's a bad crutch for the language.

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> That probably should have been the first thing to try.

The point of the post is not "how to make mmap work best with async/await" or "how to optimise mmap with async/await", but exploring the consequence of incorrect code (and thus explaining why one might need potential remedies like those). Sorry if that didn't come across!

I think it's harder for you to write "correct code" here because the crate is hiding most of the actual detail from you. I put that in quotes because there's absolutely nothing incorrect about the code, it's really just suboptimal, and most probably because it can't even use the full syscall interface.

Seriously, I hate to be a curmudgeon, but that crate looks like a particularly bad and naive wrapper around mmap. It works very hard to provide you things you don't need when the basic interface is much more flexible. Aside from having to put `unsafe` around the call and re-import the kernel header constants, there's almost no reason to even have this in a crate.

I have a feeling we’re talking at cross purposes here: I was actively trying to write incorrect code. This post isn’t about memmap2 crate specifically at all, it just happens to be a convenient way to get the exact (“incorrect”) syscall I wanted from Rust.

I see where you’re coming from but… it feels like you’re trying to convince me of something about the post? If you feel like convincing a larger audience of the limitations of the memmap2 crate specifically, I suggest writing your own blog posts and/or getting involved with it. :)

I looked into it (as part of optimizing LLM memory throughput) and hugepage file-backed maps aren't supported under linux. Only anonymous mappings.

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