Comment by newpavlov
1 day ago
>which we found in every existing futures implementation we inspected
This is exactly what I meant when I wrote about the indirect influence from other languages. People may dress it up as much as they want, but it's clear that polling was the most important model at the time (outside of the Windows world) and a lot of design consideration was put into being compatible with it. The Rust async model literally uses the polling terminology in its most fundamental interfaces!
>this approach nevertheless forces allocation at almost every point of future composition
This is only true in the narrow world of modeling async execution with futures. Do you see heap allocations in Go on each equivalent of "future composition" (i.e. every function call)? No, you do not. With the stackfull models you allocate a full stack for your task and you model function calls as plain function calls without any future composition shenaniganry.
Yes, the stackless model is more efficient memory-wise and allows for some additional useful tricks (like sharing future stacks in `join!`). But the stackfull model is perfectly efficient for 95+% of use cases, fits better with the borrow/ownership model, does not result in the `.await` noise, does not lead to the horrible ecosystem split (including split between different executors), and does not need the language-breaking hacks like `Pin` (see the `noalias` exception made for it). And I believe it's possible to close the memory efficiency gap between the models with certain compiler improvements (tracking maximum stack usage bound for functions and introducing a separate async ABI with two separate stacks).
>The existence of a usable io_uring in 2016 wouldn't have changed the fundamental calculus.
IIRC the first usable versions of io-uring very released approximately during the time when the Rust async was undergoing stabilization. I am really confident that if the async system was designed today we would've had a totally different model. Importance of completion-based models has only grown since then not only because of the sane async file IO, but also because of Spectre and Meltdown.
> But the stackfull model is
The existence of advantages doesn't change anything here. The problems is that the disadvantages made this approach a non-starter, despite a lot of effort to make it work. Tradeoffs exist in language design, and the approaches were judged accordingly. What works for Go doesn't necessarily work for Rust, because they target different domains.
> I am really confident that if the async system was designed today we would've had a totally different model
No, without solving the original problems, the outcome would be the same. The Rust devs at the time were well aware of io_uring.
What were the original problems exactly? From what I recall they effectively boiled down to size concerns due to seeing themselves as a c/c++ successor and they didn’t want to lose any adoption in the embedded systems target audience.
Have you read the article by Aaron Turon linked above? It's very informative, and if you have any questions about specific parts of it, feel free to reference them. In particular it boils down to the fact that Rust bends over backwards to avoid putting anything that requires allocation or dynamic dispatch in the core language (e.g. Rust's closures are fascinating in that they're stack-allocated, like C++'s, while also playing nicely with the borrow checker, which is quite a feat). This property extends to the current design of async, which makes async suitable for embedded devices, which is extremely cool (check out the Embassy project for the state of the art in this space).
I mean from an outsiders perspective on Rust this is how I saw it.
Rust is in a strange place because they're a systems language directly competing with C++. Async, in general, doesn't vibe with that but green threads definitely don't.
If you're gonna do green threads you might as well throw in a GC too and get a whole runtime. And now you're writing Go.
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