Comment by hmry
10 days ago
Great article! Love these types of deep dives into optimizations. Hope the project goal works out!
I've felt before that compilers often don't put much effort into optimizing the "trivial" cases.
Overly dramatic title for the content, though. I would have clicked "Async Rust Optimizations the Compiler Still Misses" too you know
So on the title, I picked this because it's simply the truth. Since async landed in 2019 or so, not much has changed.
Yes, we can have async in traits and closures now. But those are updates to the typesystem, not to the async machinery itself. Wakers are a little bit easier to work with, but that's an update to std/core.
As I understand it, the people who landed async Rust were quite burnt out and got less active and no one has picked up the torch. (Though there's 1 PR open from some google folk that will optimize how captured variables are laid out in memory, which is really nice to have) Since I and the people I work with are heavy async users, I think it's maybe up to me to do it or at least start it. Free as in puppy I guess.
So yeah, the title is a little baitey, but I do stand behind it.
Some of the burnout no doubt being due to the catastrophizing of every decision by the community and the extreme rhetoric used across the board.
Great to see people wanting to get involved with the project, though. That’s the beauty of open source: if it aggravates you, you can fix it.
As an example of this, i remember a huge debate at the time about `await foo()` vs `foo().await` syntax. The community was really divided on that one, and there was a lot of drama because that's the kind of design decision you can't really walk back from.
Retrospectively, i think everyone is satisfied with the adopted syntax.
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I think it's partially accurate, and partially a consequence of how async fractures the design space, so it will always feel like a somewhat separate thing, or at least until we figure out how to make APIs agnostic to async-ness.
I am a beginner to Rust but I've coded with gevent in Python for many years and later moved to Go. Goroutines and gevent greenlets work seamlessly with synchronous code, with no headache. I know there've been tons of blog posts and such saying they're actually far inferior and riskier but I've really never had any issues with them. I am not sure why more languages don't go with a green thread-like approach.
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Claim-1: the async versus sync distinction cannot be meaningfully dissolved at the programmer level.
Claim-2: async versus sync is a fundamental division in CS
Discuss amongst yourselves. I lean towards thinking both are probably true (P~70%, P~90%)
See: “What Color is Your Function?” by Bob Nystrom (2015). https://journal.stuffwithstuff.com/2015/02/01/what-color-is-...
As an uninterested 3rd party, it’s a wild exaggeration
> So on the title, I picked this because it's simply the truth. Since async landed in 2019 or so, not much has changed.
Hi. The article calls Rust async an MVP. You should expect strong reactions when you frame it like that.
"MVP" has a generally understood meaning; distorting that is unhelpful and confusing. Rust's async was not an MVP when it was released in 2019. It was the result of a lot of earlier work.
Rust async: (a) works well for a lot of people and orgs in production settings and (b) is arguably better designed than most (all?) other async implementations. Calling it an MVP is far from "simply the truth". It is an opinion -- and frankly a pretty clickbaity one. I appreciate your article's attention to detail, but the title is straight up shameful sensationalism.
I strive to not reflexively defend the status quo, but I get really chafed when people conveniently blur the difference between fact and opinion.
Please argue on narrowest correct claims available. The current title overstates your claims and undermines its overall credibility. Your central claim (as I read it) is that for embedded software there are opportunities for async improvement in Rust. Yeah this might sound boring, but I think it's accurate.
My other main criticism of your article is when it claims Rust async breaks the "zero cost abstraction" principle. I don't buy this claim, because you do not show that hand rolling the code provides the same guarantees. A lot of people misunderstand what "zero cost" means; your article wouldn't be the first to give the wrong impression.
Writing is hard (different audiences bring different backgrounds), and I commend anyone who puts their ideas out into the world. Please take this as constructive feedback: please agree or disagree with me on the merits. Ask and engage where I'm unclear.
> Rust's async was not an MVP when it was released in 2019
The team literally described it as such.
One of the main architects of Rust’s async/await, withoutboats, left a comment on lobsters:
> It's just the truth. Neither in the language design nor in the compiler implementation has hardly any progress been made in the now 7 years since we shipped the MVP. The people primarily involved in delivering the MVP all become less active in the project around the same time and delivery since then has stalled out.
>> I hope this person receives the support to do this work.
Rust’s async is great, and I feel you around some of the less informed criticisms. But it’s been called an MVP for a decade now, it’s not an insulting characterization. Just because it’s been an MVP does not mean it’s not good or useful.
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Agree on title. Too dramatic.
The author seems to be obsessing about the overhead for trivial functions. He's bothered by overhead for states for "panicked" and "returned". That's not a big problem. Most useful async blocks are big enough that the overhead for the error cases disappears.
He may have a point about lack of inlining. But what tends to limit capacity for large numbers of activities is the state space required per activity.
> Most useful async blocks are big enough that the overhead for the error cases disappears.
Is it really though?
In my experience many Rust applications/libraries can be quite heavy on the indirection. One of the points from the article is that contrary to sync Rust, in async Rust each indirection has a runtime cost. Example from the article:
I would naively expect the above to be a 'free' indirection, paying only a compile-time cost for the compiler to inline the code. But after reading the article I understand this is not true, and it has a runtime cost as well.
In my experience, it's not uncommon to have an async trait method for which many implementations are actually synchronous. For example, different tables in your DB need to perform some calculations, but only some tables reference other tables. In that case, the method needs to be async and take a handle to the DB as parameter, but many table entries can perform the calculation on their own without using the handle (or any async operation).
This may look like a case of over-optimization, but given how many times i've seen this pattern, i assume it builds up to a lot of unnecessary fluff in huge codebases. To be clear, in that case, the concern is not really about runtime speed (which is super fast), but rather about code bloat for compilation time and binary size.
> Most useful async blocks are big enough that the overhead for the error cases disappears.
Most useful async blocks are deeply nested, so the overhead compounds rapidly. Check the size of futures in a decently large Tokio codebase sometime
He's optimizing for embedded no-std situation. These things do matter in constrained environments.
He briefly mentions microcontrollers, but doesn't go into details. That might make sense, for microcontrollers which implement some kind of protocol talking to something.
> [...] That's not a big problem [...]
Depends somewhat on your expectations, I suppose. Compared to Python, Java, sure, but Rust off course strives to offer "zero-cost" high level concepts.
I think the critique is in the same realm of C++'s std::function. Convenience, sure, but far from zero-cost.
To the point it got replaced by std::function_ref() in C++26.
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> Agree on title. Too dramatic.
not just too dramatic
given that all the things they list are
non essential optimizations,
and some fall under "micro optimizations I wouldn't be sure rust even wants",
and given how far the current async is away from it's old MVP state,
it's more like outright dishonest then overly dramatic
like the kind of click bait which is saying the author does cares neither about respecting the reader nor cares about honest communication, which for someone wanting to do open source contributions is kinda ... not so clever
through in general I agree rust should have more HIR/MIR optimizations, at least in release mode. E.g. its very common that a async function is not pub and in all places directly awaited (or other wise can be proven to only be called once), in that case neither `Returned` nor `Panicked` is needed, as it can't be called again after either. Similar `Unresumed` is not needed either as you can directly call the code up to the first await (and with such a transform their points about "inlining" and "asyncfns without await still having a state machine" would also "just go away"TM, at least in some places.). Similar the whole `.map_or(a,b)` family of functions is IMHO a anti-pattern, introducing more function with unclear operator ordering and removal of the signaling `unwrap_` and no benefits outside of minimal shortening a `.map(b).unwrap_or(a)` and some micro opt. is ... not productive on a already complicated language. Instead guaranteed optimizations for the kind of patterns a `.map(b).unwrap_or(a)` inline to would be much better.