Futurelock: A subtle risk in async Rust

You can write code using the actor model with Tokio. But it's not natural to do so.

Getting back to Rust, even if not natural, I agree with the parent that the actor model is simply the better paradigm. Zero runtime allocation should still be possible, you just have to accept some constraints.

I think async looks simple because it looks like writing imperative code; unfortunately it is just obfuscating the complex reality underlying. The actor model makes things easier to reason about, even if it looks more complicated initially.

I tend to write most of my async Rust following the actor model and I find it natural. Alice Rhyl, a prominent Tokio contributor, has written about the specific patterns:

https://ryhl.io/blog/actors-with-tokio/

[1] https://nexte.st/docs/design/architecture/runner-loop/

1. embedded hardware, like you mentioned

2. high-performance stuff

3. "embedding" in the cross-language sense, with foreign function calls

Of course the "don't use a lot of resources" thing that makes Rust/C/C++ good for tiny hardware also tends to be helpful for performance on bigger iron. Similarly, the "don't assume much about your runtime" thing that's necessary for bare metal programming also helps a lot with interfacing with other languages. And "run on a GPU" is kind of all three of those things at once.

So yeah, which of those concerns was async Rust really designed around? All of them I guess? It's kind of like, once you put on the systems programming goggles for long enough, all of those things kind of blend together?

So its a pity that Rust async design tried so hard to avoid any explicit allocations rather than using an explicit allocator that embedding can use to preallocate and reuse objects.

There is a lot riding on that “just”. Hardware stacks are very, very unlike heap memory allocators in pretty much every possible way other than “both systems provide access to memory.”

Tons and tons of embedded code assumes the stack is, indeed, a hardware stack. It’s far from trivial to make that code “just use a dummy/static allocator with the same api as a heap”; that code may not be in Rust, and it’s ubiquitous for embedded code to not be written with abstractions in front of its allocator—why would it do otherwise, given that tons of embedded code was written for a specific compiler+hardware combination with a specific (and often automatic or compiler-assisted) stack memory management scheme? That’s a bit like complaining that a specific device driver doesn’t use a device-agnostic abstraction.

Yes, all of them. Futures needed to work on embedded platforms (so no allocation), needed to be highly optimizable (so no virtual dispatch), and need to act reasonably in the presence of code that crosses FFI boundaries (so no stack shenanigans). Once you come to terms with these constraints--and then add on Rust's other principles regarding guaranteed memory safety, references, and ownership--there's very little wiggle room for any alternative designs other than what Rust came up with. True linear types could still improve the situation, though.

.await(DEADLINE) (where deadline is any non 0 unit, and 0 is 'reference defined' but a real number) should have been the easy interface. Either it yields a value or it doesn't, then the programmer has to expressly handle failure.

Deadline would only be the minimum duration after which the language, when evaluating the future / task, would return the empty set/result.

Speaking of which, I'm kind of surprised we landed on a Waker design that requires/hand-rolls virtual dispatch. Was there an alternate universe where every `poll()` function was generic on its Waker?

At least for Linux, offhand, popular task scheduler frequencies used to be 100 and 1000hz.

Looks like the Kernel's tracking that for tasks:

https://www.kernel.org/doc/html/latest/scheduler/sched-desig...

"In CFS the virtual runtime is expressed and tracked via the per-task p->se.vruntime (nanosec-unit) value."

I imagine the .vruntime struct field is still maintained with the newer "EEVDF Scheduler".

...

A Userspace task scheduler could similarly compare the DEADLINE against that runtime value. It would still reach that deadline after the minimum wait has passed, and thus be 'background GCed' at a time of the language's choice.

This problem does not happen with a custom allocator where things to allocate are of roughly the same size and allocator uses same-sized cells to allocate.

And then the need to poll features by the runtime means that async in Rust requires non-trivial runtime going against the desire to avoid abstractions in the embedded.

Async without polling while stack-unfriendly requires less runtime. And if Rust supported type-safe region-based allocators when a bunch of things are allocated one by one and then released at once it could be a better fit for the embedded world.

Question is - when will Zig become mature enough to become a legit choice next to say, Go or Rust?

I mean for a regular dev team, not necessarily someone who works deeply along with Andrew Kelley etc like Tigerbeetle.

This appears to be misunderstanding how futures work in Rust. The language doesn't evaluate futures or tasks. A future is just a struct with a poll method, sort of like how a closure in Rust is just a struct with a call method. The await keyword just inserts yield points into the state machine that the language generates for you. If you want to actually run a future, you need an executor. The executor could implement timeouts, but it's not something that the language could possibly have any way to enforce or require.