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0 pointssaurik6y ago0 comments

Thanks for the information!!

On your last paragraph, the thing I'm concerned by is where this extra priority information is stored and propogated, as the term "task" is interesting: isn't every single separate thing being awaited its own task? There isn't (in my mental model) a concept that maps into something like a "pseudo-thread" (but maybe Rust does something like this, requiring a very structured form of concurrency?), which would let me set a "pseudo-thread" property, right?

As an example: if I am already in an asynchronous coroutine and I spawn of two asynchronous web requests as sub-tasks, the results of which will be processed potentially in parallel on various work queues, and then join those two tasks into a high-level join task that I wait on (so I want both of these things to be done before I continue), I'd want the background processing done on the results to be handled at the priority of this parent spawning task; do I have to manually propagate this information?

In C++2a, I would model this by having a promise type that is used for my prioritize-able tasks and, to interface with existing APIs (such as the web request API) that are doing I/O scheduling; I'd use await_transform to adapt their promise type into one of mine that lets me maintain my deadline across the I/O operation and then recover it in both of the subtasks that come back into my multi-threaded work queue. Everything I've seen about Rust seems to assume that there is a single task/promise type that comes from the standard library, meaning that it isn't clear to me how I could possibly do this kind of advanced scheduling work.

(Essentially, whether or not it was named for this reason--and I'm kind of assuming it wasn't, which is sad, because not enough people understand monads and I feel like it hurts a lot of mainstream programming languages... I might even say particularly Rust, which could use more monadic concepts in its error handling--await_transform is acting as a limited form of monad transformer, allowing me to take different concepts of scheduled execution and merge them together in a way that is almost entirely transparent to the code spawning sub-tasks. The co_await syntax is then acting as a somewhat-lame-but-workable-I-guess substitute for do notation from Haskell. In a perfect world, of course, this would be almost as transparent as exceptions are, which are themselves another interesting form of monad.)

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tmandry6y ago

The concept of a pseudo-thread you're referring to is a task. A task contains a whole tree of futures awaiting other futures. So no manual propagation is necessary.

Of course, it's possible for tasks to spawn other tasks that execute independently. (To be clear, if you are awaiting something from within your task, it is not a separate task.) For spawning new tasks, there's a standard API[1], which doesn't include any executor-specific stuff like priority. You'll have to decide what you want the default behavior to be when someone calls this; for example, a newly spawned task can inherit the priority of its parent.

To get more sophisticated, you could even have a "spawn policy" field for every task that your first-party code knows how to set. Any new task spawned from within that task inherits priority according to that task's policy. The executor implementation decides what tasks look like and how to spawn new ones, so you can go crazy. (Not that you necessarily should, that is.)

To summarize the Rust approach, I'd say you have 3 main extension points:

1. The executor, which controls the spawning, prioritization, and execution of tasks

2. Custom combinators (like join_all[2]), which allow you to customize the implementation of poll[3] and, say, customize how sub-futures are prioritized (This is at the same level as await, so per-Future, not per-Task.)

3. Leaf futures (like the ones that read or write to a socket). These are responsible for working with the executor to schedule their future wake-ups (with, say, epoll or some other mechanism). For more on this, see [4].

[1]: https://doc.rust-lang.org/1.28.0/std/task/trait.Executor.htm...

[2]: https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-a...

[3]: https://doc.rust-lang.org/1.28.0/std/future/trait.Future.htm...

[4]: https://boats.gitlab.io/blog/post/wakers-i/

saurikOP6y ago

Thank you so much for the context here!! And yeah: a big terminology clash is that many of the libraries for C++ come from a common lineage (almost entirely from Lewis Baker, who has been involved in the STL abstractions, wrote cppcoro, and then got allocated by Facebook to work on Folly) and use the term "task" to essentially mean "a future that you can efficiently co_await". What I'm seeing so far seems reasonably sane and arguably similar to the abstraction I have been building up using lower-level components in C++; which all makes me very happy, as I'm anticipating eventually wanting to rewrite what I have done so far in Rust.

anp6y ago

In a Rust implementation of your example, you might not necessarily spawn the two sub-operations as separate tasks. Awaiting them directly in a parent async function (probably using FuturesUnordered, like Promise.all in JS) will cause all of their work to be scheduled, prioritized, cancelled, etc. together because they’ll be a part of the same task. There’s a 1-many relation from tasks to Futures in Rust.

saurikOP6y ago

FWIW, what I meant by "join those two tasks into a high-level join task" was "call something akin to Promise.all to compose the two futures into a single one upon which I could await". It sounds like I need to learn more about the concept Rust has for "tasks", as maybe they are providing this "pseudo-thread" abstraction I was discussing in passing. I am seeing terms like "leaf Futures" and "top-level Futures" in some of the documentation I am quickly turning up.

comex6y ago

I don’t fully understand your use case, but in case this helps:

- Future is a trait, not a type, so you can write your own future types.

- Future’s poll method takes a context argument, and async/await-based futures pass that argument unchanged to any sub-futures that they await. The context argument is a pointer to a standard library type (std::task::Context), which itself contains a pointer to a “waker” object that’s meant to be supplied by the executor, with some predefined methods. There’s some room for customization here, but it’s not as flexible as it probably should be – e.g. for now, as far as I can tell, you can’t get the pointer back out of the waker, only call the standard methods on it. Thread-local storage is an option, of course.

steveklabnik6y ago

A “task” is the thing you submit to an executor, that is, the sum of all async/awaits in a single computation. Each await is not its own task.

j / k navigate · click thread line to collapse

0 pointssaurik6y ago0 comments

Thanks for the information!!

0 comments

tmandry6y ago

The concept of a pseudo-thread you're referring to is a task. A task contains a whole tree of futures awaiting other futures. So no manual propagation is necessary.

To summarize the Rust approach, I'd say you have 3 main extension points:

1. The executor, which controls the spawning, prioritization, and execution of tasks

[1]: https://doc.rust-lang.org/1.28.0/std/task/trait.Executor.htm...

[2]: https://rust-lang-nursery.github.io/futures-api-docs/0.3.0-a...

[3]: https://doc.rust-lang.org/1.28.0/std/future/trait.Future.htm...

[4]: https://boats.gitlab.io/blog/post/wakers-i/

saurikOP6y ago

anp6y ago

saurikOP6y ago

comex6y ago

I don’t fully understand your use case, but in case this helps:

- Future is a trait, not a type, so you can write your own future types.

steveklabnik6y ago

A “task” is the thing you submit to an executor, that is, the sum of all async/awaits in a single computation. Each await is not its own task.

j / k navigate · click thread line to collapse