You may think of use of an async keyword as explicit async code but that is very much not the case.
If you want to see async code without the keyword, most of the code of Linux is asynchronous.
Kernel-style async code, where everything is explicit:
* You write a poller that opens up queues and reads structs representing work
* Your functions are not tagged as "async" but they do not block
* When those functions finish, you explicitly put that struct in another queue based on the result
Async-await code, where the runtime is implicit:
* All async functions are marked and you await them if they might block
* A runtime of some sort handles queueing and runnability
Green threads, where all asynchrony is implicit:
* Functions are functions and can block
* A runtime wraps everything that can block to switch to other local work before yielding back to the kernel
which are no different from app POV from kernel threads, or any threads for that matter.
the whole async stuff came up because context switch per event is way more expensive than just shoveling down a page of file descriptor state.
thus poll, kqueue, epoll, io_uring, whatever.
think of it as batch processing
Let me try to clarify my point of view:
I don’t mean that async/await is more or less explicit than goroutines. I mean regular threaded code is more explicit than async/await code, and I prefer that.
I see colleagues struggle to correctly analyze resource usage for instance. Someone tries to parallelize some code (perhaps naiively) by converting it to async/await and then run out of memory.
Again, I don’t mean to judge anyone. I just observe that the async/await-flavor has more bugs in the code bases I work on.
More explicit in what sense? I've written both regular threaded Python and async/await Python. Only the latter shows me precisely where the context switches occur.
Everything is in a run loop that does not exist in my codebase.
The context switching points are obvious but the execution environment is opaque.
At least that's how it looks to me.
Green threads are better (IMHO), because they actually do hide all the machinery. As a developer in a language with mature green threads (Erlang), I don't have to know about the machinery[1], I just write code that blocks from my perspective and BEAM makes magic happen. As I understand it, that's the model for Java's Project Loom aka Java Green Threads 2: 2 Green 2 Threads. The first release had some issues with the machinery, but I think I read the second release was much better, and I haven't seen much since... I'm not a Cafe Babe, so I don't follow Java that closely.
[1] It's always nice to know about the machinery, but I don't have to know about it, and I was able to get started pretty quick and figure out the machinery later.
I would say that green threads still have "function coloring stuff", we just decided that every function will be async-colored.
Now, what happens if you try to cross an FFI-border and try to call a function that knows nothing about your green-thread runtime is an entirely different story...
Thank you for explaining much more clearly than I could.
> none of the function coloring stuff
And it’s this part that I don’t like (and see colleagues struggling to implement correctly at work).
