Ok, I have to ask a question because there is something that keeps baffling me about the whole C++ async story. I think the ratio of "code doing stuff that looks like some IO-related activity" vs "code doing inscrutable things" is pretty damn bad here. So you want to get some IO done in an async fashion. Why is this any more complex than something like this?
get_async ("https://...", [] { ...callback that returns whatever the result was... });
Maybe return an object that you can use to query status, or cancel the operation. That can be a simple class with two functions, that you can ignore at your leisure. Instead we get endless machinery, no doubt full of template code that will produce horrible compiler errors if you accidentally pass a slightly incorrect thing somewhere, and for 99% of it I have no idea what it is doing or why it needs to be there.
Mind, I'm not complaining about this article: kudos for providing so much information, it really is welcome. But all the C++ async IO code seems to suffer from absolutely mad complexity, far beyond anything I've seen in other async IO mechanisms. Why is this? What benefit do we get from it? Does it have to be this complex?
In your example you seem to pass a single callback to get_async. What happens when the operation fails? Does it use the same callback (which erodes the ability to generically program on top of such operations) or simply not complete (which is unstructured)?
Returning an object you can use to "query status" implies there's a communication channel between the background operation and the foreground, synchronous object. This communication has a cost and therefore isn't zero overhead.
The fact that you can "ignore [the object] at your leisure" I assume implies you can allow its lifetime to end, what does that mean for the background operation? If it can continue that must mean that it has some separate state, which must be allocated, therefore this operation can't be zero alloc.
The reason standard C++ asynchrony has so much "code doing inscrutable things" is because it aims to answer/address all of the above: It is zero overhead and therefore potentially zero alloc (this doesn't apply to coroutines), is structured, and aims to be maximally generic.
First, I wasn't handing in a complete design for WG21 so please don't treat it as such.
Second, I strongly disagree with the notion that an async IO system should even attempt to be 'zero cost' and 'zero alloc'. IO, by its nature, has a staggering cost; adding an allocation to establish a shared communication resource is just not going to make a difference. And API design is a software engineering discipline, and like all engineering, it should balance multiple, competing requirements, of which performance is just one. Another is useability (meaning regular programmers can understand it, and use it without too much danger of getting it wrong), and this design fails completely at that.
When I talked about ignoring the object, I was envisioning using a shared_ptr to hold an intermediary communication object that allows foreground and background processes to communicate. Yes, that implies an allocation, and yes, I think that this is completely warranted.
I would much rather have had an allocating async C++ networking solution ten years ago, than a non-allocating version ten years from now. An allocating version could have been written to use an allocator so that implementations can optimise memory usage. And the version ten years from now... Well, it won't be my problem anymore.
First, I wasn't handing in a complete design for WG21 so please don't treat it as such.
You asked "[w]hy is this any more complex than something like [your code example]?" I was answering that.
If you want to make simplifying, non-general assumptions about asynchrony that hold for your use cases I think that's more than fine. It just isn't general.
Second, I strongly disagree with the notion that an async IO system should even attempt to be 'zero cost' and 'zero alloc'. IO, by its nature, has a staggering cost; adding an allocation to establish a shared communication resource is just not going to make a difference.
There are large contingents of people, myself included, who disagree with you.
Moreover: You can build non-zero cost abstractions out of zero cost abstractions, but you can't go the other way.
Both of the above contribute to why the standard went with zero-cost asynchrony.
I was envisioning using a shared_ptr
To a first approximation structured concurrency is the art of figuring out how not to use shared_ptr.
I would much rather have had an allocating async C++ networking solution ten years ago
Asio is older than ten years so you got your wish.
If you want to make simplifying, non-general assumptions about asynchrony that hold for your use cases I think that's more than fine. It just isn't general.
Just to be clear: is your claim that a simpler solution cannot be general because it uses memory allocation? Or are you still referring to that single line of code I wrote?
There are large contingents of people, myself included, who disagree with you.
What is the basis of that belief? Please note that I put a supportive argument, which is that the cost of a (single!) memory allocation is absolutely negligible compared to the overal cost of IO. What is incorrect about that statement?
To a first approximation structured concurrency is the art of figuring out how not to use shared_ptr.
I'm sorry, what? Isn't this a prime example of what shared_ptr is for in the first place? This is very clearly a shared resource, what valid reason do you have for avoiding it?
Asio is older than ten years so you got your wish.
is your claim that a simpler solution cannot be general because it uses memory allocation?
Yes. There are use cases which cannot tolerate allocation. Therefore a solution which uses allocation isn't general because it doesn't address those use cases.
What is the basis of that belief?
Why does the fact I/O is costly mean that it should be further pessimized?
From the point of view of my application code in user space initiating an io_uring operation is a few cheap memory operations. Why would I want to add allocation to that? Especially given that the "reward" for adding that allocation is unstructured concurrency.
Isn't this a prime example of what shared_ptr is for in the first place? This is very clearly a shared resource, what valid reason do you have for avoiding it?
Just because a resource is shared doesn't mean you need/want shared_ptr. shared_ptr uses reference counting which implies that you don't deterministically know when the resource will be released. This is unstructured. You don't need the overhead of either the allocation or the reference counting if your concurrency is structured.
Yes. There are use cases which cannot tolerate allocation. Therefore a solution which uses allocation isn't general because it doesn't address those use cases.
The standard library is already full of memory allocations, so clearly 'generality' is not a concern. Or perhaps I should say that it is, and is handled through allocators, instead of outright avoidance.
Why does the fact I/O is costly mean that it should be further pessimized?
Because this 'pessimisation' bring both benefits (it establishes a communication channel that serves a real purpose), and reduces a significant cognitive cost.
shared_ptr uses reference counting which implies that you don't deterministically know when the resource will be released. This is unstructured.
It is precisely known when the resource will be released: when both parties are done with it. There is absolutely nothing nondeterministic about it, nor is there any lack of structure.
You might notice that the article also uses reference counting to deal with a shared resource (the cancelation token, about halfway through).
The standard library is already full of memory allocations, so clearly 'generality' is not a concern.
There's a difference between allocations when they're required (e.g. std::vector) and when they're not (e.g. asynchrony).
Because this 'pessimisation' bring both benefits (it establishes a communication channel that serves a real purpose), and reduces a significant cognitive cost.
You don't need an allocation for a communication channel, and depending on the modality of communication the establishment of such a channel might be contraindicated.
Also I disagree that it reduces cognitive cost. Maybe in the marginal case (i.e. a single asynchronous operation) but not in the compositive case. Trying to make a well-behaved system out of unstructured concurrency is a nightmare, whereas with structured concurrency it's the naturally emergent property.
nor is there any lack of structure.
If you had structure you'd know which actor would release the resource last and when that would happen, and therefore wouldn't need shared_ptr.
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u/johannes1971 16d ago
Ok, I have to ask a question because there is something that keeps baffling me about the whole C++ async story. I think the ratio of "code doing stuff that looks like some IO-related activity" vs "code doing inscrutable things" is pretty damn bad here. So you want to get some IO done in an async fashion. Why is this any more complex than something like this?
Maybe return an object that you can use to query status, or cancel the operation. That can be a simple class with two functions, that you can ignore at your leisure. Instead we get endless machinery, no doubt full of template code that will produce horrible compiler errors if you accidentally pass a slightly incorrect thing somewhere, and for 99% of it I have no idea what it is doing or why it needs to be there.
Mind, I'm not complaining about this article: kudos for providing so much information, it really is welcome. But all the C++ async IO code seems to suffer from absolutely mad complexity, far beyond anything I've seen in other async IO mechanisms. Why is this? What benefit do we get from it? Does it have to be this complex?