7 ms·

I'm waiting for the day when 'async' will be the default function type and 'await' will be the default type of function call. If anywhere down the callstack a

• drb91 8y ago
  Async being a “default” function type doesn’t make much sense unless you’re doing io. If you’re just computing, you often can’t proceed without the result and want to proceed as soon as you have the result—i.e. blocking. Both are critical to getting good performance. I’m curious if discerning which is the best type is possible to tool automatically with static or tracing analysis.
  • gpderetta 8y ago
    Generators, a-la python for example, are completely equivalent to async functions and have nothing to do with IO.
    • drb91 8y ago
      True! However, I believe rust has a separate, distinct yield/generator functionality coming that should address those needs separately. I think off-hand it’d be pretty easy to wrap them in a stream, or to join a stream into a generator, if you did want to compose them.
      • steveklabnik 8y ago
        That’s correct; async/await uses Generators under the hood, but we haven’t stabilized them yet. It’s the plan to though!
• quotemstr 8y ago
  Having made "async" the default, you've just returned to regular threading, which is what we should have stuck with all along. The thread model is actually pretty useful, the pitfalls are well understood, and the tooling very mature.
  • noobermin 8y ago
    I feel like you're making a very interesting point but it seems a bit broad and abstract for me to justify in my head. Could you clarify what you mean?
    • ploxiln 8y ago
      If you want easier concurrency with higher overhead, that's exactly what OS threads do. The OS kernel is the "event loop" in this case, and they're highly optimized for this, though there is some overhead which is hard to avoid. Rust has gone back and forth on this. Pre-1.0 versions of Rust had a "green threads" mode. It was removed in favor of OS threads because it wasn't worth the complexity. Go is a language in which "all functions are async" and it's pretty cool. But it needs a runtime, and interop with C / C++ has some real overhead related to switching to a C-style stack. Rust did not want to make those compromises, it wanted to be appropriate for bare-metal or kernel programming, and for libraries used by big high-performance C++ applications. In these cases, Go's model does not work well. I personally use Go a lot, it's great for network services and command-line tools, and that's mostly what I do.
      • deleted 8y ago
        [deleted]
      • weberc2 8y ago
        > In these cases, Go's model does not work well. I don't think there's anything about Go's concurrency model that precludes bare metal; I think it's just that the maintainers didn't want to support a bare metal runtime. In fact, I recall at least one other project that modified Go's runtime to support exactly this. I'm also not sure how much of the C interop overhead is due to the different stack models and how much is due to GC concerns or other things. But generally I agree with you--in practice Rust is well suited for these types of tasks and Go is not.
• kevingadd 8y ago
  One of the biggest advantages of await is that it's explicit. Otherwise literally any function call can block, and whether or not it blocks is dependent on the body of the function so you can end up with races appearing in random parts of your code because you update a package. It's bad. At that point you might as well just use threads and blocking calls.
  • wahern 8y ago
    Data races like that can only happen when you share mutable state across logical flows of control. Don't share mutable state, regardless of your concurrency framework. I don't see how explicit vs implicit yield points change this fact. Either way there's precisely one logical flow of control that should own your data. In a language like Rust this is strongly enforced, regardless. Futhermore, in any complex asynchronous I/O app most functions will end up being tagged async. The only ones that won't are simple leaf functions that wouldn't implicitly yield, anyhow. If someone has the bad idea to, e.g., put a yield point in non-obvious leaf functions as part of some kind of hack (e.g. logging, tracing), they're gonna do it in the async/await case, too, because they're already convinced it has value. Having to a drop a few more annotations here or there won't stop them from breaking the app. Lastly, the bugs that do occur in these sorts of cases usually have to do with unpredictable latencies violating implicit or accidental ordering assumptions. async/await doesn't mitigate that at all because latencies are just as unpredictable. The solution, as always, is to avoid these ordering dependencies by not sharing mutable state. This defense of async/await is a red herring.
• yazaddaruvala 8y ago
  This seems like the next logical step. A language where all functions are async/await. And it’ll likely pull in some nice features from academia like, building a dependency graph of async statements so it can automatically reorder async statements to get optimal concurrency.
  • arianvanp 8y ago
    This is Haskell's IO type and its do notation ! And the reordering of statements exists too. It's called ApplicativeDo[0] and is heavily used at Facebook[1] [0] https://www.microsoft.com/en-us/research/wp-content/uploads/2016/08/desugaring-haskell-haskell16.pdf https://www.microsoft.com/en-us/research/wp-content/uploads/... [1] https://youtu.be/sT6VJkkhy0o https://youtu.be/sT6VJkkhy0o
  • Peaker 8y ago
    There's a huge advantage to not everything being async/await. Knowing exactly when you yield control can give you atomicity and more determinism for free.
• noobermin 8y ago
  Not everything is a webserver/frontend/web related.
  • eikenberry 8y ago
    I'm curious as to why you think that matters? Async/Await are general purpose concurrent programming abstractions.
    • ori_b 8y ago
      Concurrency has overhead. On top of that, synchronous is easier to reason about.
    • tormeh 8y ago
      They are, but calling a function asynchronously takes a lot longer than synchronously. For small functions the execution time could easily be increased by more than 100%. It's something that only really makes sense if the function will do (slow) I/O or a lot of computation, and the program has other things it could do in the meantime. Making every function call asynchronous is likely to make most programs a lot slower.
    • noobermin 8y ago
      Alright, so I do computational plasma physics simulations for a living. Given our problem sizes often reach sizes which require teraflop level of parallelism to get done before I retire (that's my scale, I'm sure you're aware of petaflop sims the fluid/engineering guys do) we do need concurrency but it's done by chopping up the domain, literally the volume of space we simulate, onto processors. Now, we could possibly do async at some level (sub node level), but we absolutely need to be syncronized globally because we are simulating real-life physics and we need to maintain causality, especially when we pass information between nodes. Say a wave front passes across one node boundary into an adjacent one, waves must move at a finite and causal manner, of course, just as if you watched a ripple on the surface of a puddle. If we did the whole problem async (async by default as I interpret it) it would require so much bookkeeping to keep causality that would make it unreasonably difficult. For things that require causality like that, we are lucky that the default mode of computation is synchronous because it fits my problem domain perfectly. That's why I say "not everything is web" because while async maps well to problems like a server-client type application, it doesn't map well to all problems like my problem for example. Also, as someone else said, sync is easier to reason about for problems that don't require a lot of parallelism.
• barrkel 8y ago
  That model is called green threads. Continuation passing style means passing along your continuation to your callee, and is the implementation of the async model. But normal calls do pass their continuation: the return address, pushed on the stack, is the continuation function, and the stack itself is the scope, and together they are a closure. Make the stack a first class value that can be switched, and you get to continuations - and that's what the runtime gets with green threads. It means much harder interop because you can't have native frames on your stack. But you have the same problem with async.
  • gpderetta 8y ago
    Green threads is very different from CPS actually both from an implementation and semantics point of view. In a CPS model the continuation is first class and is available at every single call point. In practice this would require heap allocating every single function frame, which is expensive and and a huge problem for interoperability (a smart compiler can turn it back to a traditional stack if the continuation is only used in traditional ways) With the normall call/return model, the implicit continuation in a normal call stack can only be accessed via return. Green threads do allow accessing the continuation at specific yield points (although most green thread implementations do not expose it to the user) but because the yield point are much fewer than every single call, a whole stack can be allocated and used in one go. There aren't in fact many issues with interoperability as foregin functions or even os calls can be accommodated on this stack. Async is a tradeoff, basically the programmer is responsible of marking functions that are to be CPS transformed and that need their stack frame reified. This way no full stack is needed nor there are the performance and interoperability issues of full CPS. You end up with the blue/red functions issue though.
    • barrkel 8y ago
      If you want call/cc (for repeated executions of the continuation) you need a first class stack. A CPS execution model can use green threads right up until you want that explicit continuation in hand and split the stack. In most implementations of CPS the continuation is not in fact available for the user to execute: stack-oriented programming is the mental model, until you pull out call/cc or something similar. Note that you don't get the continuation in hand for the async-everywhere advocated by GP, only blocking operations get the continuation, and most of those are implemented by the runtime.
• Koshkin 8y ago
  > a function needs to await something According to a famous quote, the source of which escapes me, "await does not wait for anything, and async is not asynchronous." (This could be specific to C# though.)
• jeremyjh 8y ago
  Rust worked this way earlier in its development. The keyword as others mention is “green threads”. It moved away from this because this model has some overheads, and not every application is a socket server. Languages with this feature include Go, Haskell and Erlang.
  • algesten 8y ago
    And also the ABI compatibility for C interop is harder without a "classic" stack.
• pron 8y ago
  This is the situation in Scheme, Erlang and Go (although the latter two strongly bind continuations with a specific, built-in scheduling), and what we're doing in Java as part of Project Loom. All Java functions will be able to run inside delimited continuations without modification. Continuations/coroutines are thus a purely dynamic rather than a syntactic entity.
  • avodonosov 8y ago
    Any news on Project Loom? When to expect it available?
    • pron 8y ago
      We should have a public repo you can use to build Loom (a prototype at this stage) in a couple of weeks.
      • avodonosov 8y ago
        What does such build produce? A JVM? A bytecode converter which produced code for existing JVMs?
        • pron 8y ago
          A full JDK (with a VM, of course). Loom includes both changes to the VM and the core JDK libraries.
          • avodonosov 8y ago
            Is there any roadmap for inclusion into release?
            • pron 8y ago
              No. OpenJDK projects are no longer planned to target a specific release.
• weberc2 8y ago
  One thing I like about Go is that there is no dichotomy between sync and async. Every function call presents a sync interface, but under the hood all I/O is async. I really miss this in Python, where every library has an async twin, usually maintained by someone unafiliated with the sync library.
• kibwen 8y ago
  I think that being async-by-default makes sense for any new high-level language written these days, but the machinery required to make that happen is probably always going to be magic enough that there will be some space for low-level languages that will find value in giving the user choice between synchronous and asynchronous operations.
• k__ 8y ago
  So we remove async/await from our async functions and add sync/block to our sync functions? :D
  • pimeys 8y ago
    Actually Tokio already has `blocking` implemented! https://docs.rs/tokio-threadpool/0.1/tokio_threadpool/fn.blocking.html https://docs.rs/tokio-threadpool/0.1/tokio_threadpool/fn.blo...
• infogulch 8y ago
  I think it should be possible to design an async system that is generically async, i.e. whether a given function with no special annotations or syntax is async (including all of the async-able calls that function makes transitively) is entirely determined by the top-level caller at compile time. The called function doesn't need any special syntax, and in fact doesn't even need to consider being async at all: the async-capability bubbles up based on what other async-able functions it calls. Only the very ends of the chain: at the bottom doing IO via system calls, or at the very top where request handlers are first dispatched (for example) should need to think about being asynchronous or synchronous at all; everything in-between is compiled for whatever the top wants.
  • gpderetta 8y ago
    To allow for separate compilation you need to compile each function twice though: a CPS version and 'classic' one. This can be wasteful. You also need annotations in the object file to describe the frame size of each CPS function. At some point this was seriously considered for C++ and I'm sure some language implementations actually do it. Or you go with cactus stacks which have their own set of issues.
    • infogulch 8y ago
      > you need to compile each function twice ... This can be wasteful. More wasteful than writing each function twice? See C# for endless examples of libraries that have manual duplicate X() and XAsync() for every single method. If you compare the two they're almost always have the exact same body, except the async version has "async" and "await" peppered in and calls duplicate XAsync methods (which are implemented the same, except they call XAsync methods... you might be seeing a pattern here). If I'm not mistaken most rust doesn't use separate compilation and is built from source. In this case the compiler can treat it just like normal generics and just not compile it until it's used. And even with separate compilation for, e.g. a library, a very basic LTO would trivially trim out the unused methods when compiled into a final binary. And if you don't want 2x library itself, even then you can just use an escape hatch to turn it of on the crate/impl/function level and it's still better for you than the status-quo where you have to manually add a bunch of tokens and junk that the compiler already knows how to do precisely. If anything is wasted it's human effort because we're wasting our time doing something the computer can do better and faster. > "annotations in the object file to describe the frame size" Wouldn't you need this anyway for anything async, even if it's manual? This seems like an argument against async in general, not automatic async.