5 ms·

Is it time now to say that async was a mistake, a-la C++ exceptions? The recent futurelock discussion[1] more or less solidified for me that this is all just a

by ajross 1y ago

Is it time now to say that async was a mistake, a-la C++ exceptions? The recent futurelock discussion[1] more or less solidified for me that this is all just a mess. Not just that one bug, but the coloring issue mentioned in the blog post (basically async "infects" project code requiring that you end up porting or duplicating almost everything -- this is especially true in Python). The general cognitive load of debugging inside out code is likewise really high, even if the top-level expression of the loop generator or whatever is clean. And it's all for, what? A little memory for thread stacks (most of which ends up being a wash because of all the async contexts being tossed around anyway -- those are still stacks and still big!)? Some top-end performance for people chasing C10k numbers in a world that has scaled into datacenters for a decade anyway? Not worth it. IMHO it's time to put this to bed. [1] No one in that thread or post has a good summary, but it's "Rust futures consume wakeup events from fair locks that only emit one event, so can deadlock if they aren't currently being selected and will end up waiting for some other event before doing so."

amelius 1y ago
What is wrong about C++ exceptions?
- ajross 1y ago
  "Nothing", in principle. But they're bug factories in practice. It's really easy to throw "past" cleanup code in a language where manual resource management remains the norm. It's not that they can't be used productively. It's that they probably do more harm than good on balance. And I think async mania is getting there. It was a revelation when node showed it to us 15 years ago. But it's evolved in bad directions, IMHO.
  - baq 1y ago
    Yeah node showed that a native async single threaded runtime can be performant. Seems like this knowledge was lost to the world somewhere along windows vista; everyone who has had to ever develop in the cooperative world of early winapi could tell you that easily.
  - efdee 1y ago
    C# had async/await long before Javascript/node. Not that big a revelation ;-)
    - ajross 1y ago
      .NET wasn't the first either. Lisps were doing continuations in the 70's. But "invented" and "revealed" are different verbs for a reason. The release of node.js and it's pervasively async architecture changed the way a lot of people thought about how to write code. For the better in a few ways. But the resulting attempt to shoe-horn the paradigm into legacy and emerging environments that demanded it live in a shared ecosystem with traditional "blocking" primitives and imperative paradigms has been a mess.
      - mrsmrtss 1y ago
        I think you're underestimating the role of .NET in this. It was .NET that popularized this concept for the masses, and from there it spread to other languages including JavaScript, which also borrowed the exact same async/await keywords from C#.
- KerrAvon 1y ago
  For one thing, they’re expensive and viral. “Zero overhead” implementations don’t take into account the need for unwind tables. For every function/method that might be thrown across. They’re disabled in a lot of production environments for this reason.
  - amelius 1y ago
    But if you explicitly handle exceptions using IF statements then that's overhead too, right?
    - arbitrandomuser 1y ago
      yes but i think branch prediction essentialy makes them zero overhead
      - neonz80 1y ago
        That's a different type of overhead than having unwind tables. With exceptions you wouldn't need a branch after each function call at all.
        amelius 1y ago
        But a branch that is (almost) never taken has an overhead close to the overhead of a NOP instruction, which may be negligible on modern architectures.
        neonz80 1y ago
        The CPU can not remember an infinite number of branches. Also, many branches will increase code size. With exceptions the unwind tables and unwind code can be placed elsewhere and not take up valuable L1 cache.
        amelius 1y ago
        > The CPU can not remember an infinite number of branches. I suspect a modern CPU has a branch instruction saying "This branch will never be taken except in exceptions, so assume this branch is not taken". But I must admit I haven't seriously looked at assembly language for some time. (EDIT: yes, modern CPUs including x86 and ARM allow the programmer/compiler to hint if a branch is expected to be taken). > Also, many branches will increase code size. I'd like to see some data on that. Of course branches take code size, but how much is that percentage-wise? I suspect not much.
- jandrewrogers 1y ago
  There are cases in systems-y code where it is not safe to unwind the stack in the ordinary way and it is difficult to contain the side-effects. These can be non-obvious and subtle edge cases that are often difficult to see and tricky to handle correctly. C++ today is primarily used in code contexts where these kinds of issues can occur. This is why it is a standard practice to disable exceptions at build time i.e. -fno-exceptions. With the benefit of hindsight, explicit handling and unwinding has proven to be safer and more reliable.
  - amelius 1y ago
    But you can implement exceptions by using the same IF statement approach you would use for manual error handling. No need for unwinding tables and such if that optimization is a bridge too far for your specific target platform.
jayd16 1y ago
I really wish people would get over the coloring meme. Knowing if a function will yield the thread is actually extremely relevant knowledge you want available.
- pton_xd 1y ago
  Look at the node.js APIs: readFile, readFileSync, writeFile, writeFileSync ... and on and on. If that's not a meme then I don't know what is.
  - rafaelmn 1y ago
    And the alternative without async-await is ? blocking the event loop or the callback pyramid. Node is one place where async-await has zero counter arguments and every alternative is strictly worse.
    - luke5441 1y ago
      They could have added threads to Node as well? Granted, it would have been a lot of difficult work.
      - jayd16 1y ago
        You mean like with web workers or something?
        luke5441 1y ago
        With a shared interpreter/process state, like Python, Java, C, C++, ... Node is not a web page, so no reason to limit it to the same patterns. Then, the next issue would be thread safety. But that could be treated as a separate problem.
      - dns_snek 1y ago
        Losing threads and moving to the async I/O model was the motivation behind Node in the first place. https://nodejs.org/en/about https://nodejs.org/en/about
the__alchemist 1y ago
Concur. I build my own tools in rust when I have to just to avoid it. It is splitting rust into 2 ecosystems, and I wish it didn't exist because it's a big compatibility barrier. We should be moving towards fewer of these; not more. Make code and applications easier to interop; Async makes it more difficult.
- echelon 1y ago
  I can't stand this aversion to async/await. It's not a big deal. I don't understand why async code is being treated as dangerous or as rocket science. You still maintain complete control, and it's straightforward. Now that we know about the "futurelock" issue, it will be addressed. I'm sure Rust and the cargo/crates ecosystem will even grow the ability to mark crates as using async so if you really care to avoid them in your search or blow up at compile time upon import, you can. I've been asking for that feature for unsafe code and transitive dependency depth limits.
  - jeltz 1y ago
    Because async Rust is a lot harder to reason about than sync code. And I want my code to be as easy to reason about as possible.
csande17 1y ago
Your comment is downvoted as I write this, but I kind of think Zig's new design agrees with you. It uses the terms "async" and "await", but the API design looks more similar to traditional threading (like Rust's thread::spawn and join() APIs). With the fun distinction that you can choose whether your program uses actual threads, or coroutines, or just runs everything synchronously without changing any of your code.
rr808 1y ago
With Java 25 virtual threads, async definitely is no longer required and I hope it dies a slow and painful death. We have projects at work that have never more than 3 concurrent users that use rxjava and are a nightmare to work on.

by ajross 1y ago

amelius 1y ago

What is wrong about C++ exceptions?

ajross 1y ago
"Nothing", in principle. But they're bug factories in practice. It's really easy to throw "past" cleanup code in a language where manual resource management remains the norm. It's not that they can't be used productively. It's that they probably do more harm than good on balance. And I think async mania is getting there. It was a revelation when node showed it to us 15 years ago. But it's evolved in bad directions, IMHO.
- baq 1y ago
  Yeah node showed that a native async single threaded runtime can be performant. Seems like this knowledge was lost to the world somewhere along windows vista; everyone who has had to ever develop in the cooperative world of early winapi could tell you that easily.
- efdee 1y ago
  C# had async/await long before Javascript/node. Not that big a revelation ;-)
  - ajross 1y ago
    .NET wasn't the first either. Lisps were doing continuations in the 70's. But "invented" and "revealed" are different verbs for a reason. The release of node.js and it's pervasively async architecture changed the way a lot of people thought about how to write code. For the better in a few ways. But the resulting attempt to shoe-horn the paradigm into legacy and emerging environments that demanded it live in a shared ecosystem with traditional "blocking" primitives and imperative paradigms has been a mess.
    - mrsmrtss 1y ago
      I think you're underestimating the role of .NET in this. It was .NET that popularized this concept for the masses, and from there it spread to other languages including JavaScript, which also borrowed the exact same async/await keywords from C#.
KerrAvon 1y ago
For one thing, they’re expensive and viral. “Zero overhead” implementations don’t take into account the need for unwind tables. For every function/method that might be thrown across. They’re disabled in a lot of production environments for this reason.
- amelius 1y ago
  But if you explicitly handle exceptions using IF statements then that's overhead too, right?
  - arbitrandomuser 1y ago
    yes but i think branch prediction essentialy makes them zero overhead
    - neonz80 1y ago
      That's a different type of overhead than having unwind tables. With exceptions you wouldn't need a branch after each function call at all.
      - amelius 1y ago
        But a branch that is (almost) never taken has an overhead close to the overhead of a NOP instruction, which may be negligible on modern architectures.
        neonz80 1y ago
        The CPU can not remember an infinite number of branches. Also, many branches will increase code size. With exceptions the unwind tables and unwind code can be placed elsewhere and not take up valuable L1 cache.
        amelius 1y ago
        > The CPU can not remember an infinite number of branches. I suspect a modern CPU has a branch instruction saying "This branch will never be taken except in exceptions, so assume this branch is not taken". But I must admit I haven't seriously looked at assembly language for some time. (EDIT: yes, modern CPUs including x86 and ARM allow the programmer/compiler to hint if a branch is expected to be taken). > Also, many branches will increase code size. I'd like to see some data on that. Of course branches take code size, but how much is that percentage-wise? I suspect not much.
jandrewrogers 1y ago
There are cases in systems-y code where it is not safe to unwind the stack in the ordinary way and it is difficult to contain the side-effects. These can be non-obvious and subtle edge cases that are often difficult to see and tricky to handle correctly. C++ today is primarily used in code contexts where these kinds of issues can occur. This is why it is a standard practice to disable exceptions at build time i.e. -fno-exceptions. With the benefit of hindsight, explicit handling and unwinding has proven to be safer and more reliable.
- amelius 1y ago
  But you can implement exceptions by using the same IF statement approach you would use for manual error handling. No need for unwinding tables and such if that optimization is a bridge too far for your specific target platform.

jayd16 1y ago

I really wish people would get over the coloring meme. Knowing if a function will yield the thread is actually extremely relevant knowledge you want available.

pton_xd 1y ago
Look at the node.js APIs: readFile, readFileSync, writeFile, writeFileSync ... and on and on. If that's not a meme then I don't know what is.
- rafaelmn 1y ago
  And the alternative without async-await is ? blocking the event loop or the callback pyramid. Node is one place where async-await has zero counter arguments and every alternative is strictly worse.
  - luke5441 1y ago
    They could have added threads to Node as well? Granted, it would have been a lot of difficult work.
    - jayd16 1y ago
      You mean like with web workers or something?
      - luke5441 1y ago
        With a shared interpreter/process state, like Python, Java, C, C++, ... Node is not a web page, so no reason to limit it to the same patterns. Then, the next issue would be thread safety. But that could be treated as a separate problem.
    - dns_snek 1y ago
      Losing threads and moving to the async I/O model was the motivation behind Node in the first place. https://nodejs.org/en/about https://nodejs.org/en/about

the__alchemist 1y ago

Concur. I build my own tools in rust when I have to just to avoid it. It is splitting rust into 2 ecosystems, and I wish it didn't exist because it's a big compatibility barrier. We should be moving towards fewer of these; not more. Make code and applications easier to interop; Async makes it more difficult.

echelon 1y ago
I can't stand this aversion to async/await. It's not a big deal. I don't understand why async code is being treated as dangerous or as rocket science. You still maintain complete control, and it's straightforward. Now that we know about the "futurelock" issue, it will be addressed. I'm sure Rust and the cargo/crates ecosystem will even grow the ability to mark crates as using async so if you really care to avoid them in your search or blow up at compile time upon import, you can. I've been asking for that feature for unsafe code and transitive dependency depth limits.
- jeltz 1y ago
  Because async Rust is a lot harder to reason about than sync code. And I want my code to be as easy to reason about as possible.

csande17 1y ago

Your comment is downvoted as I write this, but I kind of think Zig's new design agrees with you. It uses the terms "async" and "await", but the API design looks more similar to traditional threading (like Rust's thread::spawn and join() APIs). With the fun distinction that you can choose whether your program uses actual threads, or coroutines, or just runs everything synchronously without changing any of your code.

rr808 1y ago

With Java 25 virtual threads, async definitely is no longer required and I hope it dies a slow and painful death. We have projects at work that have never more than 3 concurrent users that use rxjava and are a nightmare to work on.