Comment by munch117 | Hacker News Reader

Comment by munch117 | Hacker News Reader

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Advantage: No function colouring. Just like with threads, you don't need to declare which functions might potentially yield. There's none of that noisy async/await syntax.

Advantage: Fast switching. Switching between fibers is done by swapping in new values for the program counter and stack pointer. Compare with async, where yielding only takes you up one level of stack, so your code ends up walking a stack of coroutine invocations to get all the way back to the scheduler.

Disadvantage: Expensive setup - each fiber needs its own pre-allocated stack.

Disadvantage: Operating systems don't provide I/O systems readymade to work with fiber schedulers, so if you want to yield to a different fiber during blocking I/O, then you need to invent your own system to handle that.

PS about naming: "Fiber" is the Microsoft Windows name for their implementation of full stack coroutines, but the proper platform-independent name for it is actually "coroutine". I've kept with the word "fiber" here to avoid misunderstandings.

PS about C++: This is based on async like you find it in C# or Python, I don't really know about C++ coroutines.

A disadvantage of the ‘no function coloring’ in fibers is that it makes lockless programming harder. A nested function call can switch from under you without your knowledge, making it hard to know where the preemption points are and whether to take locks when making updates to shared state. With function coloring you immediately see whether a function might switch as there’s a different calling syntax.

I’ve programmed both fiber based systems and coroutines. I even created my own fiber libraries for Python (https://github.com/geertj/gruvi) and C++ (https://github.com/geertj/cgreenlet, mostly an experiment, and incorrectly named coroutines for C++ while it’s really fibers). In the Python version I experimented with some features to help you know whether a nested function might switch.

In the end, for me and for the problem domains I worked in, the explicit async/await co-routine style wins over fibers. It gives you most of the performance benefits of user mode switching, all of the memory benefits, while keeping your code mostly lock free.

[I assume you are using lock free in a loose way, as lacking explicit mutual exclusion].

In practice in most real world applications in C++, absence of async calls is not enough to guard against re-entrancy. Shared state might still be mutated by callbacks, by recursively re-entering the event loop or by other threads. So you need to either document explicitly all the assumptions you are making and vet every single function you are calling in your implicit critical section, or you need some other mechanism that is going to look like a critical section anyway.

Yeah I was using the term lockless loosely here, not referring to atomics.

I agree that using explicitly switched coroutines is not sufficient for mostly “lockless” programming (but I do think it’s necessary). I was coming from a thread per core perspective where you run one event loop per core and limit cross core communications (like in the Seastar framework). With this you should be able to perform most state manipulations without locks.

That is where an Actor or CSP style comes in useful, one then avoids the locks by having that state mutated in the context of one coroutine alone.

Most problems can be recast in to that form, only a small subset are better handled by mutable shared state (and hence explicit locks. In those cases one should probably be modelling the updates as 'Compare and Swap' / 'Compare and Set' types of mutable operation, hence avoiding the situation where unexpected preemption can occur while holding a lock.

From my perspective, having use both forms (manually and with language support), I generally find the coroutine (plus Actor/CSP) scheme to be easier to work with and reason about.

> A disadvantage of the ‘no function coloring’ in fibers is that it makes lockless programming harder.

But if you treat fiber-style coroutines like threads, and use locks and message queues exactly the same as you would with threads (although differently implemented, of course), then coroutines are basically better-behaved, and nicely deterministic, threads. It's a wonderful programming model, with the one big drawback that you can't take advantage of multiple cores.

C++ coroutines are based on C# async model, as they were originally proposed by Microsoft, they even use similar concepts in regards to compiler magic of Awaitable types with special methods being reckognised by the compiler, as means to create an awaitable type for any kind of data structure.

That's what I thought, thanks for confirming.

Fun fact, what you call async is also widely referred to as coroutines. Stackless coroutines vs stackful coroutines seems like the most appropriate way to disambiguate the two.

Thanks, stackful/stackless is noted for next time. I still kinda like using "async" as a shorthand for stackless coroutines though.

Before async for I/O became popular, stackless coroutines were mainly used for generator abstractions (for example yield in python), so async can be reductive.

And that's exactly why I would say "async", because then everyone will know that I'm talking about something like async in Python/C#, and not something like Python generators.

Technically, they're may well be almost the same thing, but practically, they're used very differently.

Just a note on naming: fibers, goroutines, green threads, virtual threads and the like are stackful coroutines (miniature threads, essentially). Then there are stackless coroutines, which are state machine objects that have function coloring (async/await).

I see. Seems pretty similar conceptually to C++ coroutines.

Fast switching between fibers is mainly what got me interested, of course. Thank you for the explanations.

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