Looking at Unity made me understand the point of C++ coroutines
mropert.github.io
mropert.github.io
[0] https://web.archive.org/web/20260105235513/https://www.chiar...
Also, as noted in that Simon Tatham article, Python makes choices at the language level that you have to fuss over yourself in C++. Given how different Trio is from asyncio (the async library in Python's standard library), it seems to me that making some of those basic choices wasn't actually that restrictive, so I'd guess that a lot of C++'s async complexity isn't that necessary for the problem.
[1] https://vorpus.org/blog/notes-on-structured-concurrency-or-g...
I will say that it's still not as nice as using Trio. Partly that's because it has edge-triggered cancellation (calling task.cancel() injects a single cancellation exception) rather than Trio's level-triggered cancellation (once a scope is cancelled, including the scope implicit in a nursery, it stays cancelled so future async calls all throw Cancelled unless shielded). The interaction between asyncio TaskGroup and its older task API is also really awkward (how do I update the task's cancelled count if an unrelated task I'm waiting on throws Cancelled?). But it's a huge improvement if you're forced to use asyncio.
In the context of an article about C++’s coroutines for building concurrency I think structured concurrency is out of scope. Structured concurrency is an effective and, reasonably, efficient idiom for handling a substantial percentage of concurrent workloads (which in light of your parent’s comment is probably why you brought up structured concurrency as a solution); however, C++ coroutines are pitched several levels of abstraction below where structured concurrency is implemented.
Additionally, there is the implementation requirements to have Trio style structured concurrency function. I’m almost certain a garbage collector is not required so that probably isn’t an issue, but, the implementation of the nurseries and the associated memory management required are independent implementations that C++ will almost certainly never impose as a base requirement to have concurrency. There are also some pretty effective cancelation strategies presumed in Trio which would also have to be positioned as requirements.
Not really a critique on the idiom, but I think it’s worth mentioning that a higher level solution is not always applicable given a lower level language feature’s expected usage. Particularly where implementing concurrency, as in the C++ coroutines, versus using concurrency, as in Trio.
Most people would prefer opinionated libraries that allow them to not think about the design tradeoffs. The core implementation is targeted at efficient creation of opinionated abstractions rather than providing one. This is the right choice. Every opinionated abstraction is going to be poor for some applications.
For the `tick`, the core language gets an un-opinionated iteration of the feature that is meant for compiler developers and library writers to play with. (This is why we sometimes see production compilers lagging behind in features).
For the `tock`, we try to get the standard library improved with these features to a realistic extent, and also fix wrinkles in the primary idea.
This avoids the standard library having to rely on any compiler magic (languages like swift are notorious for this), so in practice all libraries can leverage the language to the same extend.
This pattern has been broken in a few instances (std::initializer_list), and those have been widely considered to have been missteps.
I don’t doubt your point (I know enough about Swift’s generic resolution crapshow during semantic analysis to be justified in assuming the worst) but can you think of any areas worth looking into for expansion of the compiler magic issues.
I have a near reflexive revulsion for the kinds of non-composability and destruction of principled, theoretically sound language design that tends to come from compiler magic and shortcuts, so always looking for more reading to enrage myself.
This is actually a good example by itself.
Int is defined in swift with Builtin.int64 IIRC. That is not part of the swift language.
I will be adding the following to my “Primary Design Criteria” list: The core design and implementation of any language feature is explicitly targeted at the efficient creation of opinionated, composable abstractions rather than providing those abstractions at the language level.
Additionally, for those with .NET background, C++ co-routines are pretty much inspired by how they work in .NET/C#, naturally with the added hurdle there isn't a GC, and there is some memory management to take into account.
Also so even if it takes some time across ISO working processes, there is still a goal to have some capabilities on the standard library, that in Rust's case means "use tokio" instead.
Thanks for the list.
1. C++20 coros are stackless, in the general case every async "function call" heap allocates.
2. If you do your own stackful coroutines, every function can suspend/resume, you don't have to deal with colored functions.
3. (opinion) C++20 coros are very tasteless and "C++-design-commitee pilled". They're very hard to understand, implement, require the STL, they're very heavy in debug builds and you'll end up with template hell to do something as simple as Promise.all
I'm not normally keen to "well actually" people with the C standard, but .. if you're writing in assembly, you're not writing in C. And the obvious consequence is that it stops being portable. Minicoro only supports three architectures. Granted, those are the three most popular ones, but other architectures exist.
(just double checked and it doesn't do Windows/ARM, for example. Not that I'm expecting Microsoft to ship full conformance for C++23 any time soon, but they have at least some of it)
This support table is complete mess. And saying "most platforms are supported" is too optimistic or even cocky.
They are actively working on it for their VS2026 C++ compiler. I think since 2017 or so they've kept up with C++ standards reasonably? I'm not a heavy C++ guy, so maybe I'm wrong, but my understanding is they match the standards.
These days on Linux/BSD/Solaris/macOS you can use makecontext()/swapcontext() from ucontext.h and it will turn out roughly the same performance on important architectures as what everyone used to do with custom assembly. And you already have fiber functions as part of the Windows API to trampoline.
I had to support a number of architectures in libdex for Debian. This is GNOME code of course, which isn't everyone's cup of C. (It also supports BSDs/Linux/macOS/Solaris/Windows).
Also, although not likely to be removed anytime soon from existing systems, POSIX has declared the context API obsolescent a while ago (it might actually no longer be part of the standard).
It can be safely ignored for the vast majority of apps. If you're using multithreading (quite likely if you're doing coroutines), then signals are not a good fit anyway.
That's the problem with register machines, I guess. Interestingly enough, BCPL, its main implementation being a p-code interpreter of sorts, has pretty trivially supported coroutines in its "standard" library since the late seventies — as you say, all you need to save is the current stack pointer and the code pointer.
Actually you don't even need setjmp/longjmp. I've used a library (embedded environment) called protothreads (plain C) that abused the preprocessor to implement stackful coroutines.
(Defined a macro that used the __LINE__ macro coupled with another macro that used a switch statement to ensure that calling the function again made it resume from where the last YIELD macro was encountered)
This would be about 32 years ago - I don't like thinking about that ...
Passing uncaught exceptions from the coroutine up to the caller is also pretty easy, because it's all synchronous. You just need to wrap it so it can safely travel across the gap. You can restrict the exception types however you want. I chose to support only subclasses of std::exception and handle anything else as an unknown exception.
This is also how dotnet handles it, and you can choose whether to rethrow at the caller site, inspect the exception manually, or run a continuation on exception.
You could use the same trick used by glibc to implement unstoppable exceptions for POSIX cancellation: the exception rethrows itself from its destructor.
The stack save/restore happens in: https://swtch.com/libtask/asm.S
That was over 20 years ago. No idea what the current hotness is.
Which is why a solution with no (or very tiny) context switching is preferred over one that's heavy to switch.
> they must have been really, really simple stuff
Yes, because they were low-overhead it was trivial to start them for all kinds of tiny things.
> require the STL
That it has to heap-allocate if non-inlined is a misconception. This is only the default behavior.
One can define:
void *operator new(size_t sz, Foo &foo)
in the coro's promise type, and this:
- removes the implicitly-defined operator new
- forces the coro's signature to be CoroType f(Foo &foo), and forwards arguments to the "operator new" one defined
Therefore, it's pretty trivial to support coroutines even when heap cannot be used, especially in the non-recursive case.
Yes, green threads ("stackful coroutines") are more straightforward to use, however:
- they can't be arbitrarily destroyed when suspended (this would require stack unwinding support and/or active support from the green thread runtime)
- they are very ABI dependent. Among the "few registers" one has to save FPU registers. Which, in the case of older Arm architectures, and codegen options similar to -mgeneral-regs-only (for code that runs "below" userspace). Said FPU registers also take a lot of space in the stack frame, too
Really, stackless coros are just FSM generators (which is obvious if one looks at disasm)
A pure library implementation that uses on normal function call semantics obviously needs to conservatively save at least all callee-save registers, but that's not the only possible implementation. An implementation with compiler help should be able to do significantly better.
Ideally the compiler would provide a built-in, but even, for example, an implementation using GCC inline ASM with proper clobbers can do significantly better.
Also, this is not some random GitHub Repo, Chris Kohlhoff is the developer of ASIO :)
In a Unity context, the engine provides the main loop and the developer is writing behaviors for game entities.
Why? You can just as well execute all your coroutines on a single thread. Many networking applications are doing fine with just use a single ASIO thread.
Another example: you could write game behavior in C++ coroutines and schedule them on the thread that handles the game logic. If you want to wait for N seconds inside the coroutine, just yield it as a number. When the scheduler resumes a coroutine, it receives the delta time and then reschedules the coroutine accordingly. This is also a common technique in music programming languages to implement musical sequencing (e.g. SuperCollider)
Multithreaded? Nope. You can do C++ coroutines just fine in a single-threaded context.
Event loop? Only if you're wanting to do IO in your coroutines and not block other coroutines while waiting for that IO to finish.
> most people end up using coroutines with something like boost::asio
Sure. But you don't have to. Asio is available without the kitchen sink: https://think-async.com/Asio/
Coroutines are actually really approachable. You don't need boost::asio, but it certainly makes it a lot easier.
I recommend watching Daniela Engert's 2022 presentation, Contemporary C++ in Action: https://www.youtube.com/watch?v=yUIFdL3D0Vk
The most helpful resource about it is a guy on stackoverflow (sehe). No idea how to get help once SO will have closed
You can call a function that makes use of coroutines without worrying about it. That's the core intent of the design.
That is, if you currently use some blocking socket library, we could replace the implementation of that with coroutine based sockets, and everything should still work without other code changes.
It can easily and often does lead to messy rube goldberg machines.
There was a game AI talk a while back, I forget the name unfortunately, but as I recall the guy was pointing out this friction and suggesting additions we could make at the programming language level to better support that kind of time spanning logic.
Sounds interesting. If it's not too much of an effort, could you dig up a reference?
Mind you my memory may have distorted it a little beyond what it was, but it's loosely on the topic!
This what Rich Hickey (Clojure author) has termed “place oriented programming”, when the focus is mutating memory addresses and having to synchronize everything, but failing to model time as a first class concept.
I’m not aware of any general purpose programming language that successfully models time explicitly, Verilog might be the closest to that.
Step 1, solve "time" for general computing.
The difficulty here is that our periods are local out of both necessity and desire; we don't fail to model time as a first class concept, we bring time-as-first-class with us and then attempt to merge our perspectives with varying degrees of success.
We're trying to rectify the observations of Zeno, a professional turtle hunter, and a track coach with a stopwatch when each one has their own functional definition of time driven by intent.
That’s similar to most of what makes C++ tick: There’s no deep magic, it’s “just” type-checked syntactic sugar for code patterns you could already implement in C.
(Occurs to me that the exceptions to this … like exceptions, overloads, and context-dependent lookup … are where C++ has struggled to manage its own complexity.)
This is why coroutine-based frameworks (e.g., C++20 coroutines with cppcoro) have largely superseded future-chaining for async state machine work — the generated code is often equivalent, but the source code is dramatically cleaner and closer to the synchronous equivalent.
(me: ex-Visual Studio dev who worked extensively on our C++ coroutine implementation)
Do you believe that std::future is the better option?
With the coroutine approach using yield, doesn't that mean the caller needs to decide when to call it again? With the std::future approach where it's event driven by the promise being set when that state/step has completed.
> "The only 'assembly' required is creating the associated promise"
Again, that is only true for one step. For a state machine with N states you need explicit state enums or a long chain of .then() continuations. You also need to the manage the shared state across continuations (normally on the heap). You need to manage manual error propagation across each boundary and handle the cancellation tokens.
You only get a "A nice readable linear flow" using std:future when 1) using a blocking .get() on a thread, or 2) .then() chaining, which isn't "nice" by any means.
Lastly, you seem to be conflating a co_yield (generator, pull-based) with co_await (event-driven, push-based). With co_await, the coroutine is resumed by whoever completes the awaitable.
But what do I know... I only worked on implementing coroutines in cl.exe for 4 years. ;-)
What I was thinking of as a state machine with using std::future was a single function state machine, using switch (state) to the state specific dispatch of asynch ops using std::future, wait for completion then select next state.
I don't even know how to respond to that. How in the world are you using C++ professionally if you think coroutines are "daunting"? No one uses C++ for it's "convenience" factor. We use it for the power and control it affords.
> What I was thinking of as a state machine with using std::future was a single function state machine, using switch (state) to the state specific dispatch of asynch ops using std::future, wait for completion then select next state.
Uh huh. What about error propagation and all the other very real issues I mentioned that you are just ignoring? Why not just let the compiler do all the work the way it was spec'ed and implemented?
Sounds more like punishment than software design, but each to their own.
It’s pretty clear you’ve never built a production-grade async state machine.
C++ is designed to provide the plumbing, not the kitchen sink. It’s a language for building abstractions, not handing them to you — though in practice, there’s a rich ecosystem if you’d rather reuse than reinvent.
That flexibility comes at the cost of convenience, which is why most new engineers don’t start with C++.
What you call “intimidating,” I call powerful. If coroutines throw you off, you’re probably using the wrong language.
Last thought — when you run into someone who’s built the tools you rely on, ask them questions instead of trying to lecture them. I would have been more than happy to work through a pedagogical solution with you.
/ignored
Haha .. you have no idea.
FWIW I've built frameworks exactly for that, and it's highly likely that you've unwittingly used one of them.
But yeah, if you want to win arguments then arguing against yourself and your own hallucinations, is in your case the best way to go.
> computer systems whose backend implementations you are blissfully ignorant of
I am extremely confident in my "backend" knowledge (of course, an actual systems engineer would never refer to their work as "backend").
You wrote a "C++ framework" that runs in the "backend" of a "computer system"? Do I have that right? Please let me know what it is so that I can decompile it and see how it was implemented!
Probably not.
You seem to be struggling with technical language.
But sure... let's keep going while you do nothing but whine without displaying any technical acumen. This is fun!
Why don't you hop into the discussion of the early Windows ring-3-to-ring-0 transition issues [0] we had in the 90s? You know... since I am "blissfully ignorant" about internals. I can't wait to learn from your "backend" expertise!!
How old are you? 14?
Bye!
One of us worked on two of the most popular development tools of all time, and the other makes shit up and refuses to engage with technical details.
Also, since you know what I've worked on, it should be pretty obvious how old I am.
Why are people afraid of state machines? There's been sooo much effort spent on hiding them from the programmer...
For example, generators. Also known as semicoroutines.
https://langdev.stackexchange.com/a/834
This:
generator fib() {
a, b = 1, 2
while (a<100) {
b, a = a, a+b
yield a
}
yield a-1
}
struct fibState {
a,
b,
position
}
int fib(fibState state) {
switch (fibState.postion) {
case 0:
fibState.a, fibState.b = 1,2
while (a<100) {
fibState.b, fibState.a = fibState.a, fibState.a+fibState.b
// switching the context
fibState.position = 1;
return fibState.a;
case 1:
}
fibState.position = 2;
return fibState.a-1
case 2:
fibState.position = -1;
}
}
The programming language I made is a concrete example of what programming these things manually is like. I had to write every primitive as a state machine just like the one above.
https://www.matheusmoreira.com/articles/delimited-continuati...
Not of how to write a state machine based application without hiding the state machine behind abstractions.
The "ugly" version with the switch seems much preferable to me. It's simple, works, has way less moving parts and does not require complex machinery to be built into the language. I'm open to being convinced otherwise but as it stands I'm not seeing any horrible problems with it.
Edit: Nevermind, they eventually bothered.
[1] https://docs.unity3d.com/6000.3/Documentation/ScriptReferenc...
Is that a hack? Is that not just exactly what IEnumerable and IEnumerator were built to do?
Really you're generating the vague concept of a yield instruction but you can return other coroutines that are implicitly run and nest your execution... Because of this you can't wait less than a frame so things are often needlessly complicated and slow.
It's like using a key to jam a door shut. Sure a key is for keeping doors closed but...
https://discussions.unity.com/t/coreclr-scripting-and-ecs-st...
Capcom has their own fork of .NET for the Playstation, for example.
I don't know what kind of GC they implemented.
They will not be using .NET AOT probably ever though. Unity's AOT basically supports full C# (reflection etc) while .NET opted to restrict it and lean more on generated code.
I would just go straight to tbb and concurrent_unordered_map!
The challenge of parallelism does not come from how to make things parallel, but how you share memory:
How you avoid cache misses, make sure threads don't trample each other and design the higher level abstraction so that all layers can benefit from the performance without suffering turnaround problems.
My challenge right now is how do I make the JVM fast on native memory:
1) Rewrite my own JVM. 2) Use the buffer and offset structure Oracle still has but has deprecated and is encouraging people to not use.
We need Java/C# (already has it but is terrible to write native/VM code for?) with bottlenecks at native performance and one way or the other somebody is going to have to write it?
What do you mean here? Do you mean hand-writing MSIL or native interop (pinvoke) or something else?
Your stack is on the heap and it contains an instruction pointer to jump to for resume.
I recall working on a few VR projects - where it's imperative that you keep that framerate solid or risk making the user physically sick - this is where really began using coroutines for instantiating large volumes of objects and so on (and avoiding framerate stutter).
ECS/Dots & the burst compiler makes all of this unnecessary and the performance is nothing short of incredible.
This is quite understandable when you know the history behind how C++ coroutines came to be.
They were initially proposed by Microsoft, based on a C++/CX extension, that was inspired by .NET async/await implementation, as the WinRT runtime was designed to only support asynchronous code.
Thus if one knows how the .NET compiler and runtime magic works, including custom awaitable types, there will be some common bridges to how C++ co-routines ended up looking like.
I never understood the value. Just use lambdas/callbacks.
"Just" is doing a lot of work there. I've use callback-based async frameworks in C++ in the past, and it turns into pure hell very fast. Async programming is, basically, state machines all the way down, and doing it explicitly is not nice. And trying to debug the damn thing is a miserable experience
The author just chose to write it as a state machine, but you don't have to. Write it in whatever style helps you reach correctness.
Just put your state in visible instance variables of your objects, and then you will actually be able to see and even edit what state your program is in. Stop doing things that make debugging difficult and frustratingly opaque.
Lol, no thanks. People are using coroutines exactly to avoid callback hell. I have rewritten my own C++ ASIO networking code from callback to coroutines (asio::awaitable) and the difference is night and day!
For simple callback hell, not so much.
waitFrames(5); // wait 5 frames
fireProjectile();
waitFrames(15);
turnLeft(-30/*deg*/, 120); // turn left over 120 frames
waitFrames(10);
fireProjectile();
// spin and shoot
for (i of range(0, 360, 60)) {
turnRight(60, 90); // turn 60 degrees over 90 frames
fireProjectile();
}
``` int f() { a; co_yield r; b; co_return r2; } ```
this transforms into
``` auto f(auto then) { a; return then(r, [&]() { b; return then(r2); }); }; ```
You can easily extend this to arbitrarily complex statements. The main thing is that obviously, you have to worry about the capture lifetime yourself (coroutines allocate a frame separate from the stack), and the syntax causes nesting for every statement (but you can avoid that using operator overloading, like C++26/29 does for executors)
> The main thing is that obviously, you have to worry about the capture lifetime yourself
This is a big deal! The fact that the coroutine frame is kept alive and your state can just stay in local variables is one of the main selling points. I experienced this first-hand when I rewrote callback-style C++ ASIO code to the new coroutine style. No more [self=shared_from_this()] and other shenanigans!
The whole point of controlling the capture is controlling the memory layout, which is what C++ is all about.
Even with Asio, you don't really have to do this. It's just the style the examples follow, and Asio itself isn't necessarily the best design.
The default behaviour of many asynchronous systems is to extend the lifetime of context data until all the asynchronous handlers have run. You can also just bind them to the resource instead which is arguably more elegant, but which depends on how cancellation is implemented.
In a coroutine everything can just live on the stack because the stack frame itself is kept alive across the asynchronous function calls.
Don't you see the big difference regarding lifetime management?
Close the socket, all handlers are cancelled, context can be freed immediately. The socket owns the data.
What if there's more than one socket involved?
Resource ownership management 101.
Except in C++ this removes a fair amount of control given how low-level it is.
Appreciate this humor -- absurd, tasteful.
Unity's own documentation for changing scenes uses coroutines
I've been a serious Unity developer for 16 years, and I avoid coroutines like the plague, just like other architectural mistakes like stringly typed SendMessage, or UnityScript.
Unity coroutines are a huge pain in the ass, and a lazy undisciplined way to do things that are easy to do without them, using conventional portable programming techniques that make it possible to prevent edge conditions where things fall through the cracks and get forgotten, where references outlive the objects they depend on ("fire-and-forget" gatling foot-guns).
Coroutines are great -- right up until they aren’t.
They give you "nice linear code" by quietly turning control flow into a distributed state machine you no longer control. Then the object gets destroyed, the coroutine keeps running, and now you’re debugging a null ref 200 frames later in a different scene with an obfuscated call stack and no ownership.
"Just stop your coroutines" sounds good until you realize there’s no coherent ownership model. Who owns it? The MonoBehaviour? The caller? The scene? Every object it has a reference to? The thing it captured three yields ago? The cure is so much worse than the disease.
Meanwhile: No static guarantees about lifetime. No structured cancellation. Hidden allocation/GC from yield instructions. Execution split across frames with implicit state you can’t inspect.
Unity has a wonderful editor that lets you inspect and edit the state of the entire world: EXCEPT FOR COROUTINES! If you put your state into an object instead of local variables in a coroutine, you can actually see the state in the editor.
All of this to avoid writing a small explicit state machine or update loop -- Unity ALREADY has Update and FixedUpdate just for that: use those.
Coroutines aren’t "cleaner" -- they just defer the mess until it’s harder to reason about.
If you can't handle state machines, then you're even less equipped to handle coroutines.
The monobehavior that invoked the routine owns it and is capable of cancelling it at typical lifecycle boundaries.
This is not a hill I would die on. There's a lot of other battles to fight when shipping a game.
The biggest reason for using Unity is its editor. Don't do things that make the editor useless, and are invisible to it.
The problem with coroutines is that they generate invisible errors you end up shipping and fighting long after you shipped your game, because they're so hard to track down and reproduce and diagnose.
Sure you can push out fixes and updates on Steam, but how about shipping games that don't crash mysteriously and unpredictably in the first place?
I'm not advocating for the ubiquitous use of coroutines (there's a time and place), but they're like anything else: if you don't know what you're doing, you'll misuse them and cause problems. If you RTFM and understand how they work, you won't have any issues.
If you strictly require people to know exactly what they're doing and always RTFM and perfectly understand how everything works, then they already know well enough to avoid coroutines and SendMessage and UnityEvents and other footguns in the first place.
It's much easier and more efficient to avoid all of the footguns when you simply don't use any of the footguns.
It'd be like complaining about arrays being bad because if you pass a pointer to another object, nuke the original array, then try to access the data, it'll cause an error. That's kind of... your own fault? Got to manage your data better.
Unity's own developers use them for engine code. To claim it's just something for noobs is a bit of an interesting take, since, well, the engine developers are clearly using them and I doubt they're Unity noobs. They made the engine.
The same code in a coroutine hits the same lifecycle failures as Update() anyway. You don't gain any safety by moving it to Update().
> No structured cancellation.
Call StopCoroutine with the Coroutine object returned by StartCoroutine. Of course you can just pass around a cancellation token type thing as well.
> Hidden allocation/GC from yield instructions.
Hidden how? You're calling `new` or you're not.
Instead of fighting them, you should just learn how to use coroutines. They're a lot nicer than complicated logic in Update().
Again, fine for pet projects on PC :)
https://news.ycombinator.com/item?id=47110605
>1h 48m 06s, with arms spread out like Jesus H Christ on a crucifix: "Because we can dynamically put on ANY surface of the cube ANY image we like. So THAT's how we're going to surprise the world, is by giving clues about what's in the middle later on."
https://youtu.be/24AY4fJ66xA?t=6486
Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Click. Moo!
https://www.gamedeveloper.com/design/the-i-curiosity-i-exper...
>"I'm jealous that [Molyneux] made a more boring clicking game than I did." -Ian Bogost
>"I also think Curiosity was brilliant and inspired. But that doesn't make it any less selfish or brazen. Curiosity was not an experiment. 'Experiment' is a rhetorical ruse meant to distract you from the fact that it's promotional." -Ian Bogost
Green threads is just syntactic sugar around need to process multiple requests from a single real thread without blocking on io calls. It is somewhat helpful but it is a way around, not a generic recipe for performance increase and scalability
Sorry, that's about as far from truth as you can get. Threads were a thing way before multicore was available. Windows got them since Win95/NT4.0, and these didn't support multicore, nor did most of OpenVMS machines where NT thread model originated run multicore (were there any multicore VAXen at all). The two main points for threads back then was to simplify concurrent IO and to be able to run a background computation without hanging the UI. Unix was different because it was initially oriented towards timesharing aka. running a single program for user, so threads were seen as redundant complexity (as were async IO - so even today "I want to run something yet be able get more user input at any time" can sometimes be a PITA).
And year, not everything needs scalability. Scalability is actually actively harmful for everything that's not a server. E. g. UI needs to be responsive - but that doesn't necessary mean "run on multiple cores", rather "being able to preempt low-priority stuff". And usually you don't want to deal with multicore synchronization.
MCUs rarely have multiple cores. Yet multi-threading is ubiquitous, and on most RTOSes threads are much closer to "green threads" than Windows ones. Through as I keep screaming in every coroutine thread, I really wish C stackless coroutines were a thing, you can't afford a stack for everything.
There are more to this world than the boring AWS crud.
Even if I was talking to remote database with slow access ability of green threads to "handle" bazillion requests will just overload the database. There are no miracles around the laws of physics.