Green threads explained in 200 lines of Rust

> Co-routines are very useful and likely underused, but sometimes you are actually better off being able to pass the control to a given thread directly, other than having a scheduler involved.

That's almost the very definition of a coroutine--explicit transfer of control. In symmetric coroutines you must specify a coroutine for both yield and resume; in asymmetric coroutines you specify what to resume to but yield implicitly returns to whatever resumed the current coroutine. In either case the actual control flow transfer is explicitly invoked.

The term thread is more ambiguous, but it almost always implies control transfers--both the timing and target of control transfer--are implicit and not directly exposed to application logic. (Automagic control transfer might be hidden within commonly used functions (e.g. read and write), injected by the compiler (Go does this), or triggered by hardware.)

You can synthesize a threading framework with both asymmetric and symmetric stackful[1] coroutines by simply overloading the resume and yield operations to transfer control to a scheduler, and then hiding implicit resume/yield points within commonly used functions or by machine translation of the code. In languages where "yield" and "resume" are exposed as regular functions this is especially trivial. Stackful coroutines (as opposed to stackless, which are the most commonly provided type of coroutine) are a powerful enough primitive that building threads is relatively trivial, which is why the concepts are easy to conflate, but they shouldn't be confused.

LISP-y languages blur some of these distinctions as libraries can easily rewrite code; they can inject implicit control transfer and stack management in unobtrusive ways.[2] This isn't possible to the same extent in languages like C, C++, or Rust; lacking a proper control flow primitive (i.e. stackful coroutine) their "threading" frameworks[3] are both syntactically and semantically leaky.

[1] By definition a thread preserves stack state--recursive function state--and this usually implies that stack management occurs at a very low-level in the execution environment, but in any case largely hidden from the logical application code.

[2] OTOH, this is usually inefficient--stack management is a very performance critical aspect of the runtime. For example, Guile, a Scheme implementation, now provides a stackful coroutine primitive. For a good discussion of some of these issues, see https://wingolog.org/archives/2017/06/27/growing-fibers

[3] Specifically the frameworks that attempt to make asynchronous I/O network programming simple and efficient. So-called native threads are a different matter as both stack management and control transfer are largely implemented outside the purview of those languages, very much like how native processes are implemented. If you go back far enough in the literature, especially before virtual memory, the distinctions between process and thread fall away. Nowadays threads are differentiated from processes by sharing the same memory/object space.

If you have any knowledge of it, what are your thoughts about the Erlang scheduler & Erlang's concurrency model?

There was some work to give linux this functionality via a switchto* family of syscalls. https://youtu.be/KXuZi9aeGTw

it seems like in every post there's a naysayer about something. you're complaining about an article that explains a difficult topic extensively and generously - in my day you had to pay really really good money for that. who cares if the title focuses one aspect of the explanation? sure it's a little deceptive but it's such a minor cost to you that you literally spent more time complaining than they deceived you out (click link with false notion, see length, close page). LPT: being critical in and of itself does not make you look smart.

The ptr::write is needed because the stack vector contains bytes and the code wants to do a pointer cast and write a u64. It could be done in safe Rust with u64::to_ne_bytes and a copy_from_slice or something like that, but since all of this code is extravagantly unsafe anyway, I think it's reasonably clear to Just Do It :)

Fantastic (IMHO) read on the last paragraph's history and code movement: https://www.maizure.org/projects/evolution_x86_context_switc...

>Linux used to do its actual context switch in C with inline asm, and it changed to being in straight asm a while back.

Could you be a bit more specific, maybe a link to the diff e.g. THANKS!

I don't know how practical that optimization would be in this particular use case, wouldn't that mean the context switch code would have to be inlined at every call site? Then when you switch back to that context how would you generate that code correctly? You would need to know which registers were not saved by green thread A, and not touched by green thread B. So the compiler would basically need to know the runtime behavior of your program to optimize pushing and popping context records, unless I misunderstood your point?

If you set a breakpoint in say a win32 fiber switch, and look at the disassembly, it jumps to an internal function that just saves all the registers (and flags) to the active context and restores all the registers from the resumed context every time. Don't know how more optimal that can be for the general case.

Not really.

You potentially don't know who is "resuming", and so don't know what registers they will clobber. It would only be a "downside" if 1) your code uses a register 2) no other possible green threads do, and that isn't an invariant any compiler I know will promise, especially in the face of FFI calls.

If you're at the point where you want to do register allocation and spilling optimization across multitasking points, you're probably better off writing your own compiler instead of expecting a thread runtime to do it for you.

For comparison, normal threads have the same downside: the kernel saves and restores all registers (even more than what the example does), so you're not doing any worse than that.

> a simple but working example

It's very slow though, at least if you believe the Boost docs. They have various context-controlling types. One of them is called ucontext_t, and falls back to ucontext. Then there's fcontext_t, which is basically a more advanced version of this article. Boost's docs claim it is much faster than ucontext.

Why would one pick C or C++ over Rust for this project?

Thanks a lot for this!

> It does make me thinking about how this could be useful in the context of Futures and async/await.

At its core, when you pass a Future (really, a chain of futures, but in the end that's still a Future) to an executor, the executor creates a stack for it. An executor with multiple futures will then switch between them, sorta similar to the code seen here. The details depend on the exact executor, of course, but the fundamental idea is the same.

One of the nice things about the futures design is that you can know exactly how big the stack needs to be, so the resizing stuff isn't an issue.

A kernel context switch. Neither do the ones described here.