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Comment by mietek | Hacker News Reader

What a disappointment.

I like it. Platform threads should just be lightweight enough, that you don't have to patch on top of them.

And if you really need to M:N behaviour, it is still available as a library.

Agreed. Defaulting to native threading gives developers so much more freedom. After all, M:N threading is just implemented on native threads anyway. You can still do that if you want to. Rust isn't going to be a systems language used only for networking, so forcing a M:N threading model on it doesn't make sense.

This change was necessary to keep Rust as a general use systems programming language.

> Rust isn't going to be a systems language used only for networking, so forcing a M:N threading model on it doesn't make sense.

M:N threading decouples degree of concurrency from degree of parallelism, which is not desirable only for networking. And the old way wasn't about forcing M:N threading, it was about providing a common interface for concurrency independent of the relation of concurrency to parallelism in the backend implementation.

I can understand the idea that that abstraction may have been too expensive, but I can't get behind the idea that 1:1 threading as the only thing supported in the standard library somehow provides more freedom for developers.

Short-term it may look like there's less freedom since it's harder to use M:N threading (and 1:1 threading and M:N threading don't necessarily work perfectly together), but remember that the choice there is 1:1 threading with overhead and M:N threading with overhead.

Long term, we can have libraries that provide M:N threading with minimal overhead; and also libraries that provide 1:1 & M:N together like the old std (with the associated overhead).

That is, the programmer is getting more flexibility, just not right now (especially with Cargo, which makes it particularly easy to use external libraries). The team is trying to take a longer term view focusing std on safe interfaces to the OS primitives, which can then be used to build fancier/more abstract libraries more easily.

It's not like people are saying "Rust will never have M:N threading", it's that the current implementation didn't offer enough advantages for the disadvantages it brought. https://github.com/aturon/rfcs/blob/remove-runtime/active/00...

Having done some things with libgreen threads myself, they really didn't give anything over proper threads (especially without segmented stacks). So I am curious how this will evolve in the future.

The two different threading models have different costs/benifits. Native threads are much faster at context switching then green threads, however, native threads have a significant startup cost where as spawning green threads is nearly cost-free.

>Native threads are much faster at context switching then green threads

No, native threads have a much higher context switch overhead. The downside of green threads is the lack of parallelism, you can only use one CPU core. This is why decent languages multiplex green threads over OS threads (M:N). The mistake rust made was trying to make that distinction transparent.

Quote from Wikipedia article on Green Threads: "Linux native threads have slightly better performance on I/O and context switching operations."

Some of the actual context switch in native threads is performed by the CPU, where as with green threads it is not. On top of this, the whole stack is copied upon a context switch.

The paper[1] cited for that quote is specifically about PersonalJava 1.1.3 on Linux 2.2 on PowerPC. It may not be relevant to implementations from this century.

[1] http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.8.9...

Your quote lacks the context which makes it clear that it is not a general claim.

>Some of the actual context switch in native threads is performed by the CPU, where as with green threads it is not

Exactly. Native threads have a context switch, green threads don't.

Wikipedia is wrong and generally speaking you shouldn't trust it on CS matters. Or any other subject, really. Green thread context switching is much faster than native thread switching, as long as you don't do it badly. For example, on Linux, don't use setcontext/swapcontext/etc, because they do a system call to change the signal mask.

Green threads also suck for other reasons. Async file I/O doesn't really work properly on Linux or OS X, so you have to use a thread pool for that, and you can't casually use any other library you'd otherwise want to use, either.

That's why a decent language multiplexes green threads over native threads so you can have your cake and eat it too. You still need the ability to choose to create a green thread or a native thread though, it is the idea of trying to make the two interchangeable that was a problem.

Which languages multiplex green threads over OS threads in a non-transparent fashion, exactly?

Actually, green threads are much faster at context switching than native threads. They also consume less heap space for (if it's growable) stack.

Native threads are natively supported by the OS, so managing them requires calls to the OS, the main reason why it's slow. However, it comes with a few advantages. The main advantage of native threads is that they are preemptive - they can be scheduled based on ticks (i.e. number of microseconds spent running the thread), even if they are in the middle of a loop. Also, they can be run on different CPUs, the OS takes care of all CPU flags/registers/etc., they are actual OS structures, ...

Do other OSes have an equivalent to User Mode Scheduling? It seems to allow you to get the best of both worlds, if your tasks are amenable to the model.

http://msdn.microsoft.com/en-us/library/windows/desktop/dd62...

Google supposedly has implemented something similar called user-level threads, which they talked about at the 2013 Linux Plumbers Conference. If I recall the video correctly, they talk about plans to open source it, but as far as I know there hasn't been much movement in that space yet:

http://www.linuxplumbersconf.org/2013/ocw/proposals/1653 https://www.youtube.com/watch?v=KXuZi9aeGTw

Can you elaborate?

Fwiw I think we were all disappointed when all that work and attention on a promising mechanism for having it both ways ultimately came to nothing.

I concur. But I don't regret the time spent experimenting. Even negative results are valuable for advancing the state of the art.

Yeah, I'm just trying to describe the above poster as reasonable before other readers jump on the opportunity to white-out the comment for no good reason.

There is not much to say which has not already been said. I think it is sad that so much effort is going into projects which do not advance the state of the art. First Go, now Rust — these high-profile projects draw people’s attention away from actually innovative work.

As a core contributor, you will retort that Rust has different priorities than, say, Erlang, which is built around the concept of providing N:M processes (green threads) as a language primitive. My answer to that is, why not improve on Erlang, instead of settling for improving on C?

Because we actually have to release something. And you can't be all things to all people. The best we can do is try to create a powerful, safe, extensible language, low overhead language with unsafe escape hatches. The hope is that this will allow for future advances in lightweight, task based concurrency in the form of libraries, as opposed to as baked-in language features.

It is unfortunate that you think Rust is not improving the state of the art: no other language is low-level and memory safe, with the same flexibility as Rust. Rust is certainly pushing the research envelope further than Go, and, anyway, it seems to me that improving on C is something very desirable given the recent spate of horrible bugs in C libraries, caused by memory safety violations (heartbleed etc).

It's understandable that you think that I'm a core contributor, but I'm really just an informed community member. I have no vested stake in Rust, and I'm not afraid to criticize the core team when they make baffling decisions (example: the tone-deaf new syntax for fixed-length arrays).

And just because Rust has a focus on pragmatism doesn't mean that it's not blazing new territory. Neither Ada nor Cyclone can boast such extensively useful statically-safe references. And figuring out how to do unboxed closures memory-safely in a language without a garbage collector is, as far as I can tell, entirely novel (though you'd have to ask a PL researcher to be certain). I expect aspects of Rust to show up in all future systems programming languages, including future revisions of C++ (as well as brand-new efforts such as M#).

We already have a systems programming language with a garbage collector.

http://dl.acm.org/citation.cfm?id=367177.367199

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