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Comment by thomasahle | Hacker News Reader

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.

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

>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.

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.

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.

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.

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.

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

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