[swift-evolution] What're the Swift team's thoughts on Go's concurrency?

Kevin Ballard kevin at sb.org
Thu Aug 11 13:09:27 CDT 2016

AIUI, fibers are basically coroutines. Even the Naughty Dog presentation
says that fibers are run on threads, and you have to make an explicit
call to switch between fibers. Looking at Ruby's Fiber type, that's also
an explicit coroutine, where you actually yield up a value when you
yield your fiber (which is exactly what coroutines do).

So basically, green threading is preemptive multithreading where the
preempting is done in user-space by the runtime (so it only happens at
specific points where your code calls back into the runtime, but it can
happen at any of those points), and multiple green threads get scheduled
onto the same OS thread, whereas fibers is cooperative multithreading
where your code explicitly yields back to the runtime to switch fibers.

Of course I could be wrong, but that's the impression I got after
reading a few different things about Fibers.


On Thu, Aug 11, 2016, at 10:54 AM, Goffredo Marocchi wrote:
> Hello Kevin,
> I may be wrong in my equating support for fibers to green threads (and
> the runtime cost of supporting them), but I do have seen and linked to
> a presentation of the use and more than trivial benefits to Naughty
> Dog's engine in utilising the 8 Jaguar x86 cores in the PlayStation 4
> CPU. Although like you said, it did not come for free or without
> evident pain points for them.
> On Thu, Aug 11, 2016 at 6:50 PM, Kevin Ballard <kevin at sb.org> wrote:
>> __
>> I'm confused by your email. Rust is all about performance, and
>> embedded devices are one of the targets for Rust. And I can't think
>> of any language that uses green threading that is appropriate for
>> constrained devices (e.g. Go definitely isn't appropriate for that).
>> One of the arguments for getting rid of green threading in Rust is
>> that the extra runtime complexity imposed a performance cost.
>> -Kevin
>> On Thu, Aug 11, 2016, at 10:36 AM, Goffredo Marocchi wrote:
>>> Thanks Kevin, I think they have accepted that they do not need to
>>> enter every segment of computing so the extra performance they could
>>> get on some devices is not worth the risk and the complexity it
>>> brings. Not everyone is trying to cram complex 3D experiences at 60-
>>> 90+ FPS on a console like constrained devices and I guess Rust is
>>> not targeting that right now :).
>>> On Thu, Aug 11, 2016 at 6:12 PM, Kevin Ballard via swift-evolution
>>> <swift-evolution at swift.org> wrote:
>>>> For anyone interested in reading more about Rust's decisions,
>>>> here's two links:
>>>> The email about abandoning segmented stacks:
>>>> https://mail.mozilla.org/pipermail/rust-dev/2013-November/006314.html
>>>> The RFC to remove green threading, with motivation:
>>>> https://github.com/aturon/rfcs/blob/remove-runtime/active/0000-remove-runtime.md
>>>> -Kevin Ballard
>>>> On Tue, Aug 9, 2016, at 01:28 PM, Kevin Ballard wrote:
>>>> > The Rust language used to use a green thread model like Go
>>>> > (actually it exposed a configurable threading interface so you
>>>> > could choose green threads or OS threads). It also used segmented
>>>> > stacks like Go did. Over time, Rust ended up dropping the
>>>> > segmented stacks because it significantly complicated FFI without
>>>> > providing much, if any, benefit (and IIRC Go followed suite and
>>>> > dropped segmented stacks somewhere around version 1.5), and then
>>>> > a little while later Rust dropped green threads entirely. If you
>>>> > can find them, there are lots of discussions of the pros and cons
>>>> > that were documented during this process (on mailing lists, in
>>>> > IRC, possibly on Discourse, there's probably at least one post
>>>> > about it in the Rust subreddit, etc). But ultimately, it was
>>>> > determined that keeping this ability significantly complicated
>>>> > the Rust runtime and it provided almost no benefit. The OS is
>>>> > already really good at scheduling threads, and there's no memory
>>>> > savings without segmented stacks (though the OS will map virtual
>>>> > pages for the stack and only allocate the backing physical pages
>>>> > as the memory is touched, so even if you have a 2MB stack, a new
>>>> > thread will only actually allocate something like 8kb). And there
>>>> > are some pretty big downsides to green threads, such as the fact
>>>> > that it significantly complicates the runtime since all I/O
>>>> > everywhere has to be nonblocking and it has to be transparent to
>>>> > the code, and FFI ends up as a major problem (even without
>>>> > segmented stacks), because you have no idea if an FFI call will
>>>> > block. Green threading libraries end up having to allocate extra
>>>> > OS threads just to continue servicing the green threads when the
>>>> > existing threads are potentially blocked in FFI.
>>>> >
>>>> > So ultimately, green threads really only make sense when you
>>>> > control the entire ecosystem, so you can ensure the whole stack
>>>> > is compatible with green threads and won't ever issue blocking
>>>> > calls, and even there there's not much benefit and there's a lot
>>>> > of complexity involved.
>>>> >
>>>> > -Kevin Ballard
>>>> >
>>>> > On Tue, Aug 9, 2016, at 12:04 PM, Dan Stenmark via swift-
>>>> > evolution wrote:
>>>> > > I'd like to inquire as to what the Swift team thoughts on Go's
>>>> > > concurrency model are?  I'm not referring to convenience of the
>>>> > > 'go' keyword and nor am I referring to how the language handles
>>>> > > Channels, both of which being what most folks associate with
>>>> > > it.  Rather, I'd like to ask about the language's use of Green
>>>> > > Threads and how the runtime handles the heavy lifting of
>>>> > > multiplexing and scheduling them.  What are some of the
>>>> > > strengths and weaknesses the Swift team sees to Go's approach?
>>>> > >
>>>> > > Dan
>>>> > >
>>>> > > (DISCLAIMER: I'm posting this for academic reasons, not as a
>>>> > > pitch.  While the Swift team's responses may inform opinions on
>>>> > > the matter, I do not want this to turn into a 'this is how I
>>>> > > think Swift should do concurrency' debate.  That discussion
>>>> > > will come when it comes.)
>>>> > > _______________________________________________
>>>> > > swift-evolution mailing list
>>>> > > swift-evolution at swift.org
>>>> > > https://lists.swift.org/mailman/listinfo/swift-evolution
>>>> _______________________________________________
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