Re: swapcontext() slow

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

On 22-01-2016 15:24, Godmar Back wrote:
> Interesting - hadn't seen any of that.
> 
> Also found this post: https://plus.google.com/+OlegZabluda/posts/KXp5i967gG4
> and http://www.1024cores.net/home/lock-free-algorithms/tricks/fibers
> linked from there for an alternative project pursuing similar ideas of
> an almost portable interface while avoiding the sigprocmask penalty in
> swap_context.
> 
> Re: Google's proposal - Turner's slide say "google proprietary" - does
> anyone know if Google runs custom kernels with this fast usermode
> scheduling facility?
> 
> As one commenter on the YouTube page points out, it seems similar to
> Windows
> )
> I've always wondered which applications make use of UMS - is it just a
> facility that MS built because they thought it's cool, or do their
> core products rely on it?
> 
> Related, is there interest in the Linux glibc/kernel community in
> deploying some of these ideas?  (I'm not currently reading the linux
> kernel mailing list).
> 
>  - Godmar
> 

I can't really say from kernel side what kind of interested they have on
implement M:N model in userland, but from GLIBC side we try to avoid
creating architecture or system specific ABI, specially one without
a standard.

Also, to really implement coroutines/green thread with userland with all
ISO C and POSIX bits in place one need to take in consideration a lot of
possible scenarios along with our GNU extensions (signals, TLS, threads,
blocking syscalls, etc). With this in mind you will end up with the 
already existent API (swapcontext, setjmp, longjmp) or with half-baked 
solutions with a set of pitfalls (such as the various libraries out there). 

Another solution, which I consider better, is define the coroutines in
a strict virtual machine where the language and environment side effects
can be controlled (go, erlang, etc. for instance). In such environments
one can, for instance, create specific rule on how handle blocking syscall
or minimize the context switching by ignoring some set of registers 
(if the coroutines only use a subset of available register, the environment
can just save/restore them).

One can als