How does Project Loom Fibers/Green threads help in Anyway ?

Ron Pressler ron.pressler at
Wed Jan 17 11:58:09 UTC 2018

Hi Kant.

The idea of fibers is to let users use familiar, simple — to write, read, debug and profile — and backwards/legacy-compatible blocking IO calls in fibers, that would automatically be translated to nonblocking IO from the OS’s perspective. You’d write ordinary blocking code that in terms of performance, would behave the same as tasks using asychronous/nonblocking IO. So instead of choosing between simple code and performant code, you’d get both.


On January 17, 2018 at 10:32:45 AM, kant kodali (kanth909 at at wrote:

> Hi All,
> I came across the links below and I wondering how Fibers can really help ?
> when compared to using a thread pool (which is a pool of kernel threads)?
> Sure having both options is great but I am trying to understand what
> difference can it make in terms of performance ?
> Here is what I know
> 1) Kernel threads are expensive and the default stack size is in MB's
> whereas Fibers can have a small stack size somewhere in KB's so one can
> create as many of them. so what? eventually they all have to map to Kernel
> threads and thread pool exists so user don't create as many as say Fibers.
> isn't it?
> 2) Fibers have no idea about kernal threads and vice-versa. so if a Fiber
> makes a blocking system call it has not idea that the kernel thread is
> blocked and there is no upcall mechanism by any major OS.
> 3) One can argue Fibers are great when a program is making lot of async
> calls but I wonder how this is any better in terms of performance when
> compared to having small thread pool that makes async calls? If this is all
> about expressing something gets easier in Fibers than using thread pool but
> there is no performance difference then I must say this is more of a
> subjective call so everyone can have their own opinion.
> At this time, I am not seeing how Fibers can be better performant than
> having a pool of Kernel threads? if they are in some cases can someone
> please provide any examples so I can enlightened?
> Thanks!

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