[Haskell-cafe] Doubting Haskell

Stefan O'Rear stefanor at cox.net
Sat Feb 16 19:26:02 EST 2008

On Sat, Feb 16, 2008 at 06:50:03PM -0500, Cale Gibbard wrote:
> On 16/02/2008, Alan Carter <alangcarter at gmail.com> wrote:
> > Then when all this was going on, question number five appeared: What
> > the hell are these "lightweight Haskell threads"? Are they some kind
> > of green threads running non-preemptively inside a single OS thread?
> > Are they OS threads that could run concurrently on multi-core
> > hardware? If they are green threads (and it sounds like they are) then
> > this is all an academic exercise which has no production application
> > yet.
> >
> > Best wishes - and still hoping I'm wrong after all
> >
> > Alan Carter

Yes, they are green threads.  But not the stupid kind you are used to.

Consider an operating system.  You are running hundreds of threads in a
typical system.  You don't have hundreds of processors - let's be
generous and say you have 8.  Obviously these threads are in some sense
'green'.  But they are still being run with (limited) parallelism!
There is no reason to expect anything less of user-level 'green
threads', and if all the systems you have been using are incapable of
running threads in paralell, then all the systems you have been using
are toys or broken.  GHC is not a toy (in this regard), and contains a
mini-operating system that schedules how ever many millions of threads
you have onto a number of OS threads specified with the +RTS -N<x>

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