[Haskell-cafe] FFI and callbacks

Simon Marlow simonmar at microsoft.com
Tue Jul 26 04:59:14 EDT 2005

On 25 July 2005 22:41, Sebastian Sylvan wrote:

> On 7/25/05, Simon Marlow <simonmar at microsoft.com> wrote:
>> On 23 July 2005 03:38, Duncan Coutts wrote:
>>> The problem then as John noted is that the main loop of these
>>> toolkits block and so the other Haskell threads would not get a
>>> chance to schedule. So the challenge is to give the Haskell threads
>>> a chance to schedule.
>> [ good description of the multi-threaded GUI problem deleted ]
>> Thanks for describing the problem in detail, I understand it better
>> now. I think it comes down to this conflict:
>>  - you want to take advantage of the fact that GHC has lightweight
>>    "green" threads in order to do multithreading within a single OS 
>> thread, but 
>>  - our "bound threads" design does not require the implementation
>>    to support lightweight threads, and hence doesn't let the
>>    programmer take advantage of them.
> Well how about not touching the "bound threads" design at all, but
> modifying the lightweigh threads design to also include "forkIOHere"
> (or something) which would produce a lightweight thread which is
> always run in the "main" OS thread.

This is what Duncan was angling for, I think.  My point is that this
requires the implementation to support lightweight threads, which is
something we intentionally avoided in the bound threads design.

While I don't doubt that we could implement some way to fork another
thread bound to the same OS thread as the parent, I'm worried that this
would add yet more complexity to GHC's scheduler, so I think we should
fully explore other possibilities first.


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