[Haskell-cafe] threadWaitRead and threadWaitWrite on multiple fds

Mathijs Kwik bluescreen303 at gmail.com
Mon Dec 13 08:24:25 CET 2010

Yep, that's like the workaround I'm using right now.
I create an empty mvar, fire up 2 threads that will wait for an fd and
tryPutMVar afterwards.
My original thread justs gets the MVar to wait for any of the 2
fd-waiting-threads to complete.
But however light threads may be, I still think this might give some
overhead, and it's almost the same trick that System/Event/Thread is
using internally, although in that case, the putMVar is performed by
the event manager thread.
As far as I can tell, the behavior isn't hard-coded in the RTS (no
need to change that), but applications that use base will tell it to
use the mvar-trick as callback. That's why I was hoping to be able to
just tell it to use a different callback.

But indeed it's a solution for now.
I'll just post a feature-request for GHC.


On Mon, Dec 13, 2010 at 3:19 AM, Mitar <mmitar at gmail.com> wrote:
> Hi!
> On Mon, Dec 13, 2010 at 2:14 AM, Antoine Latter <aslatter at gmail.com> wrote:
>> Can you do it with forkIO? That is, have two light-weight threads,
>> each waiting on a different fd, which perform the same action when one
>> of them wakes up.
> Or you could wait for each fd in its own thread (those are really
> light-weight threads) and once some is triggered you spawn another
> thread which deals with the event, while the original thread goes back
> into the waiting. Or you can also send data over Chan to another
> thread which then processes the even (if you need to serialize
> processing).
> Mitar

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