[Haskell-cafe] STM, IO and event loops

Simon Peyton-Jones simonpj at microsoft.com
Mon Nov 28 03:57:01 EST 2005

A good design pattern, I think, is to have a Haskell thread dedicated to
each socket.  It can suck on the socket and dump what it finds into an
STM channel or buffer.  Then other threads (the ones doing the work) can
read it transactionally.  

It should be find to have lots of threads, esp if most of them are
asleep.  The only thing to watch out for is that GHC's runtime system
will consume one *OS* thread for each *blocked* foreign call.  So if you
have 10k threads each making a separate call to the OS to read from 10k
sockets, and they all block, you'll use 10k OS threads, and that will
probably fail. 


| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org
[mailto:haskell-cafe-bounces at haskell.org] On Behalf Of Joel
| Reymont
| Sent: 27 November 2005 11:12
| To: Haskell Cafe
| Subject: [Haskell-cafe] STM, IO and event loops
| Folks,
| I'm trying to build an event-driven system that includes multiple
| event channels. I would have one channel for network events and
| another one for events sent by other threads. I would like to use STM
| and `orElse` to poll the various event channels.
| Is there a reasonably safe way to do this with STM and IO?
| The network side of things currently times out after a given number
| of seconds if no input is available but can also throw exceptions.
| I'm having a hard time figuring out how to wrap this in STM.
| There's another alternative that I can think of...
| My poker bots are launched in separate threads but do not talk to
| each other right now. They just receive events from the network. I
| would like a poker bot to tell others to stop playing and exit, for
| example. I guess I could build poker bots with an event loop that
| reads from a TChan but... I would then need twice as many threads
| since each bot has a socket associated with it and one thread per bot
| would need to read from the socket and push events into the TChan.
| Are there any facilities in Haskell that can poll a set of 10k
| descriptors and retrieve input from the first available?
| I don't know if I should be concerned with launching 20k threads vs.
| 10k threads but it seems that disassociating bots from the network
| has some positive effects. I could compose packets by hand in the
| interpreter and feed them to the TChan by hand to test individual
| bots, for example.
| Any advice is appreciated!
| 	Thanks, Joel
| --
| http://wagerlabs.com/
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