On Unix with -threaded, all blocking I/O using Handles or sockets goes via the I/O manager thread, which multiplexes all the blocked I/O requests into a single select() call. If you have 20k blocked socket reads, there will be 20k lightweight Haskell threads, and probably 2 or 3 real OS threads. Without -threaded the outcome is pretty much the same, except the runtime does the select(). On Windows, unfortunately you'll get 20k OS threads for this scenario (regardless of -threaded), because we haven't implemented the I/O manager on Windows yet. So, to answer the question:

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Are there any facilities in Haskell that can poll a set of 10k descriptors and retrieve input from the first available?

Yes, you have one forkIO thread per descriptor, and you do your I/O using standard Handle I/O or Network.Socket.recvfrom/sendto. Unfortunately it'll be expensive on Windows right now, but the best way to fix this would be to implement the I/O manager (in GHC.Conc) rather than doing the multiplexing explicitly in your app code. Cheers, Simon On 28 November 2005 08:57, Simon Peyton-Jones wrote:

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.

Simon

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-----Original Message----- From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@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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