It seems that it's not the forkIO that is creating native threads (using GHC 6.10.1, -O flag) For example, the following code only has 5 threads on my Windows box, while I fork 10 lightweight threads import System.IO import Control.Concurrent import Control.Monad entry :: Char -> IO() entry c = do forever $ putChar c main = do hSetBuffering stdout NoBuffering mapM_ (forkIO . entry) ['0'..'9'] putStrLn "all forked; press ENTER to quit" getLine However, things can get bizarre. When we change the entry function into entry :: Char -> IO() entry c = do putChar c threadDelay maxBound Then on my machine 13 native threads *are* created. This might make sense because its waiting forever, but native threads are also created for the following code: entry :: Char -> IO() entry c = do putChar c forever $ threadDelay 0 This feels like overkill, so it seems threadDelay always forks a new native thread, although this is not documented. We can also get into the dark corners of GHC's lightweight thread scheduling: a thread switch only happens when memory is allocated on the GC head (but this is documented in the API) So the following code will not work, the runtime seems to get stuck in an infinite loop after the first thread is created. entry :: Char -> IO() entry c = do putChar c forever $ return () In artificial cases like this you can use yield entry :: Char -> IO() entry c = do putChar c forever yield On Mon, Apr 13, 2009 at 10:37 AM, Lych wrote:

I wrote a simple ECHO server like this: ---------------------------------------------------- import Network import System.IO import Control.Concurrent

service cs = do ln <- hGetContents cs hPutStr cs ln service cs

acceptloop ls = do (cs, host, port) <- accept ls hSetBuffering cs LineBuffering forkIO (service cs) acceptloop ls

main = do ls <- listenOn (PortNumber 10061) acceptloop ls ----------------------------------------------------

And tested it with a client that initiates as many parallel connections as possible. The number of connections can just reach to 1000+ on my machine, but I thought it should be more, since forkIO generates lightweight threads. I also examined the number of threads in this process with some tool and it proved there was one native thread per connection. When I replaced the 'forkIO' in the program with 'forkOS', I got an exactly similar result. Have I thought something wrong about forkIO? _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners