Haskell - Slow socket connection when threaded

Question

I've started with Haskell a while ago and now I'm focusing on networking. I followed some tutorials and source samples to put together a very simple echo server:

main = withSocketsDo $ do
    forkIO $ acceptor 8080    
    print "Server running ... " >> getLine >>= print


tcpSock :: IO Socket
tcpSock = socket AF_INET Stream 0

acceptor :: PortNumber -> IO ()
acceptor port = do

    -- Setup server socket
    sock <- tcpSock
    setSocketOption sock ReuseAddr 1
    bindSocket sock (SockAddrInet port iNADDR_ANY)
    listen sock 50

    -- Start with zero index
    loop sock 0
        where
        loop sock sockId = do
            -- Accept socket
            (nextSock, addr) <- accept sock

            -- Setup the socket for performance
            (_, handle) <- setupClient nextSock

            -- Run client in own thread
            forkIO $ do
                -- Get a stream of bytes
                stream <- BS.hGetContents handle

                -- Echo the first received char
                BS.hPut handle $ BS.take 1 stream

                -- Kill the socket
                SIO.hClose handle


            -- Accept next client
            loop sock (sockId + 1)


setupClient :: Socket -> IO (Socket, SIO.Handle)
setupClient sock = do
        -- Disable nagle
        setSocketOption sock NoDelay 1

        -- Disable buffering
        hdl <- socketToHandle sock SIO.ReadWriteMode
        SIO.hSetBuffering hdl SIO.NoBuffering

        return (sock, hdl)

Now, I've tested the code with the ab-Tool to benchmark the server. The code is compiled using -O2 and -threaded and the program is started using +RTS -N to use multiple OS threads.

The code creates a new lightweight thread per client and as far as I remember are these threads pretty cheap because they are scheduled by a bunch of real OS threads.

After running the tool, the results are:

ab -n 10000 -c 1000 http://localhost:8080/ ~ 500 - 1600 req/sec Yes, it does change sometimes between 500 and 1600!

At first I thought well, not bad. Then I ran the program without "+RTS -N" and results are almost every time ~20000 req/sec.

Obviously the threading kills the performance pretty badly, but why ? My guess is, that the IO manager does a pretty bad job when dealing with a lot of connections.

BTW: I use Ubuntu 13.04 and ghc 7.6, but I've tested the code under Windows 8 which gave me far worse results, but I think the IO manager is tuned for linux, which makes sense.

Am I doing something reallly stupid here ?? I know, the example is quite trivial but here is obviously something going wrong.

Regards, Chris

Answer 1

Okay, I think I semi-solved the problem, though I'm still not sure where the error is/was.

I'm now using the Network package so the accept routine is handle-based. I tried this because I noticed a memory leak after a couple of tests.

This way I solved magially two problem at once, because now the threading makes NO difference. I really dont know why this is happening, but the handle-based impl. is simpler and obviously faster/more safe.

Maybe this helps other people experiencing the same problem.