Re: [Haskell-cafe] Multicast receiver thread fails to receive last few messages

23 Sep 2014


      Indeed, this was the issue. Took Benno's advice and added a yield, and it
works as I hoped it would. Thanks!

On Tue, Sep 23, 2014 at 10:46 AM, Gregory Collins 
wrote:
...
Since this is UDP, you may be overflowing your device queue. Linux
helpfully won't return ENOBUFS in this case but will instead just drop the
packet on the floor. The threadDelay fixes this by yielding the processor
so that the consumer thread can dequeue packets.
G
On Tue, Sep 23, 2014 at 8:14 AM, Ben Gunton  wrote:
...
I am trying to send multicast messages from one thread, and receive them
on another. The sending thread reads lines from a file and sends them.
However, the receiving thread does not receive all the messages - it seems
to miss the last 10 message or so. If I run other programs to listen for
the multicast message, they receive them fine. So I think the issue is
something to do with receiving, not sending. I keep the program alive with
a threadDelay, so it shouldn't be halting prematurely. If I add a small
delay after each send, it works fine. And if I make the input file smaller,
it works fine.
On the provided data file, on my system (64-bit Ubuntu, GHC 7.8.2), the
receiver fails to receive lines 125-140. Compiled with no optimizations.
Thanks for any help!
The relevant code snippets are below, and the sample data file and full
program are attached.
-- Loop to receive all the packets
    let receiveMulticast = do
            (msg, _) <- recvFrom recvSock 32628
            putStrLn . BS.unpack $ BS.take 100 msg
            receiveMulticast
    _ <- forkIO receiveMulticast
-- Send every line from a file as multicast message
    inputFile <- openFile "data" ReadMode
    fileLines <- BS.lines <$> BS.hGetContents inputFile
    let sendMulticast msg = do
            sendTo sendSock msg addr
            -- Receiver FAILS to receive last few messages unless this
            -- thread delay exists... why?!
            -- threadDelay (1)
    mapM_ sendMulticast fileLines
    hClose inputFile
threadDelay (1000*1000*1000) -- Delay for 1000 seconds
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Gregory Collins