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

[Gregory Collins]

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 <ben.gunton at gmail.com> 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 <greg at gregorycollins.net>
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