OK, I've done some more investigation here, as much time as I can spare for now:

I'm not sure this program really is leaking forever after all, even on latest GHC. Originally I thought it was, because I was running only 2 pings / client-second as you were. If you increase this to something like 20 pings per client-second, you see the same asymptotics at first but eventually the client plateaus, at least on my machine. I left it running for an hour. The question remains as to why this program exhibits such strange GC behavior (I don't see any reason for it to slowly gobble RAM until plateauing at an arbitrary figure), maybe Simon M can comment.
The biggest thing you're spending RAM on here is stacks for the threads you create. By default the stack chunk size is 32k, you can lower this with +RTS -kcXX --- using 2kB stacks both programs use <40MB heap resident on my machine. Counting the garbage being generated, the space needed for buffers/etc, and the fact that the binaries themselves are 8MB, I don't think 20kB per active client is unreasonable.
You can reduce GC pressure somewhat by reusing the output buffer, the "io-streams" branch at my copy of your test repo does this.

On Wed, Jul 16, 2014 at 1:00 PM, Gregory Collins <greg@gregorycollins.net> wrote:

On Wed, Jul 16, 2014 at 5:46 AM, Ben Bangert <ben@groovie.org> wrote:

I've run the new server code, indeed it initially takes less memory. However, when I ctrl-C the testing client, it fails to properly close sockets now, they stay open forever (the original code I posted always closed the sockets when I killed the testing client). Did you try ctrl-c the test client and re-running it several times?

Yes once I exhibited the leaking behaviour I stopped (and didn't implement cleanup properly). It should be ok now.

I'm guessing that the recv call is blocking forever and failing somehow to note that the socket was actually closed under it, while the hGetline I was using properly detects this condition and closes the socket when it notices the other side has left.

The Handle stuff will still end up calling recv() under the hood.

I should note that when asking for even 1000 echo clients, the test client as you've changed it dies when I try to launch it (it shows Clients Connected: 0, then just exits silently).

Are you running out of FDs?

I had to use my old testing client to test the server changes you made. If the sockets would properly close when terminating the client abruptly, then its quite possible memory usage would remain lower as it definitely took much much less memory for the first 2k echo clients.

I've also updated my copy of the the test code to work with the GHC 7.4.1 that's installed on my Ubuntu box at work (64-bit) --- and I don't see the client leak there, resident heap plateaus at around 90MB.

G
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Gregory Collins <greg@gregorycollins.net>

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Gregory Collins <greg@gregorycollins.net>