[Haskell-cafe] Exceeding OS limits for simultaneous socket connections

[Alexander Kjeldaas]

On 30 January 2012 14:22, Rob Stewart <robstewart57 at gmail.com> wrote:

> Hi,
>
> I'm experiencing the "accept: resource exhausted (Too many open
> files)" exception when trying to use sockets in my Haskell program.
>
> The situation:
> - Around a dozen Linux machines running my Haskell program,
> transmitting thousands of messages to each other, sometimes within a
> small period of time.
>

...


> $ ulimit -n
> 1024
>
>
This is not an OS limit, this is your freely chosen limit.  You should not
run with this few file descriptors on a server.  Increasing this by 50x is
entirely reasonable.  However, having too many open TCP connections is not
a good thing either. 1024 was an upper limit way way back on the i386 linux
architecture for code using the select() system call, that is why it is
still a common default.

There are a few ways to get out of this situation.

1. Reuse your TCP connections.  Maybe you could even use HTTP.  An HTTP
library might do reusing of connections for you.

2.  Since you are blocking in getContents, there is a probability that it
is the senders that are being lazy in sendAll.  They opened the TCP
connection, but now they are not sending everything in sendAll, so your
receiver is having lots of threads that are blocked on reading.  Try to be
strict when *sending* so you do not have too many ongoing TCP connections.

3. On the receiver side, to be robust, you could limit the number of
threads that are allowed to do an accept() to the number of file
descriptors you have free.  You can also block on a semaphore whenever
accept returns out of resources, and signal that semaphore after every
close.


Alexander

Indeed, when I experience the "accept: resource exhausted (Too many
> open files)" exception, I check the number of open sockets, which
> exceeds 1024, by looking at the contents of the directory:
> ls -lah /proc/<prod_id>/fd
>
> It is within the getContents function that, once the lazy bytestring
> is fully received, the socket is shutdown http://goo.gl/B6XcV :
> shutdown sock ShutdownReceive
>
> There seems to be no way of limiting the number of permitted
> connection requests from remote nodes. What I am perhaps looking for
> is a mailbox implementation on top of sockets, or another way to avoid
> this error. I am looking to scale up to 100's of nodes, where the
> possibility of more than 1024 simultaneous socket connections to one
> node is increased. Merely increasing the ulimit feels like a temporary
> measure. Part of the dilemma is that the `connect' call in `sendMsg'
> does not throw an error, despite the fact that it does indeed cause an
> error on the receiving node, by pushing the number of open connections
> to the same socket on the master node, beyond the 1024 limit permitted
> by the OS.
>
> Am I missing something? One would have thought such a problem occurs
> frequently with Haskell web servers and the like.. ?
>
> --
> Rob Stewart
>
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