Closing sockets does not free them

[Lars Frantzen-2]

Hi all,

I have written a simple server written in GNU Prolog opening a socket
for a client. After the job is done I close the streams and terminate
the program.

However, after terminating, the socket stays blocked for some time (like
2 minutes), I get a:

system_error(cannot_catch_throw(error(system_error('Address already in
use'),socket_bind/2)))

I am using it under a 64bit Linux. Is this normal behaviour or is there
something wrong with my code? To set up the socket I do:

        socket('AF_INET', Des),
        socket_bind(Des, 'AF_INET'(Host,Port)),
        socket_listen(Des, 1),
        socket_accept(Des, Client, StreamIn, StreamOut),

        repeat,
        read(StreamIn, Query),
        ...
        write(StreamOut, Solution),
        fail.

To close the connection I do:

        close(StreamIn),
        close(StreamOut)

I also tried adding a:

        socket_close(Des)

But that did not help.

The full source is here:
http://www.cs.ru.nl/~lf/tools/treesolver/treeSolver.pl

Thanks for all hints, and cheers,
Lars


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[Nicolas Pelletier-2]

Hello,

On Wed, Sep 2, 2009 at 00:41, Lars Frantzen<lf@...> wrote:

>
> I have written a simple server written in GNU Prolog opening a socket
> for a client. After the job is done I close the streams and terminate
> the program.
>
> However, after terminating, the socket stays blocked for some time (like
> 2 minutes), I get a:
>
> system_error(cannot_catch_throw(error(system_error('Address already in
> use'),socket_bind/2)))

This is perfectly normal behaviour for a TCP socket. :-)

When a TCP socket is closed, it still lingers for some time to allow
for traffic that would still be directed at it and "on the fly"
somewhere in the network to die out before the socket can be
reallocated. If this were not so, quickly closing and re-opening a
socket bound to the same address and port would let it received
traffic that was part of the previous (and now irrelevant) stream.

The recommended, standard, and default time-out is 2 minutes. However,
when using fast networks and/or many TCP connections, this safety
time-out may come as a performance bottleneck (It is still quite
common to mistake the number of TCP connections established per second
for a measure of the actual TCP stack performance). This parameter is
usually settable; under Linux, man 7 tcp and the parameters in
/proc/sys/net/ipv4 will let you tweak this. Specifically,

       tcp_tw_recycle (Boolean; default: disabled)
              Enable fast recycling of TIME_WAIT sockets.  Enabling
this option is not recommended since this causes problems when working
with  NAT  (Net‐
              work Address Translation).

       tcp_tw_reuse (Boolean; default: disabled)
              Allow  to  reuse  TIME_WAIT  sockets  for  new
connections  when  it  is  safe  from  protocol  viewpoint.  It should
not be changed without
              advice/request of technical experts.

However, be prepared to debug strange network behaviour if you turn
the knobs without knowing what they do.

Hope this helps,

--
Nicolas


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[Lars Frantzen-2]

Dear Nicolas,

many thanks for this comprehensive answer, it helps a lot! Will try
playing with the options you mentioned.

Cheers,
Lars

Nicolas Pelletier wrote:

> Hello,
>
> On Wed, Sep 2, 2009 at 00:41, Lars Frantzen<lf@...> wrote:
>> I have written a simple server written in GNU Prolog opening a socket
>> for a client. After the job is done I close the streams and terminate
>> the program.
>>
>> However, after terminating, the socket stays blocked for some time (like
>> 2 minutes), I get a:
>>
>> system_error(cannot_catch_throw(error(system_error('Address already in
>> use'),socket_bind/2)))
>
> This is perfectly normal behaviour for a TCP socket. :-)
>
> When a TCP socket is closed, it still lingers for some time to allow
> for traffic that would still be directed at it and "on the fly"
> somewhere in the network to die out before the socket can be
> reallocated. If this were not so, quickly closing and re-opening a
> socket bound to the same address and port would let it received
> traffic that was part of the previous (and now irrelevant) stream.
>
> The recommended, standard, and default time-out is 2 minutes. However,
> when using fast networks and/or many TCP connections, this safety
> time-out may come as a performance bottleneck (It is still quite
> common to mistake the number of TCP connections established per second
> for a measure of the actual TCP stack performance). This parameter is
> usually settable; under Linux, man 7 tcp and the parameters in
> /proc/sys/net/ipv4 will let you tweak this. Specifically,
>
>        tcp_tw_recycle (Boolean; default: disabled)
>               Enable fast recycling of TIME_WAIT sockets.  Enabling
> this option is not recommended since this causes problems when working
> with  NAT  (Net‐
>               work Address Translation).
>
>        tcp_tw_reuse (Boolean; default: disabled)
>               Allow  to  reuse  TIME_WAIT  sockets  for  new
> connections  when  it  is  safe  from  protocol  viewpoint.  It should
> not be changed without
>               advice/request of technical experts.
>
> However, be prepared to debug strange network behaviour if you turn
> the knobs without knowing what they do.
>
> Hope this helps,
>

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