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Re: Closing sockets does not free them

From: Nicolas Pelletier
Subject: Re: Closing sockets does not free them
Date: Fri, 4 Sep 2009 19:30:39 +0900


On Wed, Sep 2, 2009 at 00:41, Lars Frantzen<address@hidden> 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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