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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 |
Hello,
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,
--
Nicolas