Most operating systems limit an amount of TCP sockets that can be used 
simultaneously. It is possible for a busy Tor relay to run into these
limits, thus being unable to fully utilize the bandwidth resources it 
has at its disposal. Following system-specific tips might be helpful
to alleviate the aforementioned problem.

Linux
-----

Use 'ulimit -n' to raise an allowed number of file descriptors to be 
opened on your host at the same time.

FreeBSD
-------

Tune the followind sysctl(8) variables:
 * kern.maxfiles - maximum allowed file descriptors (for entire system)
 * kern.maxfilesperproc - maximum file descriptors one process is allowed
   to use
 * kern.ipc.maxsockets - overall maximum numbers of sockets for entire 
   system
 * kern.ipc.somaxconn - size of listen queue for incoming TCP connections
   for entire system

See also:
 * https://www.freebsd.org/doc/handbook/configtuning-kernel-limits.html
 * https://wiki.freebsd.org/NetworkPerformanceTuning

Mac OS X
--------

Since Mac OS X is BSD-based system, most of the above hold for OS X as well.
However, launchd(8) is known to modify kern.maxfiles and kern.maxfilesperproc
when it launches tor service (see launchd.plist(5) manpage). Also, 
kern.ipc.maxsockets is determined dynamically by the system and thus is 
read-only on OS X.

OpenBSD
-------

For recent versions of OpenBSD (5.5 and 5.6, and probably older releases
as well), the maximum number of file descriptors that can be opened is
7030:

http://unix.stackexchange.com/questions/104929/does-openbsd-have-a-limit-to-the-number-of-file-descriptors/104948#104948

The maximum number of file descriptors that an OpenBSD machine can have
open is stored in the sysctl variable kern.maxfiles. This value defaults
to 7030 - to verify this, run sysctl kern.maxfiles.

To immediately change a running system's file descriptor limit to, for
example, 20,000 files, run sudo sysctl kern.maxfiles=20000. All sysctl
variables are reset upon reboot using defaults and /etc/sysctl.conf, so
to make your change permanent you must add the line kern.maxfiles=20000
to /etc/sysctl.conf.

One can also change a maximum number of allowed file descriptors for Tor
daemon alone by editing /etc/rc.d/tor and adding the following lines:

tor:\
        :openfiles-max=8192:\
        :tc=daemon:

However, there are stricter limits set on users. This is a security
feature intended to prevent one user from choking out others by opening
all possible file descriptors.

The stricter limits are set in /etc/login.conf. This config file sets
resource access rules for user classes. You should be running
Tor as a non-privileged daemon user '_tor', which belongs to the 'daemon'
class. It will therefore be subject to the 'default' and 'daemon' rules.
There are two relevant rules: openfiles-cur and openfiles-max. The prior
is the initial limit upon login - the soft limit. The latter is the maximum
limit that can be set using 'ulimit -n' or setrlimit() without editing
/etc/login.conf and rebooting. This is known as the hard limit.

Without editing /etc/login.conf, daemon-owned processes have
soft limit of 512 open files and a hard limit of 1024 open files.
Tor can increase the soft limit as needed, so you will therefore
eventually get warnings about running out of available file descriptors
once Tor reaches ~1024 open files.

To increase the hard limit, add the following line to the daemon class
rules in /etc/login.conf:

tor:\
        :openfiles-max=8192:\
        :tc=daemon:

Upon restarting the machine, Tor will be able to open up to 6500 file
descriptors.

Be aware that, by doing this, you are bypassing a security and stability
feature of the OS. If you are running your relay on a weak or old system,
watch your system load to ensure that it can handle this many open files.
Also, Tor may interfere with any other programs that open many files.

Disclaimer
----------

Do note that this document is a draft and above information may be
technically incorrect and/or incomplete. If so, please open a ticket
on https://trac.torproject.org or post to tor-relays mailing list.

Are you running a busy Tor relay? Let us know how you are solving
the out-of-sockets problem on your system.