Initial version of doc/TUNING.

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+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.
+
+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.
+