Performance > expose a few tunables

[pablos-here]

I plan on exposing the following performance tunables.

I was going to play with --txqueuelen but received the following openvpn warning in the log:

2023-02-02 15:00:52 Note: Cannot set tx queue length on tun0: Operation not permitted (errno=1)

Tunables

  --fast-io
          (Experimental) Optimize TUN/TAP/UDP I/O writes by avoiding a call  to  poll/epoll/select
          prior  to the write operation. The purpose of such a call would normally be to block un-
          til the device or socket is ready to accept the write. Such blocking is  unnecessary  on
          some  platforms which don't support write blocking on UDP sockets or TUN/TAP devices. In
          such cases, one can optimize the event loop by avoiding the poll/epoll/select call,  im-
          proving CPU efficiency by 5% to 10%.

          This  option can only be used on non-Windows systems, when --proto udp is specified, and
          when --shaper is NOT specified.

   --sndbuf size
          Set the TCP/UDP socket send buffer size. Defaults to operating system default.

   --rcvbuf size
          Set the TCP/UDP socket receive buffer size. Defaults to operating system default.