Comment 1 for bug 1420710

Also intresting

option splice-auto
no option splice-auto
  Enable or disable automatic kernel acceleration on sockets in both directions
  May be used in sections : defaults | frontend | listen | backend
                                 yes | yes | yes | yes
  Arguments : none

  When this option is enabled either on a frontend or on a backend, haproxy
  will automatically evaluate the opportunity to use kernel tcp splicing to
  forward data between the client and the server, in either direction. Haproxy
  uses heuristics to estimate if kernel splicing might improve performance or
  not. Both directions are handled independently. Note that the heuristics used
  are not much aggressive in order to limit excessive use of splicing. This
  option requires splicing to be enabled at compile time, and may be globally
  disabled with the global option "nosplice". Since splice uses pipes, using it
  requires that there are enough spare pipes.

http://www.haproxy.org/download/1.4/doc/configuration.txt