title: Throughput decreases after MTU 5000
body: <p>I am trying to test throughput between two computers directly connected through 1 GbE and testing with iperf. I am getting a throughput around 980Mbps when MTU is between 5000 and 5050, however it drastically falls to around 680Mbps anything above MTU=5050. I have checked varying window sizes, but with same result.
  Increasing the MTU should decrease the overheads and thereby should increase the bandwidth or at least should not fall.
  I can't figure out this strange behavior. By the way testing TCP throughput.
  Any help ! and thanks guys. This is my post ever post(question) on any forum :) usually I find answers....</p>
  
  <p>Additional info!
  Two centos systems
  One system is Xen 4.2 host (but it shouldn't be the problem)
  Checked with varying buffer sizes in /pro/sys/net/ipv4</p>