Why iperf dual traffic in 100Mbps envrionment is much less than 100M?

http://archive.ncsa.illinois.edu/lists/iperf-users/apr06/msg00011.html

http://archive.ncsa.illinois.edu/lists/iperf-users/apr06/msg00012.html


I cant be sure, but that sounds like you might be seeing what is called a "send stall", you are over-running the NIC queue.  Try this (assuming you are usign eth0):

 

ifconfig eth0 txqueuelen 5000

and see if that makes any difference.