I am not new to linux but I am not formally trained in networking so I need some help understanding what I am seeing. I am using iperf to check the bandwidth on a Beowulf cluster. All machines are running Linux 2.6 (CentOS 4.4), AMD, 64 bit, integrated NICs and a gigabit switch. Below is a truncated summary from iperf for each machine. I am not sure I'm reading this right. It appears that the bandwitdh on several of the machines is about 2/3 what it could be, that is, why are some rates around 626 Mbits/sec and some around 940?
- Is something wrong with my hardware or software setup? If so, what could some of the potential problems be with the slower machines that I could test?
- Is this an iperf bug?
- Am I reading the output wrong? If so, what am I misunderstanding?

I ran the test twice and the results were consistent with the below. Thanks in advance for any help you can provide.

Shawn
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.121 port 33120
[ 4] 0.0-10.0 sec 1.09 GBytes 940 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.122 port 33074
[ 4] 0.0-10.0 sec 744 MBytes 624 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.124 port 33124
[ 4] 0.0-10.0 sec 1.09 GBytes 940 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.125 port 33160
[ 4] 0.0-10.0 sec 1.09 GBytes 940 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.126 port 33001
[ 4] 0.0-10.0 sec 746 MBytes 626 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.127 port 32925
[ 4] 0.0-10.0 sec 745 MBytes 625 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.128 port 32913
[ 4] 0.0-10.0 sec 754 MBytes 633 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.129 port 32930
[ 4] 0.0-10.0 sec 743 MBytes 623 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.130 port 32917
[ 4] 0.0-10.0 sec 756 MBytes 634 Mbits/sec
[ 4] local 192.168.11.120 port 5001 connected with 192.168.11.131 port 32931
[ 4] 0.0-10.0 sec 758 MBytes 635 Mbits/sec