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qperf. On the first node, just run <tt>qperf</tt>. On the second, <tt>qperf -vvs SERVER_IP tcp_lat -m 4096</tt>.
[[File:Warning icon.svg|32px|link=]] Warning: Ubuntu has AppArmor enabled by default and it affects network latency adversely. Disable it if you want good performance. The effect of AppArmor is like the following (Intel X520-DA2):
* centos 3.10: rtt min/avg/max/mdev = 0.039/0.053/0.132/0.012 ms
* ubuntu 4.x + apparmor: rtt min/avg/max/mdev = 0.068/0.163/0.230/0.029 ms
* ubuntu 4.x: rtt min/avg/max/mdev = 0.037/0.071/0.157/0.018 ms
== Why is it so slow ==
