Fixing OpenVSwitch and GRE asymetric performances

OpenStack Havana is configured to use OpenVSwitch 1.10.2 as packaged for Ubuntu precise, with a linux-3.11 kernel.The cluster is connected to a 100Mb/s link. When sending data from an instance to the internet (using iperf), it shows ~90Mb/s. When receiving data from the internet to the instance, it is down to ~1Mb/s. After capturing the packets on the interface used by the default route on the hypervisor running the neutron router with

tcpdump -i eth0 host -w /tmp/bad.cap

wireshark /tmp/bad.cap shows a lot of retransmissions.

A similar problem was reported back in October 2013 and hints that it may be a kernel problem. Upgrading the kernel of the hypervisor running the neutron router to linux-3.13 indeed fixes the problem. The compute nodes running the instances do not need their kernel updated, they can keep using the linux-3.11 kernel with the 1.10.2 version of the OpenVSwitch datapath kernel module. The OpenVSwitch kernel part is in the linux-3.13 tree and the openvswitch-datapath-dkms is not used any longer. It will fail to compile against the linux-3.13 headers but the error can be ignored and the package uninstalled.