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2015-06-11 13:31:26 -0600 asked a question Slow iperf performance between vms


I am seeing a marked performance difference using iperf between host-host and vm-vm and was hoping someone could help me understand why this is.

I have OpenStack Juno with Neutron (vxlan) running on 2 CentOS 7 hosts. I followed the (OpenStack documentation). One host is responsible for all the services (MariaDB, keystone etc.) as well as compute, the second host is compute only. I have an Intel X710 in each host connected back-to-back. I am using this connection as the instance network. The MTU on the physical NIC is 9000. On a host-host test iperf returns: 16.7 Gbits/sec. On an MTU of 1500 I get 8.71 Gbits/sec.

When running the same iperf test vm-vm (each on a different host) I consistently get somewhere between 1.4 and 1.6 Gbits/sec. The MTU on the vms is 1454. I would expect at least something close to the 8.71 I get on host-host with that MTU size? I've confirmed with tcpdump that the iperf traffic from the vm is flowing over the X710. I've tried turning gro off on the host X710 interface, but this makes no difference to the host-host iperf or the vm-vm iperf.

I've seen suggestions that this is (to be expected), but was wondering if anyone else had any input?

I'd also like to know if anyone has a reference on how you set the MTU sizes for the instance networking that openvswitch creates or do I just set this manually after openvswitch creates all the interfaces? They are all currently set to 1500.

Thanks in advance. Emile