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Open vSwitch with DPDK datapath

This page serves as a guide for how to use the OVS with DPDK datapath functionality available in the Networking service as of the Mitaka release.

The basics

Open vSwitch (OVS) provides support for a Data Plane Development Kit (DPDK) datapath since OVS 2.2, and a DPDK-backed vhost-user virtual interface since OVS 2.4. The DPDK datapath provides lower latency and higher performance than the standard kernel OVS datapath, while DPDK-backed vhost-user interfaces can connect guests to this datapath. For more information on DPDK, refer to the DPDK website.

OVS with DPDK, or OVS-DPDK, can be used to provide high-performance networking between instances on OpenStack compute nodes.

Prerequisites

Using DPDK in OVS requires the following minimum software versions:

  • OVS 2.4
  • DPDK 2.0
  • QEMU 2.1.0
  • libvirt 1.2.13

Support of vhost-user multiqueue that enables use of multiqueue with virtio-net and igb_uio is available if the following newer versions are used:

  • OVS 2.5
  • DPDK 2.2
  • QEMU 2.5
  • libvirt 1.2.17

In both cases, install and configure Open vSwitch with DPDK support for each node. For more information, see the OVS-DPDK installation guide (select an appropriate OVS version in the Branch drop-down menu).

/contributor/internals/ovs_vhostuser for configuration of neutron OVS agent.

In case you wish to configure multiqueue, see the OVS configuration chapter on vhost-user in QEMU documentation.

The technical background of multiqueue is explained in the corresponding blueprint.

Additionally, OpenStack supports vhost-user reconnect feature starting from the Ocata release, as implementation of fix for bug 1604924. Starting from OpenStack Ocata release this feature is used without any configuration necessary in case the following minimum software versions are used:

  • OVS 2.6
  • DPDK 16.07
  • QEMU 2.7

The support of this feature is not yet present in ML2 OVN and ODL mechanism drivers.

Using vhost-user interfaces

Once OVS and neutron are correctly configured with DPDK support, vhost-user interfaces are completely transparent to the guest (except in case of multiqueue configuration described below). However, guests must request huge pages. This can be done through flavors. For example:

$ openstack flavor set m1.large --property hw:mem_page_size=large

For more information about the syntax for hw:mem_page_size, refer to the Flavors guide.

Note

vhost-user requires file descriptor-backed shared memory. Currently, the only way to request this is by requesting large pages. This is why instances spawned on hosts with OVS-DPDK must request large pages. The aggregate flavor affinity filter can be used to associate flavors with large page support to hosts with OVS-DPDK support.

Create and add vhost-user network interfaces to instances in the same fashion as conventional interfaces. These interfaces can use the kernel virtio-net driver or a DPDK-compatible driver in the guest

$ openstack server create --nic net-id=$net_id ... testserver

Using vhost-user multiqueue

To use this feature, the following should be set in the flavor extra specs (flavor keys):

$ openstack flavor set $m1.large --property hw:vif_multiqueue_enabled=true

This setting can be overridden by the image metadata property if the feature is enabled in the extra specs:

$ openstack image set --property hw_vif_mutliqueue_enabled=true IMAGE_NAME

Support of virtio-net multiqueue needs to be present in kernel of guest VM and is available starting from Linux kernel 3.8.

Check pre-set maximum for number of combined channels in channel configuration. Configuration of OVS and flavor done successfully should result in maximum being more than '1'):

$ ethtool -l INTERFACE_NAME

To increase number of current combined channels run following command in guest VM:

$ ethtool -L INTERFACE_NAME combined QUEUES_NR

The number of queues should typically match the number of vCPUs defined for the instance. In newer kernel versions this is configured automatically.

Known limitations

  • This feature is only supported when using the libvirt compute driver, and the KVM/QEMU hypervisor.
  • Huge pages are required for each instance running on hosts with OVS-DPDK. If huge pages are not present in the guest, the interface will appear but will not function.
  • Expect performance degradation of services using tap devices: these devices do not support DPDK. Example services include DVR, FWaaS, or LBaaS.