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Taikun OCP Guide

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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_multiqueue_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 and
    FWaaS.
  • When the ovs_use_veth option is set to
    True, any traffic sent from a DHCP namespace will have an
    incorrect TCP checksum. This means that if
    enable_isolated_metadata is set to True and
    metadata service is reachable through the DHCP namespace, responses from
    metadata will be dropped due to an invalid checksum. In such cases,
    ovs_use_veth should be switched to False and
    Open vSwitch (OVS) internal ports should be used instead.

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