# DPDK Bridges

Bridge must be specially configured to utilize DPDK-backed physical and virtual ports.

## Quick Example

This example demonstrates how to add a bridge that will take advantage of DPDK:

    $ ovs-vsctl add-br br0 -- set bridge br0 datapath_type=netdev

This assumes Open vSwitch has been built with DPDK support. Refer to Open vSwitch with DPDK for more information.

## Extended & Custom Statistics

The DPDK Extended Statistics API allows PMDs to expose a unique set of statistics. The Extended Statistics are implemented and supported only for DPDK physical and vHost ports. Custom statistics are a dynamic set of counters which can vary depending on the driver. Those statistics are implemented for DPDK physical ports and contain all “dropped”, “error” and “management” counters from `XSTATS`. A list of all `XSTATS` counters can be found here.

Note

vHost ports only support RX packet size-based counters. TX packet size counters are not available.

To enable statistics, you have to enable OpenFlow 1.4 support for OVS. To configure a bridge, `br0`, to support OpenFlow version 1.4, run:

    $ ovs-vsctl set bridge br0 datapath_type=netdev \
      protocols=OpenFlow10,OpenFlow11,OpenFlow12,OpenFlow13,OpenFlow14

Once configured, check the OVSDB protocols column in the bridge table to ensure OpenFlow 1.4 support is enabled:

    $ ovsdb-client dump Bridge protocols

You can also query the port statistics by explicitly specifying the `-O OpenFlow14` option:

    $ ovs-ofctl -O OpenFlow14 dump-ports br0

There are custom statistics that OVS accumulates itself and these stats has `ovs_` as prefix. These custom stats are shown along with other stats using the following command:

    $ ovs-vsctl get Interface <iface> statistics

## Simple Match Lookup

There are cases where users might want simple forwarding or drop rules for all packets received from a specific port, e.g

    in_port=1,actions=2
    in_port=2,actions=IN_PORT
    in_port=3,vlan_tci=0x1234/0x1fff,actions=drop
    in_port=4,actions=push_vlan:0x8100,set_field:4196->vlan_vid,output:3

There are also cases where complex OpenFlow rules can be simplified down to datapath flows with very simple match criteria.

In theory, for very simple forwarding, OVS doesn’t need to parse packets at all in order to follow these rules. In practice, due to various implementation constraints, userspace datapath has to match at least on a small set of packet fields. Some matching criteria (for example, ingress port) are not related to the packet itself and others (for example, VLAN tag or Ethernet type) can be extracted without fully parsing the packet. This allows OVS to significantly speed up packet forwarding for these flows with simple match criteria. Statistics on the number of packets matched in this way can be found in a Simple Match hits counter of ovs-appctl dpif-netdev/pmd-perf-show command.

## EMC Insertion Probability

By default 1 in every 100 flows is inserted into the Exact Match Cache (EMC). It is possible to change this insertion probability by setting the `emc-insert-inv-prob` option:

    $ ovs-vsctl --no-wait set Open_vSwitch . other_config:emc-insert-inv-prob=N

where:

`N`

  A positive integer representing the inverse probability of insertion, i.e. on average 1 in every `N` packets with a unique flow will generate an EMC insertion.

If `N` is set to 1, an insertion will be performed for every flow. If set to 0, no insertions will be performed and the EMC will effectively be disabled.

With default `N` set to 100, higher megaflow hits will occur initially as observed with pmd stats:

    $ ovs-appctl dpif-netdev/pmd-perf-show

For certain traffic profiles with many parallel flows, it’s recommended to set `N` to ‘0’ to achieve higher forwarding performance.

It is also possible to enable/disable EMC on per-port basis using:

    $ ovs-vsctl set interface <iface> other_config:emc-enable={true,false}

Note

This could be useful for cases where different number of flows expected on different ports. For example, if one of the VMs encapsulates traffic using additional headers, it will receive large number of flows but only few flows will come out of this VM. In this scenario it’s much faster to use EMC instead of classifier for traffic from the VM, but it’s better to disable EMC for the traffic which flows to the VM.

For more information on the EMC refer to Open vSwitch with DPDK .

## SMC cache

SMC cache or signature match cache is a new cache level after EMC cache. The difference between SMC and EMC is SMC only stores a signature of a flow thus it is much more memory efficient. With same memory space, EMC can store 8k flows while SMC can store 1M flows. When traffic flow count is much larger than EMC size, it is generally beneficial to turn off EMC and turn on SMC. It is currently turned off by default.

To turn on SMC:

    $ ovs-vsctl --no-wait set Open_vSwitch . other_config:smc-enable=true