The infrastructure used by SWITCHengines is composed of about 100 servers. Each one uses two 10 Gb/s network ports. Ideally, a given instance (virtual machine) on SWITCHengines would be able to achieve 20 Gb/s of throughput when communicating with the rest of the Internet. In the real world, however, several bottlenecks limit this rate. We are working hard to address these bottlenecks and bring actual performance closer to the theoretical optimum.

An important bottleneck in our infrastructure is the network node for each region. All logical routers are implemented on this node, using Linux network namespaces and Open vSwitch (OVS). That means that all packets between the Internet and all the instances of the region need to pass through the node.

In our architecture, the OpenStack services run inside various virtual machines (“service VMs” or “pets”) on a dedicated set of redundant “provisioning” (or “prov”) servers. This is good for serviceability and reliability, but has some overhead—especially for I/O-intensive tasks such as network packet forwarding. Our network node is one of those service VMs.

In the original configuration, a single instance would never get more than about 2 Gb/s of throughput to the Internet when measured with iperf. What’s worse, the aggregate Internet throughput for multiple VMs was not much higher, which meant that a single high-traffic VM could easily “starve” all other VMs.

We had investigated many options for improving this situation: DVR, multiple network nodes, SR-IOV, DPDK, moving work to switches etc. But each of these methods has its drawbacks such as additional complexity (and thus potential for new and exciting bugs and hard-to-debug failure modes), lock-in, and in some cases, loss of features like IPv6 support. So we stayed with our inefficient but simple configuration that has worked very reliably for us so far.

Multithreading to the Rescue!

Our network node is a VM with multiple logical CPUs. But when running e.g. “top” during high network load, we noticed that only one (virtual) core was busy forwarding packets. So we started looking for a way to distribute the work over several cores. We found that we could achieve this by enabling three things:

Multi-queue virtio-net interfaces

Our service nodes run under libvirt/Qemu/KVM and use virtio-net network devices. These interfaces can be configured to expose multiple queues. Here is an example of an interface definition in libvirt XML syntax which has been configured for eight queues:

<interface type='bridge'> <mac address='52:54:00:e0:e1:15'/> <source bridge='br11'/> <model type='virtio'/> <driver name='vhost' queues='8'/> <virtualport type='openvswitch'/> </interface>

A good rule of thumb is to set the number of queues to the number of (virtual) CPU cores of the system.

Multi-threaded forwarding in the network node VM

Within the VM, kernel threads need to be allocated to the interface queues. This can be achieved using ethtool -L:

ethtool -L eth3 combined 8

This should be done during interface initialization, for example in a “pre-up” action in /etc/network/interfaces. But it seems to be possible to change this configuration on a running interface without disruption.

Recent version of the Open vSwitch datapath

Much of the packet forwarding on the network node is performed by OVS. Its “datapath” portion is integrated into the Linux kernel. Our systems normally run Ubuntu 14.04, which includes the Linux 3.13 kernel. The OVS kernel module isn’t included with this package, but is installed separately from the openvswitch-datapath-dkms package, which corresponds to the relatively old OVS version 2.0.2. Although the OVS kernel datapath is supposed to have been multi-threaded since forever, we found that in our setup, upgrading to a newer kernel is vital for getting good (OVS) network performance.

The current Ubuntu 16.04.1 LTS release includes a fairly new Linux kernel based on 4.4. That kernel also has the OVS datapath module included by default, so that the separate DKMS package is no longer necessary. Unfortunately we cannot upgrade to Ubuntu 16.04 because that would imply upgrading all OpenStack packages to OpenStack “Mitaka”, and we aren’t quite ready for that. But thankfully, Canonical makes newer kernel packages available for older Ubuntu releases as part of their “hardware enablement” effort, so it turns out to be very easy to upgrade 14.04 to the same new kernel:

sudo apt-get install -y --install-recommends linux-generic-lts-xenial

And after a reboot, the network node should be running a fresh Linux 4.4 kernel with the OVS 2.5 datapath code inside.

Results

A simple test is to run multiple netperf TCP_STREAM tests in parallel from a single bare-metal host to six VMs running on separate nova-compute nodes behind the network node.

Each run consists of six netperf TCP_STREAM measurements started in parallel, whose throughput values are added together. Each figure is the average over ten consecutive runs with identical configuration.

The network node VM is set up with 8 vCPUs, and the two interfaces that carry traffic are configured with 8 queues each. We vary the number of queues that are actually used using ethtool -L iface combined n. (Note that even the 1-queue case does not exactly correspond to the original situation; but it’s the closest approximation that we had time to test.)

Network node running 3.13.0-95-generic kernel

1: 3.28 Gb/s

2: 3.41 Gb/s

4: 3.51 Gb/s

8: 3.57 Gb/s

Making use of multiple queues gives very little benefit.

Network node running 4.4.0-36-generic kernel

1: 3.23 Gb/s

2: 6.00 Gb/s

4: 8.02 Gb/s

8: 8.42 Gb/s (8.75 Gb/s with 12 target VMs)

Here we see that performance scales up nicely with multiple queues.

The maximum possible throughput in our setup is lower than 10 Gb/s, because the network node VM uses a single physical 10GE interface for both sides of traffic. And traffic between the network node and the hypervisors is sent encapsulated in VXLAN, which has some overhead.

Outlook

Now we know how to enable multi-core networking for hand-configured service VMs (“pets”) such as our network node. But what about the VMs under OpenStack’s control?

Starting in Liberty, Nova supports multi-queue virtio-net. Our benchmarking cluster was still running Kilo, so we could not test that yet. But stay tuned!