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Tuning the performance of the lwaftr

Adjust CPU frequency governor

To avoid power-saving heuristics causing decreased throughput and higher latency, set the CPU frequency governor to performance:

for CPUFREQ in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do
   [ -f $CPUFREQ ] || continue;
   echo -n performance > $CPUFREQ;

You might need to also go into your BIOS and verify that you have not enabled aggressive power-saving modes that could downclock your processors. A CPU in a power-saving mode typically takes some time to return to peak performance, and this latency can cause packet loss.

Avoid fragmentation

Fragment reassembly in particular is a costly operation. Make sure that MTUs are set such that fragmentation is rare.

CPU affinity

The snabb lwaftr run and snabb loadtest transient commands take a --cpu argument, which will arrange for the Snabb process to run on a particular CPU. It will also arrange to make sure that all memory used by that Snabb process is on the same NUMA node as that CPU, and it will check that any PCI device used by that Snabb process has affinity to that NUMA node, and issue a warning if anything is amiss.

Binding Snabb to a CPU and NUMA node can also be done using the numactl --membind and taskset -c commands, but we recommend the --cpu argument as it is easiest.


In a machine with multiple sockets, you usually have Non-Uniform Memory Access, or NUMA. On such a system, a PCI device or a range of memory might be "closer" to one node than another. The --cpu argument to snabb lwaftr run, described above, will issue a warning if you use a PCI device that is not local to the NUMA node that corresponds to the chosen CPU.

To determine what PCI devices are local to what NUMA nodes, you need to grovel around in /sys. For example if you are going to be working with NICs 0000:01:00.0, 0000:01:00.1, 0000:02:00.0, and 0000:02:00.1, check:

$ for device in 0000:0{1,2}:00.{0,1}; do \
    echo $device; cat /sys/bus/pci/devices/$device/numa_node; \

So all of these are on NUMA node 0. Then you can check your CPUs:

$ numactl -H | grep cpus
node 0 cpus: 0 1 2 3 4 5
node 1 cpus: 6 7 8 9 10 11

So for these we should run our binaries under --cpu CPU to bind them to CPUs in the NUMA node 0, and to arrange to use only memory that is local to that CPU.

Isolate CPUs

When running a Snabb dataplane, we don't want interference from the Linux kernel. In normal operation, a Snabb dataplane won't even make any system calls at all. You can prevent the Linux kernel from pre-empting your Snabb application to schedule other processes on its CPU by reserving CPUs via the isolcpus kernel boot setting.

To isolate CPUs, boot your Linux kernel with the isolcpus parameter. Under NixOS, edit /etc/nixos/configuration.nix to add this parameter:

boot.kernelParams = [ "isolcpus=1-5,7-11" ];

The line above prevents the kernel to schedule processes in CPUs ranging from 1 to 5 and 7 to 11. That leaves CPUs 0 and 6 for the Linux kernel. By default, the kernel will arrange deliver interrupts to the first CPU on a socket, so this isolcpus setting should also isolate the dataplane from interrupt handling as well.

After adding the isolcpus flag run nixos-rebuild switch and then reboot your workstation to enable the changes.

Ingress and egress filtering

Simply enabling ingress and/or egress filtering has a cost. Enabling all filters adds 4 apps to the Snabb graph, and there is a cost for every additional Snabb app. In our tests, while a normal run can do 10 Gbps over two interfaces in full duplex, enabling filters drops that to 8.2 Gbps before dropping packets. However we have not found that the performance depends much on the size of the filter, as the filter's branches are well-biased.


Normally Linux will handle hardware interrupts on the first core on a socket. In our case above, that would be cores 0 and 6. That works well with our isolcpus setting as well: interrupts like timers and so on will only get delivered to the cores which Linux is managing already, and won't interrupt the dataplanes.

However, some distributions (notably Ubuntu) enable irqbalanced, a daemon whose job it is to configure the system to deliver interrupts to all cores. This can increase interrupt-handling throughput, but that's not what we want in a Snabb scenario: we want low latency for the dataplane, and handling interrupts on dataplane CPUs is undesirable. When deploying on Ubuntu, be sure to disable irqbalanced.


Hyperthreads are a way of maximizing resource utilization on a CPU core, driven by the observation that a CPU is often waiting on memory or some external event, and might as well be doing something else while it's waiting. In such a situation, it can be advantageous to run a second thread on that CPU. However for Snabb that's exactly what we don't want. We do not want another thread competing for compute and cache resources on our CPU and increasing our latency. For best results and lowest latency, disable hyperthreading via the BIOS settings.

Huge pages

By default on a Xeon machine, the virtual memory system manages its allocations in 4096-byte "pages". It has a "page table" which maps virtual page addresses to physical memory addresses. Frequently-used parts of a page table are cached in the "translation lookaside buffer" (TLB) for fast access. A virtual memory mapping that describes 500 MB of virtual memory would normally require 120000 entries for 4096-byte pages. However, a TLB only has a limited amount of space and can't hold all those entries. If it is missing an entry, that causes a "TLB miss", causing an additional trip out to memory to fetch the page table entry, slowing down memory access.

To mitigate this problem, it's possible for a Xeon machine to have some "huge pages", which can be either 2 megabytes or 1 gigabyte in size. The same 500MB address space would then require only 250 entries for 2MB hugepages, or just 1 for 1GB hugepages. That's a big win! Also, memory within a huge page is physically contiguous, which is required to interact with some hardware devices, notably the Intel 82599 NICs.

However because hugepages are bigger and need to be physically contiguous, it may be necessary to pre-allocate them at boot-time. To do that, add the default_hugepagesz, hugepagesz, and hugepages parameters to your kernel boot. In NixOS, we use the following, adding on to the isolcpus setting mentioned above:

boot.kernelParams = [ "default_hugepagesz=2048K" "hugepagesz=2048K"
                      "hugepages=10000" "isolcpus=1-5,7-11" ];

Ring buffer sizes

The way that Snabb interfaces with a NIC is that it will configure the NIC to receive incoming packets into a /ring buffer/. This ring buffer is allocated by Snabb (incidentally, to a huge page; see above) and will be filled by the NIC. It has to be a power of 2 in size: so it can hold space for 64 packets, 128 packets, 256 packets, and so on. The default size is 512 packets and the maximum is 65536. The NIC will fill this buffer with packets as it receives them: first to slot 0, then to slot 1, all the way up to slot 511 (for a ring buffer of the default size), then back to slot 0, then slot 1, and so on. Snabb will periodically take some packets out of this read buffer (currently 128 at a time), process them, then come back and take some more: wash, rinse, repeat.

The ring buffer size is configurable via the --ring-buffer-size argument to snabb lwaftr run. What is the right size? Well, there are a few trade-offs. If the buffer is too big, it will take up a lot of memory and start to have too much of a cache footprint. The ring buffer is mapped into Snabb's memory as well, and the NIC arranges for the ring buffer elements that it writes to be directly placed in L3 cache. This means that receiving packets can evict other entries in L3 cache. If your ring buffer is too big, it can evict other data resident in cache that you might want.

Another down-side of having a big buffer is latency. The bigger your buffer, the more the bloat. Usually, Snabb applications always run faster than the incoming packets, so the buffer size isn't an issue when everything is going well; but in a traffic spike where packets come in faster than Snabb can process them, the buffer can remain full the whole time, which just adds latency for the packets that Snabb does manage to process.

However, too small a buffer exposes the user to a higher risk of packet loss due to jitter in the Snabb breath time. A "breath" is one cycle of processing a batch of packets, and as we mentioned it is currently up to 128 packets at a time. This processing doesn't always take the same amount of time: the contents of the packets can obviously be different, causing different control flow and different data flow. Even well-tuned applications can exhibit jitter in their breath times due to differences between what data is in cache (and which cache) and what data has to be fetched from memory. Infrequent events like reloading the binding table or dumping configuration can also cause one breath to take longer than another. Poorly-tuned applications might have other sources of latency such as garbage collection, though this is not usually the case in the lwAFTR.

So, a bigger ring buffer insulates packet processing from breath jitter. You want your ring buffer to be big enough to not drop packets due to jitter during normal operation, but not bigger than that. In our testing we usually use the default ring buffer size. In your operations you might want to increase this up to 2048 entries. We have not found that bigger ring buffer sizes are beneficial, but it depends very much on the environment.