I was asked in an OVN meeting to send out an email talking about what we're working on to make ovn-northd and ovn-controller faster. Here's my summary.
OVN is essentially a stack of compilers. At the top, the CMS dumps some configuration into the northbound database (NDBB). Then: 1. ovn-northd centrally translates the high-level NBDB description into logical flows in the southbound database (SBDB). 2. ovn-controller, on each HV, translates the SBDB logical flows into "physical" (OpenFlow) flows for the local hypervisor and passes them to ovs-vswitchd. 3. ovs-vswitchd translates OpenFlow flows into datapath flows on demand as traffic appears. Currently, OVN implements steps 1 and 2 with code that translates all input to output in one go. When any of the input changes, it re-translates all of it. This is fine for small deployments, but it scales poorly beyond about 1000 hypervisors, at which point each translation step begins to take multiple seconds. Larger deployments call for incremental computation, in which a small change in the input requires only a small amount of computation to yield a small change in the output. It is difficult to implement incremental computation in C. For ovn-controller, two attempts have been made already. The first attempt, in 2016, increased code complexity without similar benefit (https://mail.openvswitch.org/pipermail/ovs-dev/2016-August/078272.html). A recent approach, by Han Zhou shows a much bigger improvement, but it also increases complexity greatly and definitely makes maintenance more difficult. Justin and I are proposing a new approach, based on an incremental computation engine called Differential Datalog, or DDlog for short (https://github.com/ryzhyk/differential-datalog). DDlog is open source software developed at the VMware Research Group in Palo Alto by Leonid Ryzhyk, Mihai Budiu, and others. It uses an underlying engine developed by Frank McSherry at Microsoft Research, called Differential Dataflow (https://github.com/frankmcsherry/differential-dataflow). Here's a talk that Leonid gave on DDlog earlier this earlier: https://ovsorbit.org/#e58 DDlog appears suitable for steps 1 and 2, that is, for both ovn-northd and ovn-controller. Justin and I are starting with ovn-northd, because it is a simpler case, and once we've arrived at some minimum amount of success, Han is going to apply what we've learned to ovn-controller as well. Leonid and Mihai have been working very closely with us (we have literally been writing DDlog code in conference rooms in 90 minute sessions with everyone clustered around laptops) and none of it could happen without them. Here's the process we'll need to follow to get DDlog to work with ovn-northd: * DDlog needs to be able to talk to OVSDB for input (reading data from the northbound database) and output (writing data to the southbound database). Therefore, we need to write OVSDB adapters for DDlog. Leonid has already done an important part of this work. There is more work to do plumbing the adapter into ovn-northd's database connections. * We need to translate the C flow generation code in ovn-northd into DDlog's domain specific language. There are some tricky parts to this but we expect the bulk of it to be straightforward and probably easier to read in DDlog than in C. We've started with the tricky parts, which you can find at https://github.com/ryzhyk/differential-datalog/blob/northd/test/ovn/ovn_northd.dl Please don't take the code there as illustrative of what one would typically see for flow generation, because as I said, these are the hard parts. * The OVN build system will need some changes: - The DDlog compiler, which translates .dl files into Rust, is written in Haskell, so Haskell becomes an OVN build requirement but not a runtime requirement. (If that's a problem, then we can arrange to distribute the Rust output as well as the .dl input, for situations where Haskell is not available.) - OVN will require a Rust compiler at build time. Whatever libraries Rust needs becomes runtime requirements. - ovn-northd (and eventually ovn-controller) will link against the Rust object files and call into DDlog through its external API. * Initially, we plan to make DDlog optional. If Haskell and Rust are available at configure and build time, ovn-northd will build in support for DDlog. At ovn-northd runtime, command-line options will control which implementation is used; we hope to make it possible to run both in parallel to check for differences in behavior. After the DDlog implementation is proven in practice, we hope to delete the C implementation entirely. Happy to hear comments. Ben _______________________________________________ discuss mailing list disc...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-discuss