[ovs-discuss] RFC: incremental computation for OVN with DDlog

Mark Michelson mmichels at redhat.com
Wed Nov 7 13:57:00 UTC 2018

Thanks for the e-mail, Ben. I'm 100% behind this effort. The performance 
benefits and the potential drop in CPU usage of OVN components is 
absolutely worth it. I have some questions inline below with regards to 
specific points you've brought up.

On 11/02/2018 01:44 PM, Ben Pfaff wrote:
> 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.

Is this work in one of the repos you previously linked? If not, is there 
somewhere we can find the WIP?

> * 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.

Thanks for the code examples. Seeing sample DDlog is very nice, even if 
it's not necessarily illustrative of what the final product will be.

For those of us doing work right now to add new features to OVN, how 
should we approach the conversion to DDlog? As an example, I have some 
multicast work in progress that will add some new northbound data. It 
also introduces ovn-northd changes to generate logical flows and 
southbound data.

My assumption is that I should focus 100% on the C implementation for 
now. When should I consider adding the analogous DDlog changes?

Is there some sort of plan for how to keep DDlog up to date in the face 
of new C development? For instance, would we implement a policy that 
states that C changes will not be accepted without equivalent DDlog 
changes? For this initial conversion, would we declare a C feature 
freeze date that states that no new ovn-northd C changes may be added 
after that date in order to give a stable feature set for DDlog conversion?

> * 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.

It's a bit of a loaded question, but by what measure do we consider the 
DDlog implementation to be proven in practice?

Until the C implementation is erased, should we expect to develop 
features for both C and DDlog? Or should we expect to target new 
features only for DDlog?

> Happy to hear comments.
> Ben
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