[ovs-dev] [RFC 00/14] ovn-controller incremental processing.

[Han Zhou]

On Mon, Aug 6, 2018 at 10:30 PM, Han Zhou <zhouhan at gmail.com> wrote:
>
>
>
> On Mon, Aug 6, 2018 at 3:13 AM, Mark Michelson <mmichels at redhat.com>
wrote:
> >
> > Hi Han,
> >
> > I thought about this more over the weekend, and I was hoping I'd get to
respond to my own e-mail before you saw it, because I realized I had a
fundamental misunderstanding of the scope and nature of change handlers.
I'll reply to your comments in-line below.
> >
> > On 08/05/2018 03:11 PM, Han Zhou wrote:
> >>
> >> Hi Mark,
> >>
> >> Thanks for the review and very valuable comments! (I was on vacation
last week so sorry for slow response).
> >>
> >> On Tue, Jul 31, 2018 at 3:47 AM, Mark Michelson <mmichels at redhat.com
<mailto:mmichels at redhat.com>> wrote:
> >>  >
> >>  > Hi Han,
> >>  >
> >>  > I've given this patchset a look, and I was following along pretty
well until I got to about patch 11. From that point on, I had to re-read
the code more times than I care to admit before I finally understood what
was going on :)
> >>  >
> >>  > What you have is a structure (lflow_ref_list_node) that is
simultaneously in two lists. These two lists are each the data in separate
hmap nodes. The hmap nodes are in two separate hmaps. One hmap uses the
reference type and name as a key, and the other uses the lflow UUID as a
key. This way given an address set name, you can find the associated
logical flow UUIDs in the ref_lflow_table. Or given a logical flow UUID,
you can find address sets.
> >>  >
> >>  > I'm wondering if this can be simplified somehow.
> >>  >
> >>  > Right now, if logical flows change, the change handler has to
update the ref_lflow_table so that address sets no longer reference that
logical flow. If address sets change, then the lflow_ref_table is updated.
In both cases consider_logical_flow() gets called and realigns the tables
as appropriate.
> >>  >
> >>  > The problem with this is that it reeks of cross-cutting concerns,
and it seems like it wouldn't scale well (consider a 3- or 4-chain of
dependencies). Ideally, the dependency chain would make sure that the
change handler for logical flows only deals with logical flows, and the
change handler for address sets only deals with address sets.
> >>
> >> I agree that maintaining the cross reference has some overhead, but I
don't see a scaling issue in this case. Adding entries to the cross
reference table is a by-product of the consider_logical_flow() when parsing
the lflow, and deleting the entries is also efficient with O(N), N = number
of address sets used by a lflow, which in most cases should be a very small
number (correct me if I am wrong). As to memory consumption, it maintains
only the mapping between resource names and lflow uuids, so I don't expect
it to be a significant cost either. Could you explain a little more about
the "3- or 4-chain of dependencies" example?
> >
> >
> > My thought was along the lines that table A references table B, which
references table C. A change in table A might result in a change to table
B, which then results in a change to table C. In my head, I thought this
would mean having to maintain a large series of hashmaps that
cross-referenced each other. I realize this isn't correct, though, and that
as long as the dependency chain is linear, this isn't any different from
what you already have proposed here.
> >
> > In reality, I can't think of any example changes in the southbound
database that would cause such a series of events, but I may not be
thinking hard enough :)
> >
> >>
> >> For the "cross-cutting concern", I don't see it that way. I see it as
a pattern of change handler implementation. In general, output of an engine
node is the result of a "join" operation of its inputs. When there are
multiple inputs and one of them changes, for a change handler to compute
the output incrementally, we will need to use the changed row to probe all
the other inputs to update the final output. For the address-set and
port-group handlers, it is join between two tables only, and the cross
reference table is built to make the probing efficient in change handler.
The cross reference table is also generalized so that any resources
referenced by logical flows can reuse the same data structure and
interfaces, and now it is reused by both address-sets and port-groups. We
can make it more generalized to be used for other mappings if needed.
> >
> >
> > Yes, and this sort of thinking is what I had over the weekend that made
me have a "Eureka!" moment and realize what I had been missing here.
> >
> > I had been looking at the address set change handler and thinking of
the change handler as being an "owner" of address set data. The reason is
that the engine node is tied directly to updates to the address set table.
It felt like it was overstepping its boundaries by then stepping through
data that I thought was owned by the logical flow change handler. The fact
that both change handlers acted on the same data just struck me as wrong.
> >
> > However, I realized I need to stop thinking about data ownership in
that sort of way when it comes to change handlers. Engine nodes do have
ownership like I was imagining in their run() method, but change handlers
are very different. They are responsible for analyzing more than just the
data that has changed, but also for analyzing the relationship that data
has with other data. That other data may or may not be tied to other engine
nodes.
> >
> >>
> >> If we really wants to make it more generalized, I think the answer is
the datalog approach. I would be great if it can be implemented that way,
but I am pessimistic for it to be applied to ovn-controller in a practical
time line, given that ovn-controller is more complex in terms of both data
sources and processing logic compared with ovn-northd. And I think it is
practical and simple to implement the probing for most frequent scenarios
as demonstrated by this RFC.
> >
> >
> > I agree. I don't think datalog is the correct approach for
ovn-controller.
> >
> >>
> >>  >
> >>  > If we generalize things a bit, there are likely to be two ways
dependencies manifest in the database. In this particular case, text in one
row expands to data of a separate database row. The other case would be
where a database row contains the UUID (or list of UUIDs) of other database
rows.
> >>  >
> >>  > For the textual case, I think the easiest way to handle this is to
replace the text with what it expands to earlier than when we currently do
it. Consider that a logical flow references address set $foo. Currently,
the logical flow in the southbound database has the text "$foo" in it. If
$foo were replaced with the actual addresses from the address set, then
when an address set changes, the text of the logical flow would change as
well, thus resulting in a direct change of the logical flow. A less
disruptive version of this might be to use some reserved character
automatically in the logical flow match followed by a sequence number. So
for instance, if a logical flow were set up to reference address set $foo,
then the actual logical flow might be something like $foo?1. Then if
northbound address set foo changes, the logical flow could be updated to
$foo?2 by ovn-northd. Again, the textual change in the logical flow would
result in triggering the change tracker.
> >>  >
> >>
> >> This proposal is interesting and I think it is a valid alternative. It
is trying to implement the probing without maintaining a cross reference
table in ovn-controller. In fact it moves the effort of building the
reference table from ovn-controller to maintaining the sequence number for
each address-set/port-group resources in ovn-northd. I am just not sure if
this makes the system simpler or more complex. I will need to think more
about it.
> >
> >
> > Yes, having thought about this some more, I agree that this could just
be trading one complexity for another. Plus, aside from address sets and
port groups, I'm not sure that there is any other text expansion type
references in the southbound database. So engineering a big solution for
this may not have a lot of bang for your buck. It may be worth workshopping
just to see, though.
> >
> >>
> >>  > For the database referencing case, it would be nice if the IDL
change tracking code could automatically do this for us. This way if record
foo has a column that references row bar, then if bar changes, we would be
told that foo also changed. This strikes me as difficult to implement and
could result in some interesting dependency graphs within the IDL code
though.
> >>  >
> >>  > What do you think?
> >>  >
> >>
> >> For the database referencing case, it seems not directly related to
your concern regarding address sets handling (or port-group handling).
Please correct me if I misunderstood something here. But I agree with idea
of utilizing and improving the IDL capability to build the dependency graph
for table references, and this is exactly in my TODO as mentioned in the
cover letter:
> >
> >
> > You are correct that this does not apply to address set or port group
referencing by logical flows. The relationship between logical flows and
address sets and port groups is not currently expressible at the IDL level.
I was thinking ahead a bit about how other tables may refer to each other.
I foresaw similar structures in change handlers for those tables and
wondered if that could be handled at the IDL level instead.
> >
> >
> >>
> >> "For exposing the dependencies introduced by reference access, it is a
big
> >> TODO item and it is the major reason this patch series is RFC only."
> >
> >
> Hi Mark,
>
> Thank you very much for sharing the thought process. I am glad we are on
the same page :)
>
> Regards,
> Han

Hi Mark, Ben,

Table reference access is handled in the new patch series I sent:
https://mail.openvswitch.org/pipermail/ovs-dev/2018-August/351060.html
Please see the cover letter for a briefing of the changes and ideas, and me
know your thoughts.

Thanks,
Han