[ovs-dev] incremental updates (was: Re: [PATCH 2/2] ovn/TODO: Add items proposed for 2.7 in OVN IRC meeting.)

[Ben Pfaff]

On Fri, Aug 19, 2016 at 10:24:21AM -0500, Ryan Moats wrote:
> Ben Pfaff <blp at ovn.org> wrote on 08/19/2016 10:11:59 AM:
> 
> > From: Ben Pfaff <blp at ovn.org>
> > To: Ryan Moats/Omaha/IBM at IBMUS
> > Cc: Numan Siddique <nusiddiq at redhat.com>, ovs dev <dev at openvswitch.org>
> > Date: 08/19/2016 10:12 AM
> > Subject: incremental updates (was: Re: [ovs-dev] [PATCH 2/2] ovn/
> > TODO: Add items proposed for 2.7 in OVN IRC meeting.)
> >
> > On Fri, Aug 19, 2016 at 08:34:45AM -0500, Ryan Moats wrote:
> > > We've been running incremental processing in ovn-controller here for
> quite
> > > a while (even before it merged officially) and given our experience,
> > > I've been doing some hard thinking about it.
> > >
> > > The original goal of the patch set was to allow ovn-controller to use
> > > incremental updates as much as possible.  However, the range of
> possible
> > > changes in inputs resulted in ovn-controller having to continue to
> > > maintain the ability to run a full update in those cases where
> previously
> > > calculated rules are no longer correct.
> > >
> > > What we've found is that most configuration events from our CMS
> (Neutron)
> > > end up triggering a full ovn-controller update, and so the end result
> of
> > > the incremental processing code is not that ovn-controller is doing
> > > incremental updates most of the time, but rather that ovn-controller
> > > doesn't recalculate changes in-between modifications.
> > >
> > > While we still feel that the above is a win, I'm coming to the
> conclusion
> > > that the current code base has added unnecessary complexity to achieve
> > > this. Based on this, I'm thinking of the following approach:
> > >
> > > 1) going back to doing full processing every cycle, while still
> > > keeping the persistence of items where we can, because I feel that
> > > persistence has allowed us to handle cases where we need to skip a poll
> > > cycle that we didn't have before and that has improved things.
> >
> > What I'm hearing here is that we'll do full processing if any processing
> > at all is necessary.  Seems fine to me.
> >
> > > 2) introducing a new command flag to allow those that don't want to
> > > run in what I'm now calling quiet mode to continue to do full
> processing
> > > every cycle. (In retrospect, I should have proposed this up front for
> i-p
> > > to allow for better isolation of that code, but as they say, hindsight
> > > is always 20/20).
> >
> > Hmm.  I would hope that "Full processing if any processing" is fairly
> > easy to get right.  However, I'm OK with having a flag as a fail-safe.
> > In time, I would hope that we could delete the flag.
> >
> > > 3) For quiet mode, check the integration bridge and Ben's sequence
> > > number information in the SB database to determine if anything has
> > > changed since the last cycle. If something has changed, run the full
> > > processing code. If not, quiesce for a poll cycle.
> >
> > I don't think that the sequence numbers are a good way to determine
> > whether something has changed.  Without --wait, for example, ovn-nbctl
> > doesn't change the sequence number.
> 
> Yes on #1 - that's the intent.
> 
> Apologies for not being more clear that in re my thinking for suggesting
> the command line flag.  I agree that we'd deprecate it long term if we
> added it, but .... (and now I move to #3)
> 
> I know that sequence numbers are only there today if asked for, and I was
> thinking of arguing that they should always be there and that ovn-nbctl
> should *check* them only if --wait is set.  However, there might be
> other ways to figure out if the SB DBs have changed (I'll admit, I've not
> thought of any yet - anybody have any ideas?)

The IDL has its own sequence number for that, see ovsdb_idl_get_seqno().