Re: PATCH: logical_work_mem and logical streaming of largein-progress transactions

On Fri, Dec 20, 2019 at 11:47 AM Masahiko Sawada
<masahiko.sawada@2ndquadrant.com> wrote:
>
> On Mon, 2 Dec 2019 at 17:32, Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > On Sun, Dec 1, 2019 at 7:58 AM Michael Paquier <michael@paquier.xyz> wrote:
> > >
> > > On Fri, Nov 22, 2019 at 01:18:11PM +0530, Dilip Kumar wrote:
> > > > I have rebased the patch on the latest head and also fix the issue of
> > > > "concurrent abort handling of the (sub)transaction." and attached as
> > > > (v1-0013-Extend-handling-of-concurrent-aborts-for-streamin) along with
> > > > the complete patch set.  I have added the version number so that we
> > > > can track the changes.
> > >
> > > The patch has rotten a bit and does not apply anymore.  Could you
> > > please send a rebased version?  I have moved it to next CF, waiting on
> > > author.
> >
> > I have rebased the patch set on the latest head.
>
> Thank you for working on this.
>
> This might have already been discussed but I have a question about the
> changes of logical replication worker. In the current logical
> replication there is a problem that the response time are doubled when
> using synchronous replication because wal senders send changes after
> commit. It's worse especially when a transaction makes a lot of
> changes. So I expected this feature to reduce the response time by
> sending changes even while the transaction is progressing but it
> doesn't seem to be. The logical replication worker writes changes to
> temporary files and applies these changes when the worker received
> commit record (STREAM COMMIT). Since the worker sends the LSN of
> commit record as flush LSN to the publisher after applying all
> changes, the publisher must wait for all changes are applied to the
> subscriber.
>

The main aim of this feature is to reduce apply lag.  Because if we
send all the changes together it can delay there apply because of
network delay, whereas if most of the changes are already sent, then
we will save the effort on sending the entire data at commit time.
This in itself gives us decent benefits.  Sure, we can further improve
it by having separate workers (dedicated to apply the changes) as you
are suggesting and in fact, there is a patch for that as well(see the
performance results and bgworker patch at [1]), but if try to shove in
all the things in one go, then it will be difficult to get this patch
committed (there are already enough things and the patch is quite big
that to get it right takes a lot of energy).  So, the plan is
something like that first we get the basic feature and then try to
improve by having dedicated workers or things like that.  Does this
make sense to you?

[1] - https://www.postgresql.org/message-id/8eda5118-2dd0-79a1-4fe9-eec7e334de17%40postgrespro.ru

-- 
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com