Horton

Re: PATCH: logical_work_mem and logical streaming of large in-progress transactions

Masahiko Sawada <masahiko.sawada@2ndquadrant.com>

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From: Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Dilip Kumar <dilipbalaut@gmail.com>, Michael Paquier <michael@paquier.xyz>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2019-12-24T05:46:55Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Tighten the concurrent abort check during decoding.

  2. Improve hash_create()'s API for some added robustness.

  3. Use HASH_BLOBS for xidhash.

  4. Fix initialization of RelationSyncEntry for streaming transactions.

  5. Remove unused function declaration in logicalproto.h.

  6. Add additional tests to test streaming of in-progress transactions.

  7. Fix inline marking introduced in commit 464824323e.

  8. Add support for streaming to built-in logical replication.

  9. Fix the SharedFileSetUnregister API.

  10. Fix comment in procarray.c

  11. Suppress compiler warning in non-cassert builds.

  12. Extend the BufFile interface.

  13. Mark a few logical decoding related variables with PGDLLIMPORT.

  14. Implement streaming mode in ReorderBuffer.

  15. Extend the logical decoding output plugin API with stream methods.

  16. WAL Log invalidations at command end with wal_level=logical.

  17. Immediately WAL-log subtransaction and top-level XID association.

  18. Allow logical replication to transfer data in binary format.

  19. Only superuser can set sslcert/sslkey in postgres_fdw user mappings

  20. Track statistics for spilling of changes from ReorderBuffer.

  21. Add logical_decoding_work_mem to limit ReorderBuffer memory usage.

  22. logical decoding: process ASSIGNMENT during snapshot build

  23. Emit invalidations to standby for transactions without xid.

On Fri, 20 Dec 2019 at 22:30, Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> 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?
>

Thank you for explanation. The plan makes sense. But I think in the
current design it's a problem that logical replication worker doesn't
receive changes (and doesn't check interrupts) during applying
committed changes even if we don't have a worker dedicated for
applying. I think the worker should continue to receive changes and
save them to temporary files even during applying changes. Otherwise
the buffer would be easily full and replication gets stuck.

Regards,

-- 
Masahiko Sawada            http://www.2ndQuadrant.com/
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