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Re: Perform streaming logical transactions by background workers and parallel apply

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-23T03:20:58Z
Lists: pgsql-hackers 
On Thu, Dec 22, 2022 at 6:18 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Thu, Dec 22, 2022 at 7:04 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Dec 22, 2022 at 11:39 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > Thank you for updating the patch. Here are some comments on v64 patches:
> > >
> > > While testing the patch, I realized that if all streamed transactions
> > > are handled by parallel workers, there is no chance for the leader to
> > > call maybe_reread_subscription() except for when waiting for the next
> > > message. Due to this, the leader didn't stop for a while even if the
> > > subscription gets disabled. It's an extreme case since my test was
> > > that pgbench runs 30 concurrent transactions and logical_decoding_mode
> > > = 'immediate', but we might want to make sure to call
> > > maybe_reread_subscription() at least after committing/preparing a
> > > transaction.
> > >
> >
> > Won't it be better to call it only if we handle the transaction by the
> > parallel worker?
>
> Agreed. And we won't need to do that after handling stream_prepare as
> we don't do that now.
>

I think we do this for both prepare and non-prepare cases via
begin_replication_step(). Here, in both cases, as the changes are sent
to the parallel apply worker, we missed in both cases. So, I think it
is better to do in both cases.

> >
> > It seems currently we give a similar message when the logical
> > replication worker slots are finished "out of logical replication
> > worker slots" or when we are not able to register background workers
> > "out of background worker slots". Now, OTOH, when we exceed the limit
> > of sync workers "max_sync_workers_per_subscription", we don't display
> > any message. Personally, I think if any user has used the streaming
> > option as "parallel" she wants all large transactions to be performed
> > in parallel and if the system is not able to deal with it, displaying
> > a LOG message will be useful for users. This is because the
> > performance difference for large transactions between parallel and
> > non-parallel is big (30-40%) and it is better for users to know as
> > soon as possible instead of expecting them to run some monitoring
> > query to notice the same.
>
> I see your point. But looking at other parallel features such as
> parallel queries, parallel vacuum and parallel index creation, we
> don't give such messages even if the number of parallel workers
> actually launched is lower than the ideal. They also bring a big
> performance benefit.
>

Fair enough. Let's remove this LOG message.

-- 
With Regards,
Amit Kapila.

Commits

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

  1. Fix invalid memory access during the shutdown of the parallel apply worker.

  2. Fix assertion failure in apply worker.

  3. Use elog to report unexpected action in handle_streamed_transaction().

  4. Use appropriate wait event when sending data in the apply worker.

  5. Allow the logical_replication_mode to be used on the subscriber.

  6. Rename GUC logical_decoding_mode to logical_replication_mode.

  7. Display the leader apply worker's PID for parallel apply workers.

  8. Improve the code to decide and process the apply action.

  9. Document the newly added wait events added by commit 216a784829.

  10. Perform apply of large transactions by parallel workers.

  11. Wake up a subscription's replication worker processes after DDL.

  12. Add copyright notices to meson files

  13. Better document logical replication parameters

  14. Add a common function to generate the origin name.

  15. Harmonize parameter names in storage and AM code.

  16. Avoid using list_length() to test for empty list.

  17. Improve two comments related to a boolean DefElem's value

  18. Fix partition table's REPLICA IDENTITY checking on the subscriber.

  19. Fix data inconsistency between publisher and subscriber.

  20. Fix cache look-up failures while applying changes in logical replication.