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

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
Cc: Amit Kapila <amit.kapila16@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>, Peter Smith <smithpb2250@gmail.com>
Date: 2022-10-25T03:08:10Z
Lists: pgsql-hackers 
On Fri, Oct 21, 2022 at 6:32 PM houzj.fnst@fujitsu.com
<houzj.fnst@fujitsu.com> wrote:
>
> On Thursday, October 20, 2022 5:49 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > On Thu, Oct 20, 2022 at 2:08 PM Peter Smith <smithpb2250@gmail.com>
> > wrote:
> > >
> > > 7. get_transaction_apply_action
> > >
> > > > 12. get_transaction_apply_action
> > > >
> > > > I still felt like there should be some tablesync checks/comments in
> > > > this function, just for sanity, even if it works as-is now.
> > > >
> > > > For example, are you saying ([3] #22b) that there might be rare
> > > > cases where a Tablesync would call to parallel_apply_find_worker?
> > > > That seems strange, given that "for streaming transactions that are
> > > > being applied in the parallel ... we disallow applying changes on a
> > > > table that is not in the READY state".
> > > >
> > > > ------
> > >
> > > Houz wrote [2] -
> > >
> > > I think because we won't try to start parallel apply worker in table
> > > sync worker(see the check in parallel_apply_can_start()), so we won't
> > > find any worker in parallel_apply_find_worker() which means
> > > get_transaction_apply_action will return TRANS_LEADER_SERIALIZE. And
> > > get_transaction_apply_action is a function which can be invoked for
> > > all kinds of workers(same is true for all apply_handle_xxx functions),
> > > so not sure if table sync check/comment is necessary.
> > >
> > > ~
> > >
> > > Sure, and I believe you when you say it all works OK - but IMO there
> > > is something still not quite right with this current code. For
> > > example,
> > >
> > > e.g.1 the functional will return TRANS_LEADER_SERIALIZE for Tablesync
> > > worker, and yet the comment for TRANS_LEADER_SERIALIZE says "means
> > > that we are in the leader apply worker" (except we are not)
> > >
> > > e.g.2 we know for a fact that Tablesync workers cannot start their own
> > > parallel apply workers, so then why do we even let the Tablesync
> > > worker make a call to parallel_apply_find_worker() looking for
> > > something we know will not be found?
> > >
> >
> > I don't see much benefit in adding an additional check for tablesync workers
> > here. It will unnecessarily make this part of the code look bit ugly.
>
> Thanks for the review, here is the new version patch set which addressed Peter[1]
> and Kuroda-san[2]'s comments.

I've started to review this patch. I tested v40-0001 patch and have
one question:

IIUC even when most of the changes in the transaction are filtered out
in pgoutput (eg., by relation filter or row filter), the walsender
sends STREAM_START. This means that the subscriber could end up
launching parallel apply workers also for almost empty (and streamed)
transactions. For example, I created three subscriptions each of which
subscribes to a different table. When I loaded a large amount of data
into one table, all three (leader) apply workers received START_STREAM
and launched their parallel apply workers. However, two of them
finished without applying any data. I think this behaviour looks
problematic since it wastes workers and rather decreases the apply
performance if the changes are not large. Is it worth considering a
way to delay launching a parallel apply worker until we find out the
amount of changes is actually large? For example, the leader worker
writes the streamed changes to files as usual and launches a parallel
worker if the amount of changes exceeds a threshold or the leader
receives the second segment. After that, the leader worker switches to
send the streamed changes to parallel workers via shm_mq instead of
files.

Regards,

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

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.