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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: Amit Kapila <amit.kapila16@gmail.com>
Cc: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-12-15T05:21:11Z
Lists: pgsql-hackers 
On Wed, Dec 14, 2022 at 3:48 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Wed, Dec 14, 2022 at 9:50 AM houzj.fnst@fujitsu.com
> <houzj.fnst@fujitsu.com> wrote:
> >
> > On Tuesday, December 13, 2022 11:25 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > Here are comments on v59 0001, 0002 patches:
> >
> > Thanks for the comments!
> >
> > > +void
> > > +pa_increment_stream_block(ParallelApplyWorkerShared *wshared) {
> > > +        while (1)
> > > +        {
> > > +                SpinLockAcquire(&wshared->mutex);
> > > +
> > > +                /*
> > > +                 * Don't try to increment the count if the parallel
> > > apply worker is
> > > +                 * taking the stream lock. Otherwise, there would be
> > > a race condition
> > > +                 * that the parallel apply worker checks there is no
> > > pending streaming
> > > +                 * block and before it actually starts waiting on a
> > > lock, the leader
> > > +                 * sends another streaming block and take the stream
> > > lock again. In
> > > +                 * this case, the parallel apply worker will start
> > > waiting for the next
> > > +                 * streaming block whereas there is actually a
> > > pending streaming block
> > > +                 * available.
> > > +                 */
> > > +                if (!wshared->pa_wait_for_stream)
> > > +                {
> > > +                        wshared->pending_stream_count++;
> > > +                        SpinLockRelease(&wshared->mutex);
> > > +                        break;
> > > +                }
> > > +
> > > +                SpinLockRelease(&wshared->mutex);
> > > +        }
> > > +}
> > >
> > > I think we should add an assertion to check if we don't hold the stream lock.
> > >
> > > I think that waiting for pa_wait_for_stream to be false in a busy loop is not a
> > > good idea. It's not interruptible and there is not guarantee that we can break
> > > from this loop in a short time. For instance, if PA executes
> > > pa_decr_and_wait_stream_block() a bit earlier than LA executes
> > > pa_increment_stream_block(), LA has to wait for PA to acquire and release the
> > > stream lock in a busy loop. It should not be long in normal cases but the
> > > duration LA needs to wait for PA depends on PA, which could be long. Also
> > > what if PA raises an error in
> > > pa_lock_stream() due to some reasons? I think LA won't be able to detect the
> > > failure.
> > >
> > > I think we should at least make it interruptible and maybe need to add some
> > > sleep. Or perhaps we can use the condition variable for this case.
> >
>
> Or we can leave this while (true) logic altogether for the first
> version and have a comment to explain this race. Anyway, after
> restarting, it will probably be solved. We can always change this part
> of the code later if this really turns out to be problematic.
>

+1. Thank you Hou-san for adding this comment in the latest version (v61) patch!

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.