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

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "kuroda.hayato@fujitsu.com" <kuroda.hayato@fujitsu.com>
To: 'Amit Kapila' <amit.kapila16@gmail.com>
Cc: "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>, Dilip Kumar <dilipbalaut@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-09-22T11:20:08Z
Lists: pgsql-hackers 
Hi Amit,

> > I checked other flags that are set by signal handlers, their datatype seemed to
> be sig_atomic_t.
> > Is there any reasons that you use normal bool? It should be changed if not.
> >
> 
> It follows the logic similar to ParallelMessagePending. Do you see any
> problem with it?

Hmm, one consideration is:
what will happen if the signal handler HandleParallelApplyMessageInterrupt() is fired during "ParallelApplyMessagePending = false;"?
IIUC sig_atomic_t has been needed to avoid writing to same data at the same time.

According to C99 standard(I grepped draft version [1]), the behavior seems to be undefined if the signal handler
attaches to not "volatile sig_atomic_t" data.
...But I'm not sure whether this is really problematic in the current system, sorry...

```
If the signal occurs other than as the result of calling the abort or raise function,
the behavior is undefined if the signal handler refers to any object with static storage duration other than by assigning a value to an object declared as volatile sig_atomic_t,
or the signal handler calls any function in the standard library other than the abort function,
the _Exit function, or the signal function with the first argument equal to the signal number corresponding to the signal that caused the invocation of the handler.
```

> > a.
> > I was not sure when the cell should be cleaned. Currently we clean up
> ParallelApplyWorkersList() only in the parallel_apply_start_worker(),
> > but we have chances to remove such a cell like HandleParallelApplyMessages()
> or HandleParallelApplyMessage(). How do you think?
> >
> 
> Note that HandleParallelApply* are invoked during interrupt handling,
> so it may not be advisable to remove it there.
> 
> >
> > 12. ConfigureNamesInt
> >
> > ```
> > +       {
> > +               {"max_parallel_apply_workers_per_subscription",
> > +                       PGC_SIGHUP,
> > +                       REPLICATION_SUBSCRIBERS,
> > +                       gettext_noop("Maximum number of parallel apply
> workers per subscription."),
> > +                       NULL,
> > +               },
> > +               &max_parallel_apply_workers_per_subscription,
> > +               2, 0, MAX_BACKENDS,
> > +               NULL, NULL, NULL
> > +       },
> > ```
> >
> > This parameter can be changed by pg_ctl reload, so the following corner case
> may be occurred.
> > Should we add a assign hook to handle this? Or, can we ignore it?
> >
> 
> I think we can ignore this as it will eventually start respecting the threshold.

Both of you said are reasonable for me.

[1]: https://www.open-std.org/JTC1/SC22/WG14/www/docs/n1256.pdf

Best Regards,
Hayato Kuroda
FUJITSU LIMITED

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.