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

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: "houzj.fnst@fujitsu.com" <houzj.fnst@fujitsu.com>
Cc: "wangw.fnst@fujitsu.com" <wangw.fnst@fujitsu.com>, Peter Smith <smithpb2250@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "shiy.fnst@fujitsu.com" <shiy.fnst@fujitsu.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2022-08-09T05:39:27Z
Lists: pgsql-hackers 
On Mon, Aug 8, 2022 at 11:41 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Mon, Aug 8, 2022 at 10:18 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > > Based on above, we plan to first introduce the patch to perform streaming
> > > logical transactions by background workers, and then introduce parallel apply
> > > normal transaction which design is different and need some additional handling.
> >
> > Yeah I think that makes sense.  Since the streamed transactions are
> > sent to standby interleaved so we can take advantage of parallelism
> > and along with that we can also avoid the I/O so that will also
> > speedup.
>
> Some review comments on the latest version of the patch.
>
> 1.
> +/* Queue size of DSM, 16 MB for now. */
> +#define DSM_QUEUE_SIZE    160000000
>
> Why don't we directly use 16 *1024 * 1024, that would be exactly 16 MB
> so it will match with comments and also it would be more readable.
>
> 2.
> +/*
> + * There are three fields in message: start_lsn, end_lsn and send_time. Because
> + * we have updated these statistics in apply worker, we could ignore these
> + * fields in apply background worker. (see function LogicalRepApplyLoop)
> + */
> +#define SIZE_STATS_MESSAGE (3 * sizeof(uint64))
>
> Instead of assuming you have 3 uint64 why don't directly add 2 *
> sizeof(XLogRecPtr) + sizeof(TimestampTz) so that if this data type
> ever changes
> we don't need to track that we will have to change this as well.
>
> 3.
> +/*
> + * Entry for a hash table we use to map from xid to our apply background worker
> + * state.
> + */
> +typedef struct ApplyBgworkerEntry
> +{
> +    TransactionId xid;
> +    ApplyBgworkerState *wstate;
> +} ApplyBgworkerEntry;
>
> Mention in the comment of the structure or for the member that xid is
> the key of the hash.  Refer to other such structures for the
> reference.
>
> I am doing a more detailed review but this is what I got so far.

Some more comments

+    /*
+     * Exit if any relation is not in the READY state and if any worker is
+     * handling the streaming transaction at the same time. Because for
+     * streaming transactions that is being applied in apply background
+     * worker, we cannot decide whether to apply the change for a relation
+     * that is not in the READY state (see should_apply_changes_for_rel) as we
+     * won't know remote_final_lsn by that time.
+     */
+    if (list_length(ApplyBgworkersFreeList) !=
list_length(ApplyBgworkersList) &&
+        !AllTablesyncsReady())
+    {
+        ereport(LOG,
+                (errmsg("logical replication apply workers for
subscription \"%s\" will restart",
+                        MySubscription->name),
+                 errdetail("Cannot handle streamed replication
transaction by apply "
+                           "background workers until all tables are
synchronized")));
+
+        proc_exit(0);
+    }

How this situation can occur? I mean while starting a background
worker itself we can check whether all tables are sync ready or not
right?

+    /* Check the status of apply background worker if any. */
+    apply_bgworker_check_status();
+

What is the need to checking each worker status on every commit?  I
mean if there are a lot of small transactions along with some
steamiing transactions
then it will affect the apply performance for those small transactions?


-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.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.