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

On Wed, Dec 21, 2022 at 2:32 PM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Wed, Dec 21, 2022 9:07 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
> > FYI - applying v63-0001 using the latest master does not work.
> >
> > git apply ../patches_misc/v63-0001-Perform-streaming-logical-transactions-by-
> > parall.patch
> > error: patch failed: src/backend/replication/logical/meson.build:1
> > error: src/backend/replication/logical/meson.build: patch does not apply
> >
> > Looks like a recent commit [1] to add copyrights broke the patch
>
> Thanks for your reminder.
> Rebased the patch set.
>
> Attach the new patch set which also includes some
> cosmetic comment changes.
>

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.

---
+ if (pg_atomic_read_u32(&MyParallelShared->pending_stream_count) == 0)
+ {
+ if (pa_has_spooled_message_pending())
+ return;
+
+ elog(ERROR, "invalid pending streaming block number");
+ }

I think it's helpful if the error message shows the invalid block number.

---
On Wed, Dec 7, 2022 at 10:13 PM houzj(dot)fnst(at)fujitsu(dot)com
<houzj(dot)fnst(at)fujitsu(dot)com> wrote:
>
> On Wednesday, December 7, 2022 7:51 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > ---
> > If a value of max_parallel_apply_workers_per_subscription is not
> > sufficient, we get the LOG "out of parallel apply workers" every time
> > when the apply worker doesn't launch a worker. But do we really need
> > this log? It seems not consistent with
> > max_sync_workers_per_subscription behavior. I think we can check if
> > the number of running parallel workers is less than
> > max_parallel_apply_workers_per_subscription before calling
> > logicalrep_worker_launch(). What do you think?
>
> (Sorry, I missed this comment in last email)
>
> I personally feel giving a hint might help user to realize that the
> max_parallel_applyxxx is not enough for the current workload and then they can
> adjust the parameter. Otherwise, user might have an easy way to check if more
> workers are needed.
>

Sorry, I missed this comment.

I think the number of concurrent transactions on the publisher could
be several hundreds, and the number of streamed transactions among
them could be several tens. I agree setting
max_parallel_apply_workers_per_subscription to a value high enough is
ideal but I'm not sure we want to inform users immediately that the
setting value is not enough. I think that with the default value
(i.e., 2), it will not be enough for many systems and the server logs
could be flood with the LOG "out of parallel apply workers". If we
want to give a hint to users, we can probably show the statistics on
pg_stat_subscription_stats view such as the number of streamed
transactions that are handled by the leader and parallel workers.

Regards,

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