Re: Force streaming every change in logical decoding

On Tue, Dec 13, 2022 at 2:33 PM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> On Tue, Dec 6, 2022 at 5:23 PM shiy(dot)fnst(at)fujitsu(dot)com
> <shiy(dot)fnst(at)fujitsu(dot)com> wrote:
> >
> > Hi hackers,
> >
> > In logical decoding, when logical_decoding_work_mem is exceeded, the changes are
> > sent to output plugin in streaming mode. But there is a restriction that the
> > minimum value of logical_decoding_work_mem is 64kB. I tried to add a GUC to
> > allow sending every change to output plugin without waiting until
> > logical_decoding_work_mem is exceeded.
> >
> > This helps to test streaming mode. For example, to test "Avoid streaming the
> > transaction which are skipped" [1], it needs many XLOG_XACT_INVALIDATIONS
> > messages. With the new option, it can be tested with fewer changes and in less
> > time. Also, this new option helps to test more scenarios for "Perform streaming
> > logical transactions by background workers" [2].
> >
> > [1] https://www.postgresql.org/message-id/CAFiTN-tHK=7LzfrPs8fbT2ksrOJGQbzywcgXst2bM9-rJJAAUg@mail.gmail.com
> > [2] https://www.postgresql.org/message-id/flat/CAA4eK1%2BwyN6zpaHUkCLorEWNx75MG0xhMwcFhvjqm2KURZEAGw%40mail.gmail.com
> >
>
> Hi, I've been doing some testing that makes use of this new developer
> GUC `force_stream_mode`.
>
> IIUC this GUC is used by the walsender during the logic of the
> ReorderBufferCheckMemoryLimit(). Also, AFAIK the only way that the
> walsender is going to know this GUC value is by inheritance from the
> parent publisher at the time the walsender process gets launched.
>
> I may be overthinking this. Isn't there potential for this to become
> quite confusing depending on the timing of when this GUC is modified?
>
> E.g.1 When the walsender is launched, it will use whatever is the
> current value of this GUC.
> E.g.2 But if the GUC is changed after the walsender is already
> launched, then that will have no effect on the already running
> walsender.
>
> Is that understanding correct?
>

I think I was mistaken above. It looks like even the already-launched
walsender gets the updated GUC value via a SIGHUP on the parent
publisher.

2022-12-13 16:31:33.453 AEDT [1902] LOG: received SIGHUP, reloading
configuration files
2022-12-13 16:31:33.455 AEDT [1902] LOG: parameter
"force_stream_mode" changed to "true"

Sorry for the noise.

------
Kind Regards,
Peter Smith.
Fujitsu Australia