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In-Reply-To: <0253b822-e8fd-4067-ab24-23493c115a2a@app.fastmail.com>
From: Arseniy Mukhin <arseniy.mukhin.dev@gmail.com>
Date: Thu, 13 Nov 2025 09:36:22 +0300
Message-ID: 
 <CAE7r3MJXH8Phf1s5KKvh5zUG4=jnzeM+EnAx=KakvPd_djjrtw@mail.gmail.com>
Subject: Re: Optimize LISTEN/NOTIFY
To: Joel Jacobson <joel@compiler.org>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
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 <https://www.postgresql.org/message-id/CAE7r3MJXH8Phf1s5KKvh5zUG4%3DjnzeM%2BEnAx%3DKakvPd_djjrtw%40mail.gmail.com>
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On Thu, Nov 13, 2025 at 9:28=E2=80=AFAM Joel Jacobson <joel@compiler.org> w=
rote:
>
> On Wed, Nov 12, 2025, at 17:57, Arseniy Mukhin wrote:
> > IIUC it's impossible for the listener to stop somewhere in between
> > queueHeadBeforeWrite and queueHeadAfterWrite. If the listener has
> > managed to read the first notification from the notifier, it means the
> > notifier transaction is complete and the listener should stop only
> > after reading all notifications (so we should always see pos =3D
> > queueHeadAfterWrite or further).
> >
> > So If I haven't missed anything, I think we can use QUEUE_POS_EQUAL as
> > direct advancement condition:
> >
> >     if (!QUEUE_BACKEND_IS_ADVANCING(i) && QUEUE_POS_EQUAL(pos,
> > queueHeadBeforeWrite))
> >     {
> >            QUEUE_BACKEND_POS(i) =3D queueHeadAfterWrite;
> >     }
>
> I added some logging just to test the hypothesis:
>
> @@ -2072,6 +2082,12 @@ SignalBackends(void)
>             {
>                 Assert(!QUEUE_POS_PRECEDES(pos, queueHeadBeforeWrite));
>
> +               if (!QUEUE_POS_EQUAL(pos, queueHeadBeforeWrite))
> +                   elog(LOG, "Direct advancement: PID %d from pos (%lld,=
%d) to queueHeadAfterWrite (%lld,%d)",
> +                        pid,
> +                        (long long) QUEUE_POS_PAGE(pos), QUEUE_POS_OFFSE=
T(pos),
> +                        (long long) QUEUE_POS_PAGE(queueHeadAfterWrite),=
 QUEUE_POS_OFFSET(queueHeadAfterWrite));
> +
>                 QUEUE_BACKEND_POS(i) =3D queueHeadAfterWrite;
>             }
>         }
>
> And I'm getting a lot of such log entries when benchmarking
> `./pg_async_notify_test --listeners 1 --notifiers 1 --channels 50`
>
> I think this confirms that listeners can actually stop somewhere in betwe=
en
> queueHeadBeforeWrite and queueHeadAfterWrite.


Ahh, yes, I think you are right. I missed that notifiers update the
head when they move to the next page. Thank you for the detailed
example and sorry for taking your time with it. I agree that
QUEUE_POS_PRECEDES(pos, queueHeadAfterWrite) is correct and covers
more cases where we can do direct advancement.


Best regards,
Arseniy Mukhin