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Date: Thu, 13 Nov 2025 07:28:03 +0100
From: "Joel Jacobson" <joel@compiler.org>
To: "Arseniy Mukhin" <arseniy.mukhin.dev@gmail.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
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Subject: Re: Optimize LISTEN/NOTIFY
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Precedence: bulk

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 =
> 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) = 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_OFFSET(pos),
+                        (long long) QUEUE_POS_PAGE(queueHeadAfterWrite), QUEUE_POS_OFFSET(queueHeadAfterWrite));
+
                QUEUE_BACKEND_POS(i) = 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 between
queueHeadBeforeWrite and queueHeadAfterWrite.

/Joel