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Date: Wed, 12 Nov 2025 21:53:17 +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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 <https://www.postgresql.org/message-id/6070e73e-9a1d-48e2-bd5f-d69623dc8e1f%40app.fastmail.com>
Precedence: bulk

On Wed, Nov 12, 2025, at 21:37, Joel Jacobson wrote:
> Sequence of events:
>
> 1. In the notifier, PreCommit_Notify calls asyncQueueAddEntries,
> which updates QUEUE_HEAD when the page is full,
> (and sets queueHeadAfterWrite to this value).
>
> 2. At this time, a listener wakes up and asyncQueueAddEntries

Correction:
I meant "asyncQueueReadAllNotifications" here, not "asyncQueueAddEntries".

> reads the current QUEUE_HEAD value and stores it
> in its local `head` variable, and starts reading up to this pos.
>
> 3. In the notifier, PreCommit_Notify calls asyncQueueAddEntries
> the second time, which updates QUEUE_HEAD,
> and sets queueHeadAfterWrite to the final value
> before returning.
>
> For this reason, I think the listener could actually stop
> in between queueHeadBeforeWrite and queueHeadAfterWrite,
> since it's local `head` variable could get the intermediary
> QUEUE_HEAD value, when a page is full.

/Joel