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From: Joel Jacobson <[email protected]>
To: Arseniy Mukhin <[email protected]>
Cc: pgsql-hackers <[email protected]>
Subject: Re: Optimize LISTEN/NOTIFY
Date: Wed, 12 Nov 2025 21:53:17 +0100
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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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

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