From: Joel Jacobson <joel@compiler.org>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async queue
Date: Wed, 29 Oct 2025 18:13:16 +0100
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On Wed, Oct 29, 2025, at 17:48, Tom Lane wrote:
> However ... that won't actually work, the reason being that
> asyncQueueProcessPageEntries() doesn't work directly from an SLRU page
> but from a local copy. Even if it were to modify the state of that
> copy, no other backend would see the effects.
>
> The reason it's like that is stated in the comments:
>
> * The current page must have been fetched into page_buffer from shared
> * memory. (We could access the page right in shared memory, but that
> * would imply holding the SLRU bank lock throughout this routine.)
>
> The patch proposed here likewise appears to involve holding an SLRU
> bank lock throughout what could be a significant number of
> TransactionIdDidCommit tests. That seems like it could result in a
> pretty bad "burp" in NOTIFY throughput. That problem is ameliorated
> by only doing it when VACUUM is trying to advance datfrozenxid, but
> still I wonder if we can't find a less concurrency-unfriendly answer.
...
> All of this is a problem mainly because of the presumption that
> holding an SLRU bank lock for a long time is bad. I wonder how
> dangerous that really is.
Ops. Sounds scary.
I don't know if others have looked at the v12-vacuum_notify_queue_cleanup
approach; if it's bad, it would be helpful to understand why.
/Joel
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To: pgsql-hackers@postgresql.org
Cc: joel@compiler.org, tgl@sss.pgh.pa.us, hlinnaka@iki.fi
Subject: Re: LISTEN/NOTIFY bug: VACUUM sets frozenxid past a xid in async queue
In-Reply-To: <9d93c1ee-5225-44ee-822f-e097190f1ac2@app.fastmail.com>
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