Message-ID: <8926bc6a-cab3-4edf-acf2-02b4cbeb7c9b@iki.fi>
Date: Tue, 2 Dec 2025 15:38:34 +0200
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Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
To: Andres Freund <andres@anarazel.de>
Cc: Melanie Plageman <melanieplageman@gmail.com>,
 Matthias van de Meent <boekewurm+postgres@gmail.com>,
 pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>,
 Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>,
 Michael Paquier <michael.paquier@gmail.com>
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From: Heikki Linnakangas <hlinnaka@iki.fi>
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On 02/12/2025 15:20, Andres Freund wrote:
> On 2025-12-02 10:01:06 +0200, Heikki Linnakangas wrote:
>> But speaking of that: why do we not allow two processes to concurrently set
>> hint bits on a page anymore?
> 
> It'd make the locking a lot more complicated without much of a benefit.
> 
> The new share-exclusive lock mode only requires one additional bit of lock
> state, for the single allowed holder. If we wanted a new lockmode that
> prevented the page from being written out concurrently, but could be held
> multiple times, we'd need at least MAX_BACKENDS bits for the lock level
> allowing hint bits to be set and another lock level to acquire while writing
> out the buffer.
> 
> At the same time, there seems to be little benefit in setting hint bits on a
> page concurrently. A very common case is that the same hint bit(s) would be
> set by multiple backends, we don't gain anything from that. And in the cases
> where hint bits were intended to be set for different tuples, the window in
> which that is not allowed is very narrow, and the cost of not setting right in
> that moment is pretty small and the cost of not setting the hint bit right
> then and there isn't high.
> 
> Makes sense?

Yep, makes sense. Would be good to put that in a comment somewhere, and 
in the commit message. "We could allow multiple backends to set hint 
bits concurrently, but it'd make the lock implementation more complicated"

- Heikki