Feedback-ID: id4a34324:Fastmail
Date: Thu, 18 Dec 2025 12:03:36 -0500
From: Andres Freund <andres@anarazel.de>
To: Heikki Linnakangas <hlinnaka@iki.fi>
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>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
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Hi,

On 2025-12-17 09:54:32 -0500, Andres Freund wrote:
> On 2025-12-17 11:25:50 +0200, Heikki Linnakangas wrote:
> > - LWLockWaitListLock() uses pg_atomic_read_u32() after spinning,
> > LockBufHdr() retries directly with pg_atomic_fetch_or_u32().
> 
> I think here LWLockWaitListLock() is likely right - but it seems like a change
> to LockBufHdr() that I would probably make in a separate commit?

FWIW, I couldn't come up with a scenario where it makes a performance
difference - exclusive content locks just aren't *that* frequent. And because
of that the wait list lock doesn't have similar contention as some non-content
lwlocks (like XidGenLock). The most extreme workload I could think of was
pgbench hammering a single sequence across many sessions. While the exclusive
locks show up in wait events, the buffer header spinlock itself doesn't..

So I'm inclined to not change anything about this for now.

Greetings,

Andres Freund