Feedback-ID: id4a34324:Fastmail
Date: Wed, 17 Dec 2025 09:54:32 -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 11:25:50 +0200, Heikki Linnakangas wrote:
> On 03/12/2025 02:47, Andres Freund wrote:
> > On 2025-11-25 11:54:00 -0500, Andres Freund wrote:
> > > Thanks a lot for that detailed review!  A few questions and comments, before I
> > > try to address the comments in the next version.
> > 
> > Here's that new new version, with the following changes
> > 
> > - Some more micro-optimizations, most importantly adding a commit that doesn't
> >    initialize the delay in LockBufHdr() unless needed. With those I don't see a
> >    consistent slowdown anymore (slight speedup on one workstation, slight
> >    slowdown on another, in an absurdly adverse workload)
> 
> +1
> 
> I'm comparing the patched LockBufHdr() with LWLockWaitListLock(), which does
> pretty much the same thing, and LWLockWaitListLock() already did the
> initialization of the delay that way. But there are some small differences:
> 
> - LockBufHdr() uses unlikely() in the initial attempt, LWLockWaitListLock()
> does not

I think we probably ought to do that in LWLockWaitListLock() too.


> - 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?


> Are there reasons for the differences, or is it just that they were
> developed separately and ended up looking slightly different?

I think it's just the latter...


Thanks for reviewing,

Andres Freund