Feedback-ID: id4a34324:Fastmail
Date: Wed, 3 Dec 2025 11:03:40 -0500
From: Andres Freund <andres@anarazel.de>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Matthias van de Meent <boekewurm+postgres@gmail.com>,
	pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>,
	Heikki Linnakangas <hlinnaka@iki.fi>, 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 2025-12-02 19:47:35 -0500, 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)
> 
> - Tried to address Melanie's feedback, with some exceptions (some noted below,
>   but I also need to make another pass through the reviews)
> 
> - re-implemented AssertNotCatalogBufferLock() in the new world
> 
> - Substantially expanded comments around setting hint bits (in buffer/README,
>   heapam_visibility.c and bufmgr.c)
> 
> - split out the change to fsm_vacuum_page() to start to lock the page into is
>   own commit
> 
> - reordered patch series so that smaller changes are before the 64bit-state
>   and "Implement buffer content locks independently of" commits, so they can
>   be committed while we finish cleaning the later changes
> 
> - I didn't invest much in cleaning up the later patches ("Don't copy pages
>   while writing out" and "Make UnlockReleaseBuffer() more efficient") yet,
>   wanted to focus on the earlier patches first
> 
> 
> Todo:
> 
> - still need to rename ResOwnerReleaseBufferPin(). Wondering about what to
>   rename ResourceOwnerDesc.name to. "buffer ownership" maybe? Not great...
> 
> - gistkillitems() complaint by Melanie
> 
> - amortize vs batch vs SetHintBits comment + SHB_* names
> 
> - for the next version I'll remove the BATCHMVCC_FEWER_ARGS conditionals from
>   0010. I don't love needing BatchMVCCState but I don't really see an
>   alternative, the performance difference is pretty persistent.
> 
> 
> Questions:
> - ForEachLWLockHeldByMe() and LWLockDisown() aren't used anymore, should we
>   remove them?

I'm planning to work on committing 0001, 0002, 0003, 0008 soon-ish, unless
somebody sees a reason to hold off on that.  After that I think 0005, 0006
would be next.  I think 0004 is a clear improvement, but nobody has looked at
it yet...

For 0007, I wished ConditionalLockBuffer() accepted the lock level, there's no
point in waiting for the lock in the use case. I'm on the fence about whether
it's worth changing the ~12 users of ConditionalLockBuffer()...

Greetings,

Andres Freund