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In-Reply-To: <js7t2fs6it2aljskgqzpqwj4uqc5geb4ztlcojsyh6bpzgaqew@nixo44lyx5vn>
From: Melanie Plageman <melanieplageman@gmail.com>
Date: Mon, 1 Dec 2025 15:41:07 -0500
Message-ID: 
 <CAAKRu_bFyKsGcoCznoT78sF9EYbVeWsXPfhdJnGLsHzoA01Sww@mail.gmail.com>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
To: Andres Freund <andres@anarazel.de>
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>
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On Mon, Dec 1, 2025 at 3:28=E2=80=AFPM Andres Freund <andres@anarazel.de> w=
rote:
>
> > I noticed that the BUF_STATE_GET_REFCOUNT and BUF_STATE_GET_USAGECOUNT
> > macros cast the return value to a uint32. We won't use the extra bits
> > but we did bother to keep the macro result sized to the field width
> > before so keeping it uint32 is probably more confusing now that state
> > is 64 bit.
>
> I can't really follow - why would we want to return a 64bit value if none=
 of
> the values ever can get anywhere near that big?

Well, usagecount could never have reached anything close to a uint32
either, so I thought that this cast was meant to match the datatype. I
found it rather confusing otherwise.

> > 0007
> > needs a commit message. overall seems fine though.
>
> This is what I've since written:
>
>     bufmgr: Add one-entry cache for private refcount
>
>     The private refcount entry for a buffer is often looked up repeatedly=
 for the
>     same buffer, e.g. to pin and then unpin a buffer. Benchmarking shows =
that it's
>     worthwhile to have a one-entry cache for that case. With that cache i=
n place,
>     it's worth splitting GetPrivateRefCountEntry() into a small inline
>     portion (for the cache hit case) and an out-of-line helper for the re=
st.
>
>     This is helpful for some workloads today, but becomes more important =
in an
>     upcoming patch that will utilize the private refcount infrastructure =
to also
>     store whether the buffer is currently locked, as that increases the r=
ate of
>     lookups substantially.

Sounds good based on what I recall of the patch.

- Melanie