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Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
From: Chao Li <li.evan.chao@gmail.com>
In-Reply-To: <cj5mcjdpucvw4a54hehslr3ctukavrbnxltvuzzhqnimvpju5e@cy3g3mnsefwz>
Date: Thu, 15 Jan 2026 08:04:21 +0800
Cc: Kirill Reshke <reshkekirill@gmail.com>,
 Heikki Linnakangas <hlinnaka@iki.fi>,
 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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To: Andres Freund <andres@anarazel.de>
Archived-At: <https://www.postgresql.org/message-id/58821140-0182-4FF3-9A4F-5B168DDB9243%40gmail.com>
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> On Jan 15, 2026, at 07:37, Andres Freund <andres@anarazel.de> wrote:
>=20
> Hi,
>=20
> On 2026-01-15 07:20:27 +0800, Chao Li wrote:
>>> On Jan 15, 2026, at 00:30, Andres Freund <andres@anarazel.de> wrote:
>>> On 2026-01-14 11:41:19 +0800, Chao Li wrote:
>>>> Basically, code changes in 0003 is straightforward, just a couple =
of small comments:
>>>>=20
>>>> 1
>>>> ```
>>>> - * refcounts in buf_internals.h.  This limitation could be lifted =
by using a
>>>> - * 64bit state; but it's unlikely to be worthwhile as 2^18-1 =
backends exceed
>>>> - * currently realistic configurations. Even if that limitation =
were removed,
>>>> - * we still could not a) exceed 2^23-1 because inval.c stores the =
ProcNumber
>>>> - * as a 3-byte signed integer, b) INT_MAX/4 because some places =
compute
>>>> - * 4*MaxBackends without any overflow check.  We check that the =
configured
>>>> - * number of backends does not exceed MAX_BACKENDS in =
InitializeMaxBackends().
>>>> + * refcounts in buf_internals.h.  This limitation could be lifted, =
but it's
>>>> ```
>>>>=20
>>>> Before this patch, there was room for lifting the limitation. With =
this
>>>> patch, state is 64bit already, but the significant 32bit will be =
used for
>>>> buffer locking as stated in buf_internals.h, in other words, there =
is no
>>>> room for lifting the limitation now. If that=E2=80=99s true, then I =
think we can
>>>> remove the statements about lifting limitation.
>>>=20
>>> I'm not following - there's plenty space for more bits if we need =
that:
>>>=20
>>> * State of the buffer itself (in order):
>>> * - 18 bits refcount
>>> * - 4 bits usage count
>>> * - 12 bits of flags
>>> * - 18 bits share-lock count
>>> * - 1 bit share-exclusive locked
>>> * - 1 bit exclusive locked
>>>=20
>>> That's 54 bits in total. Which part is in the lower and which in the =
upper
>>> 32bit isn't relevant for anything afaict?
>>=20
>> Because I saw the comment in buf_internals.h:
>> ```
>> * NB: A future commit will use a significant portion of the remaining =
bits to
>> * implement buffer locking as part of the state variable.
>> ```
>> That seems to indicate all the significant 32 bits will be used for =
buffer locking.
>=20
> A significant portion !=3D all. As the above excerpt from the comment =
shows, the
> locking uses 20 bits. We could increase max backends by 5 bits without =
running
> out of bits (we'd need space both in the refcount bitspace as well as =
the
> share-lock bitspace).

Make sense. I think I misread the comment.

>=20
>=20
>> Also, there is an assert that concretes the impression:
>> ```
>> StaticAssertDecl(BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS + =
BUF_FLAG_BITS =3D=3D 32,
>>       "parts of buffer state space need to equal 32");
>> ```
>=20
> You can see that being relaxed in the subsequent commit, when we start =
to use
> more bits.
>=20

Sure. I plan to review 0003-0005 today. I believe I will get better =
understanding.

So, 0001 and 0002 LGTM now.

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/