Re: Buffer locking is special (hints, checksums, AIO writes)

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From: Andres Freund <[email protected]>
To: Chao Li <[email protected]>
Cc: Kirill Reshke <[email protected]>
Cc: Heikki Linnakangas <[email protected]>
Cc: Melanie Plageman <[email protected]>
Cc: Matthias van de Meent <[email protected]>
Cc: [email protected]
Cc: Thomas Munro <[email protected]>
Cc: Noah Misch <[email protected]>
Cc: Robert Haas <[email protected]>
Cc: Michael Paquier <[email protected]>
Subject: Re: Buffer locking is special (hints, checksums, AIO writes)
Date: Wed, 14 Jan 2026 18:37:54 -0500
Message-ID: <cj5mcjdpucvw4a54hehslr3ctukavrbnxltvuzzhqnimvpju5e@cy3g3mnsefwz> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
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	<[email protected]>

Hi,

On 2026-01-15 07:20:27 +0800, Chao Li wrote:
> > On Jan 15, 2026, at 00:30, Andres Freund <[email protected]> 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:
> >> 
> >> 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
> >> ```
> >> 
> >> 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’s true, then I think we can
> >> remove the statements about lifting limitation.
> > 
> > I'm not following - there's plenty space for more bits if we need that:
> > 
> > * 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
> > 
> > That's 54 bits in total. Which part is in the lower and which in the upper
> > 32bit isn't relevant for anything afaict?
> 
> 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.

A significant portion != 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).


> Also, there is an assert that concretes the impression:
> ```
> StaticAssertDecl(BUF_REFCOUNT_BITS + BUF_USAGECOUNT_BITS + BUF_FLAG_BITS == 32,
>        "parts of buffer state space need to equal 32");
> ```

You can see that being relaxed in the subsequent commit, when we start to use
more bits.


Greetings,

Andres Freund

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