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

On 02/12/2025 15:20, Andres Freund wrote:
> On 2025-12-02 10:01:06 +0200, Heikki Linnakangas wrote:
>> But speaking of that: why do we not allow two processes to concurrently set
>> hint bits on a page anymore?
>
> It'd make the locking a lot more complicated without much of a benefit.
>
> The new share-exclusive lock mode only requires one additional bit of lock
> state, for the single allowed holder. If we wanted a new lockmode that
> prevented the page from being written out concurrently, but could be held
> multiple times, we'd need at least MAX_BACKENDS bits for the lock level
> allowing hint bits to be set and another lock level to acquire while writing
> out the buffer.
>
> At the same time, there seems to be little benefit in setting hint bits on a
> page concurrently. A very common case is that the same hint bit(s) would be
> set by multiple backends, we don't gain anything from that. And in the cases
> where hint bits were intended to be set for different tuples, the window in
> which that is not allowed is very narrow, and the cost of not setting right in
> that moment is pretty small and the cost of not setting the hint bit right
> then and there isn't high.
>
> Makes sense?

Yep, makes sense. Would be good to put that in a comment somewhere, and
in the commit message. "We could allow multiple backends to set hint
bits concurrently, but it'd make the lock implementation more complicated"

- Heikki