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
