On 02/04/2026 19:13, Ashutosh Bapat wrote:
On Thu, Apr 2, 2026 at 7:44 PM Heikki Linnakangas <[email protected]> wrote:
I think we should highlight the change in default in the release notes
though. The users which use default configuration will notice an
increase in the memory. If they are using a custom value, they will
think of bumping it up. Can we give them some ballpark % by which they
should increase their max_locks_per_transaction? E.g. double the
number or something?
I don't think people who are using the defaults will notice. I'm worried
about the people who have set max_locks_per_transactions manually, and
now effectively get less lock space for the same setting. Yeah, doubling
the previous value is a good rule of thumb.
Users who have set max_locks_per_transaction to a non-default value
but have tuned their application to respect those limits are safe even
after this change, since their lock hash table never used wiggle room.
Those users who weren't careful to respect those limits will need to
bump their setting.
That's technically true, but in practice it's very hard for someone to
carefully tune the setting. It's difficult to know how many locks a
particular set of queries take. In practice what people do is they bump
up the setting if the get the "out of shared memory" error, until the
error goes away. If you do the tuning that way, it's quite possible that
you are relying the "wiggle room" without realizing it.
I think the release notes should "nudge" all the
users who use non-default max_locks_per_transaction to increase it if
they see "out of memory" errors. I don't think it should provide a
blanket advise to double their locks
How about:
"If you had previously set max_locks_per_transaction, you might need to
set it to a higher value in v19 to avoid "out of shared memory" errors.
If you are unsure what to set it to and don't mind the increased memory
usage, you can double the value to ensure that you can acquire at least
as many locks as before"
TODO: do some more calculations and testing of how exactly the doubling
rule works with different values. Is it guaranteed to be enough in all
cases?
- Heikki
