Re: allow changing autovacuum_max_workers without restarting

Hi,

On 2024-06-03 13:52:29 -0500, Nathan Bossart wrote:
> Here is an updated patch that uses 256 as the upper limit.

I don't have time to read through the entire thread right now - it'd be good
for the commit message of a patch like this to include justification for why
it's ok to make such a change. Even before actually committing it, so
reviewers have an easier time catching up.

Why do we think that increasing the number of PGPROC slots, heavyweight locks
etc by 256 isn't going to cause issues? That's not an insubstantial amount of
memory to dedicate to something that will practically never be used.

ISTM that at the very least we ought to exclude the reserved slots from the
computation of things like the number of locks resulting from
max_locks_per_transaction. It's very common to increase
max_locks_per_transaction substantially, adding ~250 to the multiplier can be
a good amount of memory. And AV workers should never need a meaningful number.

Increasing e.g. the size of the heavyweight lock table has consequences
besides the increase in memory usage, the size increase can make it less
likely for the table to fit largely into L3, thus decreasing performance.

Greetings,

Andres Freund