Re: allow changing autovacuum_max_workers without restarting

On Wed, Apr 10, 2024 at 04:23:44PM -0500, Nathan Bossart wrote:
> The attached proof-of-concept patch demonstrates what I have in mind.
> Instead of trying to dynamically change the global process table, etc., I'm
> proposing that we introduce a new GUC that sets the effective maximum
> number of autovacuum workers that can be started at any time.  This means
> there would be two GUCs for the number of autovacuum workers: one for the
> number of slots reserved for autovacuum workers, and another that restricts
> the number of those slots that can be used.  The former would continue to
> require a restart to change its value, and users would typically want to
> set it relatively high.  The latter could be changed at any time and would
> allow for raising or lowering the maximum number of active autovacuum
> workers, up to the limit set by the other parameter.
> 
> The proof-of-concept patch keeps autovacuum_max_workers as the maximum
> number of slots to reserve for workers, but I think we should instead
> rename this parameter to something else and then reintroduce
> autovacuum_max_workers as the new parameter that can be adjusted without
> restarting.  That way, autovacuum_max_workers continues to work much the
> same way as in previous versions.

When I thought about this, I considered proposing to add a new GUC for
"autovacuum_policy_workers".

autovacuum_max_workers would be the same as before, requiring a restart
to change.  The policy GUC would be the soft limit, changable at runtime
up to the hard limit of autovacuum_max_workers (or maybe any policy
value exceeding autovacuum_max_workers would be ignored).

We'd probably change autovacuum_max_workers to default to a higher value
(8, or 32 as in your patch), and have autovacuum_max_workers default to
3, for consistency with historic behavior.  Maybe
autovacuum_policy_workers=-1 would mean to use all workers.

There's the existing idea to change autovacuum thresholds during the
busy period of the day vs. off hours.  This would allow something
similar with nworkers rather than thresholds: if the goal were to reduce
the resource use of vacuum, the admin could set max_workers=8, with
policy_workers=2 during the busy period.

-- 
Justin