Re: Track the amount of time waiting due to cost_delay

On Mon, Jun 10, 2024 at 06:05:13AM +0000, Bertrand Drouvot wrote:
> During the last pgconf.dev I attended Robert´s presentation about autovacuum and
> it made me remember of an idea I had some time ago: $SUBJECT

This sounds like useful information to me.  I wonder if we should also
surface the effective cost limit for each autovacuum worker.

> Currently one can change [autovacuum_]vacuum_cost_delay and
> [auto vacuum]vacuum_cost_limit but has no reliable way to measure the impact of
> the changes on the vacuum duration: one could observe the vacuum duration
> variation but the correlation to the changes is not accurate (as many others
> factors could impact the vacuum duration (load on the system, i/o latency,...)).

IIUC you'd need to get information from both pg_stat_progress_vacuum and
pg_stat_activity in order to know what percentage of time was being spent
in cost delay.  Is that how you'd expect for this to be used in practice?

>  		pgstat_report_wait_start(WAIT_EVENT_VACUUM_DELAY);
>  		pg_usleep(msec * 1000);
>  		pgstat_report_wait_end();
> +		/* Report the amount of time we slept */
> +		if (VacuumSharedCostBalance != NULL)
> +			pgstat_progress_parallel_incr_param(PROGRESS_VACUUM_TIME_DELAYED, msec);
> +		else
> +			pgstat_progress_incr_param(PROGRESS_VACUUM_TIME_DELAYED, msec);

Hm.  Should we measure the actual time spent sleeping, or is a rough
estimate good enough?  I believe pg_usleep() might return early (e.g., if
the process is signaled) or late, so this field could end up being
inaccurate, although probably not by much.  If we're okay with millisecond
granularity, my first instinct is that what you've proposed is fine, but I
figured I'd bring it up anyway.

-- 
nathan