Re: Track the amount of time waiting due to cost_delay

Hi,

On Mon, Jun 10, 2024 at 08:12:46PM +0000, Imseih (AWS), Sami wrote:
> >> This sounds like useful information to me.
>
> > Thanks for looking at it!
>
> The VacuumDelay is the only visibility available to
> gauge the cost_delay. Having this information
> advertised by pg_stat_progress_vacuum as is being proposed
> is much better.

Thanks for looking at it!

> However, I also think that the
> "number of times" the vacuum went into delay will be needed
> as well. Both values will be useful to tune cost_delay and cost_limit.

Yeah, I think that's a good idea. With v1 one could figure out how many times
the delay has been triggered but that does not work anymore if: 1) cost_delay
changed during the vacuum duration or 2) the patch changes the way time_delayed
is measured/reported (means get the actual wait time and not the theoritical
time as v1 does).

>
> It may also make sense to accumulate the total_time in delay
> and the number of times delayed in a cumulative statistics [0]
> view to allow a user to trend this information overtime.
> I don't think this info fits in any of the existing views, i.e.
> pg_stat_database, so maybe a new view for cumulative
> vacuum stats may be needed. This is likely a separate
> discussion, but calling it out here.

+1

> >> 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?
>
> > Yeah, one could use a query such as:
>
> > select p.*, now() - a.xact_start as duration from pg_stat_progress_vacuum p JOIN pg_stat_activity a using (pid)
>
> Maybe all progress views should just expose the "beentry->st_activity_start_timestamp "
> to let the user know when the current operation began.

Yeah maybe, I think this is likely a separate discussion too, thoughts?

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com