Re: [HACKERS] Block level parallel vacuum

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On Thu, Dec 5, 2019 at 12:27 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> I agree that there is no point is first to spawn more workers to get
> the work done faster and later throttle them. Basically, that will
> lose the whole purpose of running it in parallel.

Right. I mean if you throttle something that would have otherwise
kept 3 workers running full blast back to the point where it uses the
equivalent of 2.5 workers, that might make sense. It's a little
marginal, maybe, but sure. But once you throttle it back to <= 2
workers, you're just wasting resources.

I think my concern here is ultimately more about usability than
whether or not we allow throttling. I agree that there are some
possible cases where throttling a parallel vacuum is useful, so I
guess we should support it. But I also think there's a real risk of
people not realizing that throttling is happening and then being sad
because they used parallel VACUUM and it was still slow. I think we
should document explicitly that parallel VACUUM is still potentially
throttled and that you should consider setting the cost delay to a
higher value or 0 before using it.

We might even want to add a FAST option (or similar) to VACUUM that
makes it behave as if vacuum_cost_delay = 0, and add something to the
examples section for VACUUM that suggests e.g.

VACUUM (PARALLEL 3, FAST) my_big_table
Vacuum my_big_table with 3 workers and with resource throttling
disabled for maximum performance.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company