Re: [HACKERS] Block level parallel vacuum

On Thu, 5 Dec 2019 at 19:54, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> [ Please trim excess quoted material from your replies. ]
>
> 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.
>

Please find some review comments for v35 patch set

1.
+ /* Return immediately when parallelism disabled */
+ if (max_parallel_maintenance_workers == 0)
+ return 0;
+
Here, we should add check of max_worker_processes because if
max_worker_processes is set as 0, then we can't launch any worker so
we should return from here.

2.
+ /* cap by max_parallel_maintenace_workers */
+ parallel_workers = Min(parallel_workers, max_parallel_maintenance_workers);
+
Here also, we should consider max_worker_processes to calculate
parallel_workers. (by default, max_worker_processes = 8)

Thanks and Regards
Mahendra Thalor
EnterpriseDB: http://www.enterprisedb.com