Re: [HACKERS] Block level parallel vacuum

On Sat, Oct 5, 2019 at 4:36 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> Few more comments
> ----------------------------
>
> 1.
> +static int
> +compute_parallel_workers(Relation onerel, int nrequested, int nindexes)
> +{
> + int parallel_workers;
> + bool leaderparticipates = true;
>
> Seems like this function is not using onerel parameter so we can remove this.
>

Fixed.

>
> 2.
> +
> + /* Estimate size for dead tuples -- PARALLEL_VACUUM_KEY_DEAD_TUPLES */
> + maxtuples = compute_max_dead_tuples(nblocks, true);
> + est_deadtuples = MAXALIGN(add_size(SizeOfLVDeadTuples,
> + mul_size(sizeof(ItemPointerData), maxtuples)));
> + shm_toc_estimate_chunk(&pcxt->estimator, est_deadtuples);
> + shm_toc_estimate_keys(&pcxt->estimator, 1);
> +
> + /* Finally, estimate VACUUM_KEY_QUERY_TEXT space */
> + querylen = strlen(debug_query_string);
>
> for consistency with other comments change
> VACUUM_KEY_QUERY_TEXT to PARALLEL_VACUUM_KEY_QUERY_TEXT
>

Fixed.

>
> 3.
> @@ -2888,6 +2888,8 @@ table_recheck_autovac(Oid relid, HTAB *table_toast_map,
> (!wraparound ? VACOPT_SKIP_LOCKED : 0);
> tab->at_params.index_cleanup = VACOPT_TERNARY_DEFAULT;
> tab->at_params.truncate = VACOPT_TERNARY_DEFAULT;
> + /* parallel lazy vacuum is not supported for autovacuum */
> + tab->at_params.nworkers = -1;
>
> What is the reason for the same? Can we explain in the comments?

I think it's just that we don't want to support parallel auto vacuum
because it can consume more CPU resources in spite of background job,
which might be an unexpected behavior of autovacuum. I've changed the
comment.

Regards,

--
Masahiko Sawada