Re: [HACKERS] Block level parallel vacuum

On Wed, Oct 2, 2019 at 7:29 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
wrote:

> On Sat, Sep 21, 2019 at 9:31 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
> wrote:
> > *
> > In function compute_parallel_workers, don't we want to cap the number
> > of workers based on maintenance_work_mem as we do in
> > plan_create_index_workers?
> >
> > The basic point is how do we want to treat maintenance_work_mem for
> > this feature. Do we want all workers to use at max the
> > maintenance_work_mem or each worker is allowed to use
> > maintenance_work_mem? I would prefer earlier unless we have good
> > reason to follow the later strategy.
> >
> > Accordingly, we might need to update the below paragraph in docs:
> > "Note that parallel utility commands should not consume substantially
> > more memory than equivalent non-parallel operations. This strategy
> > differs from that of parallel query, where resource limits generally
> > apply per worker process. Parallel utility commands treat the
> > resource limit <varname>maintenance_work_mem</varname> as a limit to
> > be applied to the entire utility command, regardless of the number of
> > parallel worker processes."
>
> I'd also prefer to use maintenance_work_mem at max during parallel
> vacuum regardless of the number of parallel workers. This is current
> implementation. In lazy vacuum the maintenance_work_mem is used to
> record itempointer of dead tuples. This is done by leader process and
> worker processes just refers them for vacuuming dead index tuples.
> Even if user sets a small amount of maintenance_work_mem the parallel
> vacuum would be helpful as it still would take a time for index
> vacuuming. So I thought we should cap the number of parallel workers
> by the number of indexes rather than maintenance_work_mem.
>
>
Isn't that true only if we never use maintenance_work_mem during index
cleanup? However, I think we are using during index cleanup, see forex.
ginInsertCleanup. I think before reaching any conclusion about what to do
about this, first we need to establish whether this is a problem. If I am
correct, then only some of the index cleanups (like gin index) use
maintenance_work_mem, so we need to consider that point while designing a
solution for this.

> > *
> > + keys++;
> > +
> > + /* Estimate size for dead tuples -- PARALLEL_VACUUM_KEY_DEAD_TUPLES */
> > + maxtuples = compute_max_dead_tuples(nblocks, true);
> > + est_deadtuples = MAXALIGN(add_size(sizeof(LVDeadTuples),
> > + mul_size(sizeof(ItemPointerData), maxtuples)));
> > + shm_toc_estimate_chunk(&pcxt->estimator, est_deadtuples);
> > + keys++;
> > +
> > + shm_toc_estimate_keys(&pcxt->estimator, keys);
> > +
> > + /* Finally, estimate VACUUM_KEY_QUERY_TEXT space */
> > + querylen = strlen(debug_query_string);
> > + shm_toc_estimate_chunk(&pcxt->estimator, querylen + 1);
> > + shm_toc_estimate_keys(&pcxt->estimator, 1);
> >
> > The code style looks inconsistent here. In some cases, you are
> > calling shm_toc_estimate_keys immediately after shm_toc_estimate_chunk
> > and in other cases, you are accumulating keys. I think it is better
> > to call shm_toc_estimate_keys immediately after shm_toc_estimate_chunk
> > in all cases.
>
> Fixed. But there are some code that call shm_toc_estimate_keys for
> multiple keys in for example nbtsort.c and parallel.c. What is the
> difference?
>
>
We can do it, either way, depending on the situation. For example, in
nbtsort.c, there is an if check based on which 'number of keys' can vary.
I think here we should try to write in a way that it should not confuse the
reader why it is done in a particular way. This is the reason I told you
to be consistent.

> >
> > *
> > +void
> > +heap_parallel_vacuum_main(dsm_segment *seg, shm_toc *toc)
> > +{
> > ..
> > + /* Open table */
> > + onerel = heap_open(lvshared->relid, ShareUpdateExclusiveLock);
> > ..
> > }
> >
> > I don't think it is a good idea to assume the lock mode as
> > ShareUpdateExclusiveLock here. Tomorrow, if due to some reason there
> > is a change in lock level for the vacuum process, we might forget to
> > update it here. I think it is better if we can get this information
> > from the master backend.
>
> So did you mean to declare the lock mode for lazy vacuum somewhere as
> a global variable and use it in both try_relation_open in the leader
> process and relation_open in the worker process? Otherwise we would
> end up with adding something like shared->lmode =
> ShareUpdateExclusiveLock during parallel context initialization, which
> seems not to resolve your concern.
>
>
I was thinking that if we can find a way to pass the lockmode we used in
vacuum_rel, but I guess we need to pass it through multiple functions which
will be a bit inconvenient. OTOH, today, I checked nbtsort.c
(_bt_parallel_build_main) and found that there also we are using it
directly instead of passing it from the master backend. I think we can
leave it as you have in the patch, but add a comment on why it is okay to
use that lock mode?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com