Re: Parallel heap vacuum - Mailing list pgsql-hackers

On Mon, Mar 31, 2025 at 2:05 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> On Thu, Mar 27, 2025 at 11:40 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Thu, Mar 27, 2025 at 8:55 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > >
> > > On Wed, Mar 26, 2025 at 1:00 PM Melanie Plageman
> > > <melanieplageman@gmail.com> wrote:
> > > >
> > > > On Mon, Mar 24, 2025 at 7:58 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > > > >
> > > > > You're right. I've studied the read stream code and figured out how to
> > > > > use it. In the attached patch, we end the read stream at the end of
> > > > > phase 1 and start a new read stream, as you suggested.
> > > >
> > > > I've started looking at this patch set some more.
> > >
> > > Thank you for reviewing the patch!
> > >
> > > >
> > > > In heap_vac_scan_next_block() if we are in the SKIP_PAGES_THRESHOLD
> > > > codepath and run out of unskippable blocks in the current chunk and
> > > > then go back to get another chunk (goto retry) but we are near the
> > > > memory limit so we can't get another block
> > > > (!dead_items_check_memory_limit()), could we get an infinite loop?
> > > >
> > > > Or even incorrectly encroach on another worker's block? Asking that
> > > > because of this math
> > > >             end_block = next_block +
> > > >
> > > > vacrel->plvstate->scanworker->pbscanwork.phsw_chunk_remaining + 1;
> > >
> > > You're right. We should make sure that reset next_block is reset to
> > > InvalidBlockNumber at the beginning of the retry loop.
> > >
> > > >
> > > > if vacrel->plvstate->scanworker->pbscanwork.phsw_chunk_remaining is 0
> > > > and we are in the goto retry loop, it seems like we could keep
> > > > incrementing next_block even when we shouldn't be.
> > >
> > > Right. Will fix.
> > >
> > > >
> > > > I just want to make sure that the skip pages optimization works with
> > > > the parallel block assignment and the low memory read stream
> > > > wind-down.
> > > >
> > > > I also think you do not need to expose
> > > > table_block_parallelscan_skip_pages_in_chunk() in the table AM. It is
> > > > only called in heap-specific code and the logic seems very
> > > > heap-related. If table AMs want something to skip some blocks, they
> > > > could easily implement it.
> > >
> > > Agreed. Will remove it.
> > >
> > > >
> > > > On another topic, I think it would be good to have a comment above
> > > > this code in parallel_lazy_scan_gather_scan_results(), stating why we
> > > > are very sure it is correct.
> > > >         Assert(TransactionIdIsValid(data->NewRelfrozenXid));
> > > >         Assert(MultiXactIdIsValid(data->NewRelminMxid));
> > > >
> > > >         if (TransactionIdPrecedes(data->NewRelfrozenXid,
> > > > vacrel->scan_data->NewRelfrozenXid))
> > > >             vacrel->scan_data->NewRelfrozenXid = data->NewRelfrozenXid;
> > > >
> > > >         if (MultiXactIdPrecedesOrEquals(data->NewRelminMxid,
> > > > vacrel->scan_data->NewRelminMxid))
> > > >             vacrel->scan_data->NewRelminMxid = data->NewRelminMxid;
> > > >
> > > >         if (data->nonempty_pages < vacrel->scan_data->nonempty_pages)
> > > >             vacrel->scan_data->nonempty_pages = data->nonempty_pages;
> > > >
> > > >         vacrel->scan_data->skippedallvis |= data->skippedallvis;
> > > >
> > > > Parallel relfrozenxid advancement sounds scary, and scary things are
> > > > best with comments. Even though the way this works is intuitive, I
> > > > think it is worth pointing out that this part is important to get
> > > > right so future programmers know how important it is.
> > > >
> > > > One thing I was wondering about is if there are any implications of
> > > > different workers having different values in their GlobalVisState.
> > > > GlobalVisState can be updated during vacuum, so even if they start out
> > > > with the same values, that could diverge. It is probably okay since it
> > > > just controls what tuples are removable. Some workers may remove fewer
> > > > tuples than they absolutely could, and this is probably okay.
> > > >
> > >
> > > Good point.
> > >
> > > > And if it is okay for each worker to have different GlobalVisState
> > > > then maybe you shouldn't have a GlobalVisState in shared memory. If
> > > > you look at GlobalVisTestFor() it just returns the memory address of
> > > > that global variable in the backend. So, it seems like it might be
> > > > better to just let each parallel worker use their own backend local
> > > > GlobalVisState and not try to put it in shared memory and copy it from
> > > > one worker to the other workers when initializing them. I'm not sure.
> > > > At the very least, there should be a comment explaining why you've
> > > > done it the way you have done it.
> > >
> > > Agreed. IIUC it's not a problem even if parallel workers use their own
> > > GlobalVisState. I'll make that change and remove the 0004 patch which
> > > exposes GlobalVisState.
> > >
> > > I'll send the updated patch soon.
> >
> > I've attached the updated patches. This version includes the comments
> > from Melanie, some bug fixes, and comment updates.
>
> Rebased the patches as they conflicted with recent commits.
>

I've attached the new version patch. There are no major changes; I
fixed some typos, improved the comment, and removed duplicated codes.
Also, I've updated the commit messages.

Regards,

--
Masahiko Sawada
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