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Re: [PoC] Improve dead tuple storage for lazy vacuum

Masahiko Sawada <sawada.mshk@gmail.com>

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From: Masahiko Sawada <sawada.mshk@gmail.com>
To: John Naylor <johncnaylorls@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>, Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>
Date: 2024-03-21T02:37:16Z
Lists: pgsql-hackers

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On Wed, Mar 20, 2024 at 11:19 PM John Naylor <johncnaylorls@gmail.com> wrote:
>
> On Wed, Mar 20, 2024 at 8:30 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > I forgot to report the results. Yes, I did some tests where I inserted
> > many TIDs to make the tidstore use several GB memory. I did two cases:
> >
> > 1. insert 100M blocks of TIDs with an offset of 100.
> > 2. insert 10M blocks of TIDs with an offset of 2048.
> >
> > The tidstore used about 4.8GB and 5.2GB, respectively, and all lookup
> > and iteration results were expected.
>
> Thanks for confirming!
>
> > While reviewing the codes again, the following two things caught my eyes:
> >
> > in check_set_block_offset() function, we don't take a lock on the
> > tidstore while checking all possible TIDs. I'll add
> > TidStoreLockShare() and TidStoreUnlock() as follows:
> >
> > +           TidStoreLockShare(tidstore);
> >             if (TidStoreIsMember(tidstore, &tid))
> >                 ItemPointerSet(&items.lookup_tids[num_lookup_tids++],
> > blkno, offset);
> > +           TidStoreUnlock(tidstore);
>
> In one sense, all locking in the test module is useless since there is
> only a single process. On the other hand, it seems good to at least
> run what we have written to run it trivially, and serve as an example
> of usage. We should probably be consistent, and document at the top
> that the locks are pro-forma only.

Agreed.

>
> > Regarding TidStoreMemoryUsage(), IIUC the caller doesn't need to take
> > a lock on the shared tidstore since dsa_get_total_size() (called by
> > RT_MEMORY_USAGE()) does appropriate locking. I think we can mention it
> > in the comment as follows:
> >
> > -/* Return the memory usage of TidStore */
> > +/*
> > + * Return the memory usage of TidStore.
> > + *
> > + * In shared TidStore cases, since shared_ts_memory_usage() does appropriate
> > + * locking, the caller doesn't need to take a lock.
> > + */
> >
> > What do you think?
>
> That duplicates the underlying comment on the radix tree function that
> this calls, so I'm inclined to leave it out. At this level it's
> probably best to document when a caller _does_ need to take an action.

Okay, I didn't change it.

>
> One thing I forgot to ask about earlier:
>
> +-- Add tids in out of order.
>
> Are they (the blocks to be precise) really out of order? The VALUES
> statement is ordered, but after inserting it does not output that way.
> I wondered if this is platform independent, but CI and our dev
> machines haven't failed this test, and I haven't looked into what
> determines the order. It's easy enough to hide the blocks if we ever
> need to, as we do elsewhere...

It seems not necessary as such a test is already covered by
test_radixtree. I've changed the query to hide the output blocks.

I've pushed the tidstore patch after incorporating the above changes.
In addition to that, I've added the following changes before the push:

- Added src/test/modules/test_tidstore/.gitignore file.
- Removed unnecessary #include from tidstore.c.

The buildfarm has been all-green so far.

I've attached the latest vacuum improvement patch.

I just remembered that the tidstore cannot still be used for parallel
vacuum with minimum maintenance_work_mem. Even when the shared
tidstore is empty, its memory usage reports 1056768 bytes, a bit above
1MB (1048576 bytes). We need something discussed on another thread[1]
in order to make it work.

Regards,

[1] https://www.postgresql.org/message-id/CAD21AoCVMw6DSmgZY9h%2BxfzKtzJeqWiwxaUD2T-FztVcV-XibQ%40mail.gmail.com

-- 
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com

Commits

  1. radixtree: Fix SIGSEGV at update of embeddable value to non-embeddable.

  2. Get rid of anonymous struct

  3. Teach radix tree to embed values at runtime

  4. Teach TID store to skip bitmap for small numbers of offsets

  5. Use bump context for TID bitmaps stored by vacuum

  6. Fix alignment of stack variable

  7. Use TidStore for dead tuple TIDs storage during lazy vacuum.

  8. Rethink create and attach APIs of shared TidStore.

  9. Fix inconsistent function prototypes with function definitions.

  10. Fix a calculation in TidStoreCreate().

  11. Fix potential integer handling issue in radixtree.h.

  12. Add TIDStore, to store sets of TIDs (ItemPointerData) efficiently.

  13. Fix link error for test_radixtree module on Windows

  14. Blind attempt to fix ODR violations

  15. Fix incorrect format specifier for int64

  16. Fix redefinition of typedefs

  17. Add template for adaptive radix tree

  18. Fix signedness error in 9f225e992 for gcc

  19. Introduce helper SIMD functions for small byte arrays

  20. Optimize vacuuming of relations with no indexes.

  21. Add bound check before bsearch() for performance

  22. Allocate consecutive blocks during parallel seqscans