Horton

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: 2024-03-14T12:03:27Z
Lists: pgsql-hackers 
On Thu, Mar 14, 2024 at 6:55 PM John Naylor <johncnaylorls@gmail.com> wrote:
>
> On Thu, Mar 14, 2024 at 12:06 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> >
> > On Thu, Mar 14, 2024 at 1:29 PM John Naylor <johncnaylorls@gmail.com> wrote:
> > > Okay, here's an another idea: Change test_lookup_tids() to be more
> > > general and put the validation down into C as well. First we save the
> > > blocks from do_set_block_offsets() into a table, then with all those
> > > blocks lookup a sufficiently-large range of possible offsets and save
> > > found values in another array. So the static items structure would
> > > have 3 arrays: inserts, successful lookups, and iteration (currently
> > > the iteration output is private to check_set_block_offsets(). Then
> > > sort as needed and check they are all the same.
> >
> > That's a promising idea. We can use the same mechanism for randomized
> > tests too. If you're going to work on this, I'll do other tests on my
> > environment in the meantime.
>
> Some progress on this in v72 -- I tried first without using SQL to
> save the blocks, just using the unique blocks from the verification
> array. It seems to work fine.

Thanks!

>
> - Since there are now three arrays we should reduce max bytes to
> something smaller.

Agreed.

> - Further on that, I'm not sure if the "is full" test is telling us
> much. It seems we could make max bytes a static variable and set it to
> the size of the empty store. I'm guessing it wouldn't take much to add
> enough tids so that the contexts need to allocate some blocks, and
> then it would appear full and we can test that. I've made it so all
> arrays repalloc when needed, just in case.

How about using work_mem as max_bytes instead of having it as a static
variable? In test_tidstore.sql we set work_mem before creating the
tidstore. It would make the tidstore more controllable by SQL queries.

> - Why are we switching to TopMemoryContext? It's not explained -- the
> comment only tells what the code is doing (which is obvious), but not
> why.

This is because the tidstore needs to live across the transaction
boundary. We can use TopMemoryContext or CacheMemoryContext.

> - I'm not sure it's useful to keep test_lookup_tids() around. Since we
> now have a separate lookup test, the only thing it can tell us is that
> lookups fail on an empty store. I arranged it so that
> check_set_block_offsets() works on an empty store. Although that's
> even more trivial, it's just reusing what we already need.

Agreed.

Regards,

-- 
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