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 <john.naylor@enterprisedb.com>
Cc: Nathan Bossart <nathandbossart@gmail.com>, Andres Freund <andres@anarazel.de>, Matthias van de Meent <boekewurm+postgres@gmail.com>, Yura Sokolov <y.sokolov@postgrespro.ru>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2022-09-22T06:26:14Z
Lists: pgsql-hackers 
On Thu, Sep 22, 2022 at 1:46 PM John Naylor
<john.naylor@enterprisedb.com> wrote:
>
>
> On Thu, Sep 22, 2022 at 1:01 AM Nathan Bossart <nathandbossart@gmail.com> wrote:
> >
> > On Wed, Sep 21, 2022 at 01:17:21PM +0700, John Naylor wrote:
> >
> > > In short, this code needs to be lower level so that we still have full
> > > control while being portable. I will work on this, and also the related
> > > code for node dispatch.
> >
> > Is it possible to use approach #2 here, too?  AFAICT space is allocated for
> > all of the chunks, so there wouldn't be any danger in searching all them
> > and discarding any results >= node->count.
>
> Sure, the caller could pass the maximum node capacity, and then check if the returned index is within the range of the node count.
>
> > Granted, we're depending on the
> > number of chunks always being a multiple of elements-per-vector in order to
> > avoid the tail path, but that seems like a reasonably safe assumption that
> > can be covered with comments.
>
> Actually, we don't need to depend on that at all. When I said "junk" above, that can be any bytes, as long as we're not reading off the end of allocated memory. We'll never do that here, since the child pointers/values follow. In that case, the caller can hard-code the  size (it would even happen to work now to multiply rt_node_kind by 16, to be sneaky). One thing I want to try soon is storing fewer than 16/32 etc entries, so that the whole node fits comfortably inside a power-of-two allocation. That would allow us to use aset without wasting space for the smaller nodes, which would be faster and possibly would solve the fragmentation problem Andres referred to in
>
> https://www.postgresql.org/message-id/20220704220038.at2ane5xkymzzssb%40awork3.anarazel.de
>
> While on the subject, I wonder how important it is to keep the chunks in the small nodes in sorted order. That adds branches and memmove calls, and is the whole reason for the recent "pg_lfind_ge" function.

Good point. While keeping the chunks in the small nodes in sorted
order is useful for visiting all keys in sorted order, additional
branches and memmove calls could be slow.

Regards,

-- 
Masahiko Sawada
PostgreSQL Contributors Team
RDS Open Source Databases
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