Horton

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

Nathan Bossart <nathandbossart@gmail.com>

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From: Nathan Bossart <nathandbossart@gmail.com>
To: John Naylor <john.naylor@enterprisedb.com>
Cc: Masahiko Sawada <sawada.mshk@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: 2022-09-21T18:01:26Z
Lists: pgsql-hackers 
On Wed, Sep 21, 2022 at 01:17:21PM +0700, John Naylor wrote:
> In trying to wrap the SIMD code behind layers of abstraction, the latest
> patch (and Nathan's cleanup) threw it away in almost all cases. To explain,
> we need to talk about how vectorized code deals with the "tail" that is too
> small for the register:
> 
> 1. Use a one-by-one algorithm, like we do for the pg_lfind* variants.
> 2. Read some junk into the register and mask off false positives from the
> result.
> 
> There are advantages to both depending on the situation.
> 
> Patch v5 and earlier used #2. Patch v6 used #1, so if a node16 has 15
> elements or less, it will iterate over them one-by-one exactly like a
> node4. Only when full with 16 will the vector path be taken. When another
> entry is added, the elements are copied to the next bigger node, so there's
> a *small* window where it's fast.
> 
> 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.  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.

-- 
Nathan Bossart
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