Horton

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

John Naylor <john.naylor@enterprisedb.com>

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From: John Naylor <john.naylor@enterprisedb.com>
To: Masahiko Sawada <sawada.mshk@gmail.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: 2022-11-30T05:51:03Z
Lists: pgsql-hackers

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There are a few things up in the air, so I'm coming back to this list to
summarize and add a recent update:

On Mon, Nov 14, 2022 at 7:59 PM John Naylor <john.naylor@enterprisedb.com>
wrote:
>
> - See how much performance we actually gain from tagging the node kind.

Needs a benchmark that has enough branch mispredicts and L2/3 misses to
show a benefit. Otherwise either neutral or worse in its current form,
depending on compiler(?). Put off for later.

> - Try additional size classes while keeping the node kinds to only four.

This is relatively simple and effective. If only one additional size class
(total 5) is coded as a placeholder, I imagine it will be easier to rebase
shared memory logic than using this technique everywhere possible.

> - Optimize node128 insert.

I've attached a rough start at this. The basic idea is borrowed from our
bitmapset nodes, so we can iterate over and operate on word-sized (32- or
64-bit) types at a time, rather than bytes. To make this easier, I've moved
some of the lower-level macros and types from bitmapset.h/.c to
pg_bitutils.h. That's probably going to need a separate email thread to
resolve the coding style clash this causes, so that can be put off for
later. This is not meant to be included in the next patchset.  For
demonstration purposes, I get these results with a function that repeatedly
deletes the last value from a mostly-full node128 leaf and re-inserts it:

select * from bench_node128_load(120);

v11

NOTICE:  num_keys = 14400, height = 1, n1 = 0, n4 = 0, n15 = 0, n32 = 0,
n61 = 0, n128 = 121, n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_sparseload_ms
--------+-------+------------------+------------------
    120 | 14400 |           208304 |               56

v11 + 0006 addendum

NOTICE:  num_keys = 14400, height = 1, n1 = 0, n4 = 0, n15 = 0, n32 = 0,
n61 = 0, n128 = 121, n256 = 0
 fanout | nkeys | rt_mem_allocated | rt_sparseload_ms
--------+-------+------------------+------------------
    120 | 14400 |           208816 |               34

I didn't test inner nodes, but I imagine the difference is bigger. This
bitmap style should also be used for the node256-leaf isset array simply to
be consistent and avoid needing single-use macros, but that has not been
done yet. It won't make a difference for performance because there is no
iteration there.

> - Try templating out the differences between local and shared memory.

I hope to start this sometime after the crashes on 32-bit are resolved.

--
John Naylor
EDB: http://www.enterprisedb.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