Horton

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

Matthias van de Meent <boekewurm+postgres@gmail.com>

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From: Matthias van de Meent <boekewurm+postgres@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2021-07-07T14:25:28Z
Lists: pgsql-hackers 
On Wed, 7 Jul 2021 at 13:47, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> Hi all,
>
> Index vacuuming is one of the most time-consuming processes in lazy
> vacuuming. lazy_tid_reaped() is a large part among them. The attached
> the flame graph shows a profile of a vacuum on a table that has one index
> and 80 million live rows and 20 million dead rows, where
> lazy_tid_reaped() accounts for about 47% of the total vacuum execution
> time.
>
> [...]
>
> Overall, 'rtbm' has a much better lookup performance and good memory
> usage especially if there are relatively many dead tuples. However, in
> some cases, 'intset' and 'array' have a better memory usage.

Those are some great results, with a good path to meaningful improvements.

> Feedback is very welcome. Thank you for reading the email through to the end.

The current available infrastructure for TIDs is quite ill-defined for
TableAM authors [0], and other TableAMs might want to use more than
just the 11 bits in use by max-BLCKSZ HeapAM MaxHeapTuplesPerPage to
identify tuples. (MaxHeapTuplesPerPage is 1169 at the maximum 32k
BLCKSZ, which requires 11 bits to fit).

Could you also check what the (performance, memory) impact would be if
these proposed structures were to support the maximum
MaxHeapTuplesPerPage of 1169 or the full uint16-range of offset
numbers that could be supported by our current TID struct?

Kind regards,

Matthias van de Meent

[0] https://www.postgresql.org/message-id/flat/0bbeb784050503036344e1f08513f13b2083244b.camel%40j-davis.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