Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@gmail.com>, Tomas Vondra <tomas@vondra.me>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-03-17T18:52:02Z
Lists: pgsql-hackers 
On Mon, Mar 17, 2025 at 3:44 AM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
>
> dunno, I've just asked if it isn't suspicious to anyone except me else
> that e_io_c > m_io_c rather than e_io_c <= m_io_c. My understanding
> was always that to tune max IO queue depth you would do this:
> a. up to N backends (up to max_connections; usually much lower) * e_io_c
> b. autovacuum_max_workers * m_io_c
> c. just one (standby/recovering) * m_io_c
>
> The thing (for me) is: if we are allowing for much higher IOPS "a"
> scenario, then why standby cannot use just the same (if not higher)
> IOPS for prefetching in "c" scenario. After all, it is a much more
> critical and sensitive thing (lag).

This sounds quite reasonable to me. Given I just changed the default
effective_io_concurrency to 16, what value would you say is reasonable
for maintenance_io_concurrency? I based the eic change off of
experimentation -- seeing where the benefits flatlined for a certain
class of query on a couple different kinds of machines with different
IO latencies. I don't feel strongly that we need to be as rigorous for
maintenance_io_concurrency, but I'm also not sure 160 seems reasonable
(which would be the same ratio as before).

- Melanie

Commits

  1. Fix bitmapheapscan incorrect recheck of NULL tuples

  2. Increase default maintenance_io_concurrency to 16

  3. Separate TBM[Shared|Private]Iterator and TBMIterateResult

  4. Improve read_stream.c advice for dense streams.

  5. Increase default effective_io_concurrency to 16

  6. Delay extraction of TIDBitmap per page offsets

  7. Add lossy indicator to TBMIterateResult

  8. Move BitmapTableScan per-scan setup into a helper

  9. Add and use BitmapHeapScanDescData struct

  10. Fix bitmap table scan crash on iterator release

  11. Bitmap Table Scans use unified TBMIterator

  12. Add common interface for TBMIterators

  13. Make table_scan_bitmap_next_block() async-friendly

  14. Move EXPLAIN counter increment to heapam_scan_bitmap_next_block

  15. Refactor tidstore.c iterator buffering.

  16. BitmapHeapScan: Remove incorrect assert and reset field

  17. Change BitmapAdjustPrefetchIterator to accept BlockNumber

  18. BitmapHeapScan: Use correct recheck flag for skip_fetch

  19. BitmapHeapScan: Push skip_fetch optimization into table AM

  20. BitmapHeapScan: postpone setting can_skip_fetch

  21. BitmapHeapScan: begin scan after bitmap creation

  22. Fix EXPLAIN Bitmap heap scan to count pages with no visible tuples

  23. Remove redundant snapshot copying from parallel leader to workers

  24. Remove some obsolete smgrcloseall() calls.

  25. Remove the "snapshot too old" feature.

  26. Compute XID horizon for page level index vacuum on primary.