Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Tomas Vondra <tomas@vondra.me>, Melanie Plageman <melanieplageman@gmail.com>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Thomas Munro <thomas.munro@gmail.com>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-02-14T17:14:20Z
Lists: pgsql-hackers 
Hi,

On 2025-02-14 10:04:41 +0100, Jakub Wartak wrote:
> Is there any reason we couldn't have new pg_test_iorates (similiar to
> other pg_test_* proggies), that would literally do this and calibrate
> best e_io_c during initdb and put the result into postgresql.auto.conf
> (pg_test_iorates --adjust-auto-conf) , that way we would avoid user
> questions on how to come with optimal value?

Unfortunately I think this is a lot easier said than done:

- The optimal depth depends a lot on your IO patterns, there's a lot of things
  between fully sequential and fully random.

  You'd really need to test different IO sizes and different patterns. The
  test matrix for that gets pretty big.

- The performance characteristics of storage heavily changes over time.

  This is particularly true in cloud environments, where disk/VM combinations
  will be "burstable", allowing higher throughput for a while, but then not
  anymore.  Measureing during either of those states will not be great for the
  other state.

- e_io_c is per-query-node, but impacts the whole system. If you set
  e_io_c=1000 on a disk with a metered IOPS of say 1k/s you might get a
  slightly higher throughput for a bitmap heap scan, but also your commit
  latency in concurrent will go through the roof, because your fdatasync()
  will be behind a queue of 1k reads that all are throttled.

Greetings,

Andres Freund

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.