Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Melanie Plageman <melanieplageman@gmail.com>

View thread View on postgr.es
From: Melanie Plageman <melanieplageman@gmail.com>
To: Jakub Wartak <jakub.wartak@enterprisedb.com>
Cc: Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas@vondra.me>, Robert Haas <robertmhaas@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: 2025-02-20T16:51:15Z
Lists: pgsql-hackers 
On Wed, Feb 19, 2025 at 8:29 AM Jakub Wartak
<jakub.wartak@enterprisedb.com> wrote:
>
> On Fri, Feb 14, 2025 at 7:16 PM Andres Freund <andres@anarazel.de> wrote:
>
> > Melanie has worked on this a fair bit, fwiw.
> >
> > My current thinking is that we'd want something very roughly like TCP
> > BBR. Basically, it predicts the currently available bandwidth not just via
> > lost packets - the traditional approach - but also by building a continually
> > updated model of "bytes in flight" and latency and uses that to predict what
> > the achievable bandwidth is.[..]
>
> Sadly that doesn't sound like PG18, right? (or I missed some thread,
> I've tried to watch Melanie's presentation though )

Yes, I spent about a year researching this. My final algorithm didn't
make it into a presentation, but the basic idea was to track how the
prefetch distance was affecting throughput on a per IO basis and push
the prefetch distance up when doing so was increasing throughput and
down when throughput isn't increasing. Doing this in a sawtooth
pattern ultimately would allow the prefetch distance to adapt to
changing system resources.

The actual algorithm was more complicated than this, but that was the
basic premise. It worked well in simulations.

- 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.