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: Andres Freund <andres@anarazel.de>
Cc: 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: 2025-03-12T20:01:59Z
Lists: pgsql-hackers 
Thanks for taking a look. I've pushed the patch to increase the
default effective_io_concurrency.

On Tue, Mar 11, 2025 at 8:07 PM Andres Freund <andres@anarazel.de> wrote:
>
> On 2025-03-10 19:45:38 -0400, Melanie Plageman wrote:
> > From 7b35b1144bddf202fb4d56a9b783751a0945ba0e Mon Sep 17 00:00:00 2001
> > From: Melanie Plageman <melanieplageman@gmail.com>
> > Date: Mon, 10 Mar 2025 17:17:38 -0400
> > Subject: [PATCH v35 1/5] Increase default effective_io_concurrency to 16
>
> > Moreover, when bitmap heap scan is converted to using the read stream
> > API, a prefetch distance of 1 will prevent read combining which is quite
> > detrimental to performance.
>
> Hm? This one surprises me. Doesn't the read stream code take some pains to
> still perform IO combining when effective_io_concurrency=1? It does work for
> seqscans, for example?

I went back and tried to figure out what I meant by this. When Thomas
and I were investigating various BHS regressions, we did see that
effective_io_concurrency 1 performed particularly badly on the read
stream branch (as compared to master). One thing we noticed was very
little read combining (especially for parallel bitmap heap scan, but
that's a separate bundle of issues). Re-reading the read stream code,
though, you are right that we should have no issue read combining when
eic is 1. In that case, with the default io_combine_limit, max pinned
buffers will be 32, which is definitely enough for read combining.

Anyway, I've removed this text from the commit message.

> > diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
> > index d2fa5f7d1a9..8c4409fc8bf 100644
> > --- a/doc/src/sgml/config.sgml
<-- snip-->
> > +         <literal>0</literal> to disable issuance of asynchronous I/O requests.
> > +         The default is <literal>16</literal> on supported systems, otherwise
> > +         <literal>0</literal>. Currently, this setting only affects bitmap heap
> > +         scans.
> >          </para>
>
> I'd probably use this as an occasion to remove "Currently, this setting only
> affects bitmap heap" sentence - afaict it's been wrong for a while and got
> more wrong since vacuum started to use read streams...

Well, technically vacuum uses maintenance_io_concurrency. Bitmap heap
scan is the only one using effective_io_concurrency for prefetching
(since sequential scan doesn't do prefetching). So, it is technically
true. Sequential scans will try to combine IOs only up to the
io_combine_limit so effective_io_concurrency doesn't really matter for
them. I removed this sentence anyway, though. Other read stream users
(including non-in-core ones) could start using
effective_io_concurrency and then it might be more confusing anyway.

- 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.