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: Melanie Plageman <melanieplageman@gmail.com>
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: 2025-03-12T00:07:38Z
Lists: pgsql-hackers 
Hi,

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
> 
> The default effective_io_concurrency has been 1 since it was introduced
> in b7b8f0b6096d2ab6e. Referencing the associated discussion [1], it
> seems 1 was chosen as a conservative value that seemed unlikely to cause
> regressions.

16 years...


> Experimentation on high latency cloud storage as well as fast, local
> nvme storage (see Discussion link) shows that even slightly higher values
> improve query timings substantially. 1 actually performs worse than 0.
> With effective_io_concurrency 1, we are not prefetching enough to avoid
> I/O stalls, but we are issuing extra syscalls.

Makes sense.


> 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?


> The new default is 16, which should be more appropriate in the general
> case while still avoiding flooding low IOPs devices with I/O requests.

Maybe s/in the general case/for common hardware/?


> [1] https://www.postgresql.org/message-id/flat/FDDBA24E-FF4D-4654-BA75-692B3BA71B97%40enterprisedb.com
> 
> Discussion: https://postgr.es/m/CAAKRu_Z%2BJa-mwXebOoOERMMUMvJeRhzTjad4dSThxG0JLXESxw%40mail.gmail.com
> ---
>  doc/src/sgml/config.sgml                      | 38 +++++++++----------
>  src/backend/utils/misc/postgresql.conf.sample |  2 +-
>  src/include/storage/bufmgr.h                  |  2 +-
>  3 files changed, 19 insertions(+), 23 deletions(-)
> 
> diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
> index d2fa5f7d1a9..8c4409fc8bf 100644
> --- a/doc/src/sgml/config.sgml
> +++ b/doc/src/sgml/config.sgml
> @@ -2577,36 +2577,32 @@ include_dir 'conf.d'
>           Sets the number of concurrent disk I/O operations that
>           <productname>PostgreSQL</productname> expects can be executed
>           simultaneously.  Raising this value will increase the number of I/O
> -         operations that any individual <productname>PostgreSQL</productname> session
> -         attempts to initiate in parallel.  The allowed range is 1 to 1000,
> -         or zero to disable issuance of asynchronous I/O requests. Currently,
> -         this setting only affects bitmap heap scans.
> +         operations that any individual <productname>PostgreSQL</productname>
> +         session attempts to initiate in parallel.  The allowed range is
> +         <literal>1</literal> to <literal>1000</literal>, or
> +         <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...


>          <para>
> -         For magnetic drives, a good starting point for this setting is the
> -         number of separate
> -         drives comprising a RAID 0 stripe or RAID 1 mirror being used for the
> -         database.  (For RAID 5 the parity drive should not be counted.)
> -         However, if the database is often busy with multiple queries issued in
> -         concurrent sessions, lower values may be sufficient to keep the disk
> -         array busy.  A value higher than needed to keep the disks busy will
> -         only result in extra CPU overhead.
> -         SSDs and other memory-based storage can often process many
> -         concurrent requests, so the best value might be in the hundreds.

Afaict this whole paragraph was *never* correct... Obviously that's not
criticism of your removing it ;)


> +         Higher values will have the most impact on higher latency storage
> +         where queries otherwise experience noticeable I/O stalls and on
> +         devices with high IOPs. Higher values than needed to satisfy the query
> +         or keep the device busy can be expected to only introduce extra CPU
> +         overhead.
>          </para>

I'd say unnecessarily high values also can increase IO latency.

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.