Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Tomas Vondra <tomas@vondra.me>

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From: Tomas Vondra <tomas@vondra.me>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Nazir Bilal Yavuz <byavuz81@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Thomas Munro <thomas.munro@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2025-02-16T01:15:15Z
Lists: pgsql-hackers

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On 2/14/25 18:50, Tomas Vondra wrote:
> On 2/14/25 18:31, Melanie Plageman wrote:
>> io_combine_limit 1, effective_io_concurrency 16, read ahead kb 16
>>
>> On Fri, Feb 14, 2025 at 12:18 PM Tomas Vondra <tomas@vondra.me> wrote:
>>>
>>> Based on off-list discussion with Melanie, I ran a modified version of
>>> the benchmark, with these changes:
>>
>> Thanks! It looks like the worst offender is io_combine_limit 1 (128
>> kB), effective_io_concurrency 16, read_ahead_kb 16. This is a
>> combination that people shouldn't run in practice I think -- an
>> io_combine_limit larger than read ahead.
>>
>> But we do still see regressions with io_combine_limit 1,
>> effective_io_concurrency 16 at other read_ahead_kb values. Therefore
>> I'd be interested to see that subset (ioc 1, eic 16, diff read ahead
>> values) with the attached patch.
>>
>>> FWIW this does not change anything in the detection of sequential access
>>> patterns, discussed nearby, because the benchmarks started before Andres
>>> looked into that. If needed, I can easily rerun these tests, I just need
>>> a patch to apply.
>>
>> Attached a patch that has all the commits squashed + removes
>> sequential detection.
>>
> 
> OK, it's running.
> 
> I didn't limit the benchmarks any further, it seems useful to have
> "full" comparison with the last run. I expect to have the results in ~48
> hours, i.e. by Monday.
> 

OK, I've uploaded the results to the github repository as usual

  https://github.com/tvondra/bitmapscan-tests/tree/main/20250214-184807

and I've generated the same PDF reports, with the colored comparison.

If you compare the pivot tables (I opened the "same" PDF from the two
runs and flip between them using alt-tab, which makes the interesting
regions easy to spot), the change is very clear.

Disabling the sequential detection greatly reduces the scope of
regressions. That looks pretty great, IMO.

It also seems to lose some speedups, especially with io_combine_limit=1
and eic=1. I'm not sure why, if that's expected, etc.

There still remain areas of regression, but most of them are for cases
that'd use index scan (tiny fraction of rows scanned), or with
read-ahead=4096 (and not for the lower settings).

The read-ahead dependence is actually somewhat interesting, because I
realized the RAID array has this set to 8192 by default, i.e. even
higher than 4096 where it regresses. I suppose mdadm does that, or
something, I don't know how the default is calculated. But I assume it
depends on the number of devices, so larger arrays might have even
higher read-ahead values.

I'm running the benchmarks with ra=8192, shouldn't take more than a
couple hours.


regards

-- 
Tomas Vondra

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.