Horton

Re: BitmapHeapScan streaming read user and prelim refactoring

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Dilip Kumar <dilipbalaut@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@gmail.com>
Date: 2024-03-14T21:39:30Z
Lists: pgsql-hackers 
On Thu, Mar 14, 2024 at 5:26 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 3/14/24 19:16, Melanie Plageman wrote:
> > On Thu, Mar 14, 2024 at 03:32:04PM +0200, Heikki Linnakangas wrote:
> >> ...
> >>
> >> Ok, committed that for now. Thanks for looking!
> >
> > Attached v6 is rebased over your new commit. It also has the "fix" in
> > 0010 which moves BitmapAdjustPrefetchIterator() back above
> > table_scan_bitmap_next_block(). I've also updated the Streaming Read API
> > commit (0013) to Thomas' v7 version from [1]. This has the update that
> > we theorize should address some of the regressions in the bitmapheapscan
> > streaming read user in 0014.
> >
>
> Should I rerun the benchmarks with these new patches, to see if it
> really helps with the regressions?

That would be awesome!

I will soon send out a summary of what we investigated off-list about
0010 (though we didn't end up concluding anything). My "fix" (leaving
BitmapAdjustPrefetchIterator() above table_scan_bitmap_next_block())
eliminates the regression in 0010 on the one example that I repro'd
upthread, but it would be good to know if it eliminates the
regressions across some other tests.

I think it would be worthwhile to run the subset of tests which seemed
to fare the worst on 0010 against the patches 0001-0010-- cyclic
uncached on your xeon machine with 4 parallel workers, IIRC -- even
the 1 million scale would do the trick, I think.

And then separately run the subset of tests which seemed to do the
worst on 0014. There were several groups of issues across the
different tests, but I think that the uniform pages data test would be
relevant to use. It showed the regressions with eic 0.

As for the other regressions showing with 0014, I think we would want
to see at least one with fully-in-shared-buffers and one with fully
uncached. Some of the fixes were around pinning fewer buffers when the
blocks were already in shared buffers.

- 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.