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: Andres Freund <andres@anarazel.de>, Pg Hackers <pgsql-hackers@postgresql.org>, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Nazir Bilal Yavuz <byavuz81@gmail.com>
Date: 2024-03-02T23:39:31Z
Lists: pgsql-hackers 
On Sat, Mar 2, 2024 at 5:51 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 3/2/24 23:11, Melanie Plageman wrote:
> > On Fri, Mar 1, 2024 at 2:31 PM Melanie Plageman
> > <melanieplageman@gmail.com> wrote:
> >>
> >> ...
> >>
> >> Hold the phone on this one. I realized why I moved
> >> BitmapAdjustPrefetchIterator after table_scan_bitmap_next_block() in
> >> the first place -- master calls BitmapAdjustPrefetchIterator after the
> >> tbm_iterate() for the current block -- otherwise with eic = 1, it
> >> considers the prefetch iterator behind the current block iterator. I'm
> >> going to go through and figure out what order this must be done in and
> >> fix it.
> >
> > So, I investigated this further, and, as far as I can tell, for
> > parallel bitmapheapscan the timing around when workers decrement
> > prefetch_pages causes the performance differences with patch 0010
> > applied. It makes very little sense to me, but some of the queries I
> > borrowed from your regression examples are up to 30% slower when this
> > code from BitmapAdjustPrefetchIterator() is after
> > table_scan_bitmap_next_block() instead of before it.
> >
> >         SpinLockAcquire(&pstate->mutex);
> >         if (pstate->prefetch_pages > 0)
> >             pstate->prefetch_pages--;
> >         SpinLockRelease(&pstate->mutex);
> >
> > I did some stracing and did see much more time spent in futex/wait
> > with this code after the call to table_scan_bitmap_next_block() vs
> > before it. (table_scan_bitmap_next_block()) calls ReadBuffer()).
> >
> > In my branch, I've now moved only the parallel prefetch_pages-- code
> > to before table_scan_bitmap_next_block().
> > https://github.com/melanieplageman/postgres/tree/bhs_pgsr
> > I'd be interested to know if you see the regressions go away with 0010
> > applied (commit message "Make table_scan_bitmap_next_block() async
> > friendly" and sha bfdcbfee7be8e2c461).
> >
>
> I'll give this a try once the runs with MAX_BUFFERS_PER_TRANSFER=1
> complete. But it seems really bizarre that simply moving this code a
> little bit would cause such a regression ...

Yes, it is bizarre. It also might not be a reproducible performance
difference on the cases besides the one I was testing (cyclic dataset,
uncached, eic=8, matches 16+, distinct=100, rows=100000000, 4 parallel
workers). But even if it only affects that one case, it still had a
major, reproducible performance impact to move those 5 lines before
and after table_scan_bitmap_next_block().

The same number of reads and fadvises are being issued overall.
However, I did notice that the pread calls are skewed when the those
lines of code are after table_scan_bitmap_next_block()  -- fewer of
the workers are doing more of the reads. Perhaps this explains what is
taking longer. Why those workers would end up doing more of the reads,
I don't quite know.

- 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.