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Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>, Tomas Vondra <tomas@vondra.me>, David Rowley <dgrowleyml@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, Xuneng Zhou <xunengzhou@gmail.com>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-04-05T15:49:15Z
Lists: pgsql-hackers 
Hi,

On 2026-04-05 11:42:19 -0400, Melanie Plageman wrote:
> On Fri, Apr 3, 2026 at 1:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
> >
> > I've come across an interesting failure produced starting from 378a21618:
> > when using a build made with CFLAGS="-DRELCACHE_FORCE_RELEASE" and
> > echo "io_method = sync" >/tmp/temp.config, the test run:
> > TEMP_CONFIG=/tmp/temp.config TESTS=temp make -s check-tests
> >
> > fails as below:
> > --- .../src/test/regress/expected/temp.out   2026-02-13 06:15:55.887368624 +0200
> > +++ .../src/test/regress/results/temp.out    2026-04-03 07:51:36.735504156 +0300
> > @@ -493,11 +493,7 @@
> >
> >   -- Check that read streams deal with lower number of pins available
> >   SELECT count(*), max(a) max_a, min(a) min_a, max(cnt) max_cnt FROM test_temp;
> > - count | max_a | min_a | max_cnt
> > --------+-------+-------+---------
> > - 10000 | 10000 |     1 |       0
> > -(1 row)
> > -
> > +ERROR:  no empty local buffer available
> >   ROLLBACK;
> 
> It has to do with the query needing an additional pin for the VM
> during on-access pruning and the read stream reading ahead until there
> is only one remaining buffer pin in the local pin limit (the cursor
> above is already consuming much of the backend local pin limit). We
> could perhaps fix this test by decreasing the pages in the relation or
> increasing the backend local pin limit, but I wonder if we need to do
> something more invasive to ensure that we can pin at least two
> buffers.

I think we should probably just have GetLocalPinLimit() return something
considerably smaller than num_temp_buffers, e.g. num_temp_buffers / 4 or
so.

There always may be more than one scan going on, so we can't ever promise that
there's at least a certain number of pins available. The main goal of the
shared pin limit is to prevent one backend from pinning disproportionally much
of s_b.  And for that eventually scaling down to just needing 1-2 pins per
scan is sufficient.

Greetings,

Andres Freund

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Stabilize btree_gist test against on-access VM setting

  2. Stabilize plancache test against on-access VM setting

  3. Make local buffers pin limit more conservative

  4. Update FSM during prune/freeze replay even if freespace is zero

  5. Pass down information on table modification to scan nodes

  6. Thread flags through begin-scan APIs

  7. Remove PlannedStmt->resultRelations in favor of resultRelationRelids

  8. Make it cheap to check if a relation is modified by a query

  9. Keep newest live XID up-to-date even if page not all-visible

  10. Add pruning fast path for all-visible and all-frozen pages

  11. Detect and fix visibility map corruption in more cases

  12. Save vmbuffer in heap-specific scan descriptors for on-access pruning

  13. Avoid BufferGetPage() calls in heap_update()

  14. Initialize missing fields in CreateExecutorState()

  15. Prefix PruneState->all_{visible,frozen} with set_

  16. Add PageGetPruneXid() helper

  17. Move commonly used context into PruneState and simplify helpers

  18. Remove table_scan_analyze_next_tuple unneeded parameter OldestXmin

  19. Simplify visibility check in heap_page_would_be_all_visible()

  20. Rename LVRelState VM-related logging counters

  21. Fix flakiness in the pg_visibility VM-only vacuum test by using a temporary table.

  22. Refactor lazy_scan_prune() VM clear logic into helper

  23. Eliminate use of cached VM value in lazy_scan_prune()

  24. Combine visibilitymap_set() cases in lazy_scan_prune()

  25. Fix const qualification in prune_freeze_setup()

  26. Simplify vacuum visibility assertion

  27. Split heap_page_prune_and_freeze() into helpers

  28. Assert that cutoffs are provided if freezing will be attempted

  29. Split PruneFreezeParams initializers to one field per line

  30. Refactor heap_page_prune_and_freeze() parameters into a struct

  31. Make heap_page_is_all_visible independent of LVRelState

  32. Inline TransactionIdFollows/Precedes[OrEquals]()

  33. Add helper for freeze determination to heap_page_prune_and_freeze

  34. Bump XLOG_PAGE_MAGIC after xl_heap_prune change

  35. Correct prune WAL record opcode name in comment

  36. Add error codes when vacuum discovers VM corruption

  37. Remove unused xl_heap_prune member, reason

  38. Remove unneeded VM pin from VM replay

  39. Add assert and log message to visibilitymap_set

  40. Stamp HEAD as 19devel.

  41. Add error codes to some corruption log messages