Horton

Re: AIO v2.5

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Antonin Houska <ah@cybertec.at>, Noah Misch <noah@leadboat.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>
Date: 2025-03-19T14:47:50Z
Lists: pgsql-hackers

Commits

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

  1. aio: Fix assertion, clarify README

  2. aio: Fix reference to outdated name

  3. aio: Fix possible state confusions due to interrupt processing

  4. aio: Improve debug logging around waiting for IOs

  5. aio: Fix crash potential for pg_aios views due to late state update

  6. Increase BAS_BULKREAD based on effective_io_concurrency

  7. localbuf: Add Valgrind buffer access instrumentation

  8. aio: Make AIO more compatible with valgrind

  9. aio: Avoid spurious coverity warning

  10. tests: Fix incompatibility of test_aio with *_FORCE_RELEASE

  11. tests: Cope with WARNINGs during failed CREATE DB on windows

  12. aio: Add errcontext for processing I/Os for another backend

  13. aio: Add README.md explaining higher level design

  14. aio: Minor comment improvements

  15. aio: Add test_aio module

  16. aio: Add pg_aios view

  17. docs: Add acronym and glossary entries for I/O and AIO

  18. Enable IO concurrency on all systems

  19. read_stream: Introduce and use optional batchmode support

  20. docs: Reframe track_io_timing related docs as wait time

  21. bufmgr: Use AIO in StartReadBuffers()

  22. bufmgr: Implement AIO read support

  23. aio: Add WARNING result status

  24. Let caller of PageIsVerified() control ignore_checksum_failure

  25. pgstat: Allow checksum errors to be reported in critical sections

  26. Add errhint_internal()

  27. localbuf: Track pincount in BufferDesc as well

  28. aio, bufmgr: Comment fixes/improvements

  29. Fix mis-attribution of checksum failure stats to the wrong database

  30. aio: Implement support for reads in smgr/md/fd

  31. aio: Add io_method=io_uring

  32. aio: Add liburing dependency

  33. aio: Rename pgaio_io_prep_* to pgaio_io_start_*

  34. aio: Pass result of local callbacks to ->report_return

  35. aio: Be more paranoid about interrupts

  36. Redefine max_files_per_process to control additionally opened files

  37. aio: Change prefix of PgAioResultStatus values to PGAIO_RS_

  38. bufmgr: Improve stats when a buffer is read in concurrently

  39. aio: Add io_method=worker

  40. aio: Infrastructure for io_method=worker

  41. aio: Add core asynchronous I/O infrastructure

  42. aio: Basic subsystem initialization

  43. tests: Expand temp table tests to some pin related matters

  44. localbuf: Introduce FlushLocalBuffer()

  45. localbuf: Introduce TerminateLocalBufferIO()

  46. localbuf: Fix dangerous coding pattern in GetLocalVictimBuffer()

  47. localbuf: Introduce StartLocalBufferIO()

  48. localbuf: Introduce InvalidateLocalBuffer()

  49. Allow lwlocks to be disowned

  50. Make jsonb casts to scalar types translate JSON null to SQL NULL.

  51. bufmgr/smgr: Don't cross segment boundaries in StartReadBuffers()

  52. Use aux process resource owner in walsender

  53. bufmgr: Return early in ScheduleBufferTagForWriteback() if fsync=off

On Tue, Mar 18, 2025 at 4:12 PM Andres Freund <andres@anarazel.de> wrote:
>
> Attached is v2.10

This is a review of 0002:  bufmgr: Improve stats when buffer is read
in concurrently

In the commit message, it might be worth distinguishing that
pg_stat_io and vacuum didn't double count reads, they under-counted
hits. pgBufferUsage and relation-level stats (pg_stat_all_tables etc)
overcounted reads and undercounted hits.

Quick example:
On master, if we try to read 7 blocks and 3 were hits and 2 were
completed by someone else then
- pg_stat_io and VacuumCostBalance would record 3 hits and 2 reads,
which looks like 2 misses.
- pgBufferUsage would record 3 hits and 4 reads, which looks like 4 misses.
- pg_stat_all_tables would record 3 hits and 7 reads, which looks like 4 misses.

The correct number of misses is 2 misses comprising 5 hits and 2 reads
(or 7 reads and 5 hits for pg_stat_all_tables which does the math
later).

@@ -1463,8 +1450,13 @@ WaitReadBuffers(ReadBuffersOperation *operation)
        if (!WaitReadBuffersCanStartIO(buffers[i], false))
        {
            /*
-            * Report this as a 'hit' for this backend, even though it must
-            * have started out as a miss in PinBufferForBlock().
+            * Report and track this as a 'hit' for this backend, even though
+            * it must have started out as a miss in PinBufferForBlock().
+            *
+            * Some of the accesses would otherwise never be counted (e.g.
+            * pgBufferUsage) or counted as a miss (e.g.
+            * pgstat_count_buffer_hit(), as we always call
+            * pgstat_count_buffer_read()).
             */

I think this comment should be changed. It reads like something
written when discovering this problem and not like something useful in
the future. I think you can probably drop the whole second paragraph.

You could make it even more clear by mentioning that the other backend
will count it as a read.

Otherwise, LGTM


- Melanie