Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Matthias van de Meent <boekewurm+postgres@gmail.com>
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: 2025-07-09T14:59:41Z
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 aio: Fix reference to outdated name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

Hi,

On 2025-07-09 13:26:09 +0200, Matthias van de Meent wrote:
> I've been going through the new AIO code as an effort to rebase and
> adapt Neon to PG18. In doing so, I found the following
> items/curiosities:
> 
> 1. In aio/README.md, the following code snippet is found:
> 
> [...]
> pgaio_io_set_handle_data_32(ioh, (uint32 *) buffer, 1);
> [...]
> 
> I believe it would be clearer if it took a reference to the buffer:
> 
> pgaio_io_set_handle_data_32(ioh, (uint32 *) &buffer, 1);
> 
> The main reason here is that common practice is to have a `Buffer
> buffer;` whereas a Buffer * is more commonly plural.

It's also just simply wrong as-is :/. Interpreting the buffer id as a pointer
obviously makes no sense...


> 
> 2. In aio.h, PgAioHandleCallbackID is checked to fit in
> PGAIO_RESULT_ID_BITS (though the value of PGAIO_HCB_MAX).
> However, the check is off by 1:
> 
> ```
> #define PGAIO_HCB_MAX    PGAIO_HCB_LOCAL_BUFFER_READV
> StaticAssertDecl(PGAIO_HCB_MAX <= (1 << PGAIO_RESULT_ID_BITS), [...])
> ```
> 
> This static assert will not trigger when PGAIO_HCB_MAX is equal to
> 2^PGAIO_RESULT_ID_BITS, but its value won't fit in those 6 bits and
> instead overflow to 0.
> To fix this, I suggest the `<=` is replaced with `<` to make that
> work, or the definition of PGAIO_HCB_MAX to be updated to
> PGAIO_HCB_LOCAL_BUFFER_READV + 1.

Ooops.



> 3. I noticed that there is AIO code for writev-related operations
> (specifically, pgaio_io_start_writev is exposed, as is
> PGAIO_OP_WRITEV), but no practical way to excercise that code: it's
> not called from anywhere in the project, and there is no way for
> extensions to register the relevant callbacks required to make writev
> work well on buffered contents. Is that intentional?

Yes.  We obviously do want to support writes eventually, and it didn't seem
useful to not have the most basic code for writes in the AIO infrastructure.

You could still use it to e.g. write out temporary file data or such.


> and there is no way for extensions to register the relevant callbacks
> required to make writev work well on buffered contents. Is that intentional?

FWIW, the problem with writev for buffered IO is not so much with the AIO
infrastructure, but with buffer locking and sync.c...




> Relatedly, the rationale for using enum PgAioHandleCallbackID rather
> than function pointers in the documentation above its definition says
> that function pointer instability across backends is one of the 2 main
> reasons.
> Is there any example of OS or linker behaviour that does not start
> PostgreSQL with stable function pointer addresses across backends of
> the same PostgreSQL binary? Or is this designed with extensibility
> and/or cross-version EXEC_BACKEND in mind, but with extensibility not
> yet been implemented due to $constraints?

It's due to EXEC_BACKEND - function pointers aren't stable with EXEC_BACKEND
even *without* cross-version issues. Due to ASLR function/data pointers may be
shifted around.

Even without EXEC_BACKEND we probably would want to have something like
PgAioHandleCallbackID, just to save space. But it'd be a lot easier to make it
extensible, because we could add entries once, rather than having to do so in
every process.


I suspect to make this extensible in the face of EXEC_BACKEND we'd want an
array of dynamically registered callbacks, in shared memory, that has the
library_name/symbol_name for the location of each callback struct, to be
registered in _PG_init(), which then would be entered into the backend-local
table in pgaio_init_backend().

Personally I'm not particularly interested in investing the energy to make
that work, I'd rather wait till we have threading and focus on the
infrastructure work for AIO writes etc. But then I don't work on an extension
these days, so I'm not directly affected by this not being extensible - I'm
not opposed to somebody else doing that work.

Greetings,

Andres Freund