Horton

Re: AIO v2.3

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: pgsql-hackers@postgresql.org
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Noah Misch <noah@leadboat.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>
Date: 2025-01-23T04:29:02Z
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 aio: Fix reference to outdated name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

Attachments

Hi,

Attached is v2.3.

There are a lot of changes - primarily renaming things based on on-list and
off-list feedback. But also some other things


Functional:

- Added pg_aios view

- md.c registering sync requests, that was previously omitted
  - This triggered stats issues during shutdown, as it can lead to IO workers
    emitting stats in some corner cases. I've written a patch series to
    address this [1]. For now I've included them in this patchset, but I would
    like to push the reordering patches soon.


- Testing error handling for temp table IO made me realize that the previous
  pattern of just tracking the refcount held by the IO subsystem in the
  LocalRefCount array leads to spurious buffer leak warnings [2].  I attached
  a prototype patch to deal with this by bringing localbuf.c more in line with
  bufmgr.c, but it needs some cleanup.

  That's in v2.3-0020-WIP-localbuf-Track-pincount-in-BufferDesc-as-we.patch


- Wait for all IOs to finish during shutdown. This is primarily required to
  ensure there aren't IOs initiated by a prior "owner" of a ProcNumber when a
  new backend starts. But there are also some kernels that don't like when
  exiting while IO is in flight.


- Re-armed local completion callbacks, they're required for correctness of
  temporary table IO


- Added a bunch of central debug helpers that only lead to output if
  PGAIO_VERBOSE is defined. That did make code a good bit more readable.



Polishing:

- Lots of copy editing, a lot of it thanks to feedback by Noah and Heikki


- Renamed the previous concept of a "subject" of an IO (i.e. what the IO is
  executed on, an smgr relation, a WAL file, ...) to "target".  I'm not in
  love with that name, but I went through dozens of variations, and it does
  seem better than subject.

  Not sure anymore how I ended up with subject, it's grammatically off and not
  very descriptive to boot.


- Renamed "PgAioHandleRef" and related functions to
  PgAioWaitRef/pgaio_wref_*(), that seems a lot more descriptive.


- Renamed pgaio_io_get() to pgaio_io_acquire()


- Renamed the IO handle states (PREPARED to STAGED, IN_FLIGHT to SUBMITTED,
  REAPED to COMPLETED_IO).

  Particularly the various COMPLETED state names aren't necessarily final,
  I've been debating a bunch of variations with Thomas and Robert


- Renamed aio_ref.h to aio_types.h, moved a few more types into it.


- Renamed completion callbacks to not use "shared" anymore - ->prepare was not
  really shared and now local callbacks are back (in a restricted form).

  s/PgAioHandleSharedCallback/PgAioHandleCallback/
  s/pgaio_io_add_shared_cb/pgaio_io_register_callbacks/

  Not entirely sure *register_callbacks is the best, happy to adjust.


- Renamed the ->error IO handle callback to ->report

  Also renamed s/pgaio_result_log/pgaio_result_report/g


- Renamed the ->prepare IO handle callback to ->stage

  Also renamed s/pgaio_result_log/pgaio_result_report/g


- Partially addressed request to reorder aio/README.md


- Determine shared memory allocation size with PG_IOV_MAX not io_combine_limit

  io_combine_limit is USERSET, so it's not correct to use it for shmem
  allocations. I chose PG_IOV_MAX instead of MAX_IO_COMBINE_LIMIT because this
  is a more generic limit than bufmgr.c IO.


- Prefix PgAio* enums with PGAIO_, global variables with pgaio_*


- Split out callback related cod from aio_subject.c (now aio_target.c) into
  aio_callback.c. The target specific code is rather small, so this makes a
  lot more sense.


- Distributed functions into more appropriate .c files, documented the choice
  in aio.h, reorder them


- Disowned lwlock: More consistent naming, reduce diff size, resume interrupts

  Heikki asked to clear ->owner when disowning the lock - but we currently
  *never* clear it doesn't seem right to do so when disowning the lock.


- IO data that can be set on a handle (to e.g. transport an array of Buffers
  to the completion callbacks) is now done with
  pgaio_io_(get|set)_handle_data(). Mainly to distinguish it from data that's
  actually the target/source of a read/write.

  Heikki suggested to make this per-callback data, but I don't think there's
  currently a use case for that, and it'd add a fair bit of memory overhead. I
  added a comment documenting this.


- Lots of other cleanups, added comments and the like



Todo:

- Reorder README further


- Make per backend state not indexed by ProcNumber, as that requires reserving
  per-backend state for IO workers, which will never need them


- Clean up localbuf.c "preparation" patches


- Add more tests - I had hoped to get to this, but got sidetracked with a
  bunch of things I found while testing


- I started looking into having a distinct type for the public pgaio_io_*
  related functions that can be used just by the issuer of the IO. It does
  make things a bit easier to understand, but also complicates naming. Not
  sure if it's worth it yet.


- Need to define (and test) the behavior when an IO worker fails to reopen the
  file for an IO


- Heikki doesn't love pgaio_submit_staged(), suggested pgaio_kick_staged() or
  such. I don't love that name though.


- There's some duplicated code in aio_callback.c, it'd be nice to deduplicate
  the callback invaction of the different callbacks


- Local callbacks are triggered from within pgaio_io_reclaim(), that's not
  exactly pretty. But it's currently the most central place to deal with the
  case of IOs for which the shared completion callback was called in another
  backend.


- As Jakub suggested (below [3]), when io_method=io_uring is used, we can run
  of out file descriptors much more easily. At the very least we need a good
  error message, perhaps also some rlimit adjusting (probably as a second
  step, if so).


- Thomas is working on the read_stream.c <-> bufmgr.c integration piece


- Start to write docs adjustments



[1] https://postgr.es/m/kgng5nrvnlv335evmsuvpnh354rw7qyazl73kdysev2cr2v5zu%40m3cfzxicm5kp
[2] https://postgr.es/m/j6hny5ivrfqw356ugoy3ti5ccadamluekxod4k6amao5snew6c%40t5h3bwhrgfqx
[3] https://postgr.es/m/tp63m6tcbi7mmsjlqgxd55sghhwvjxp3mkgeljffkbaujezvdl%40fvmdr3c6uhat