Horton

Re: AIO v2.5

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Noah Misch <noah@leadboat.com>
Cc: Antonin Houska <ah@cybertec.at>, 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: 2025-03-23T16:59:43Z
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 aio: Fix reference to outdated name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 2025-03-23 08:55:29 -0700, Noah Misch wrote:
> On Sun, Mar 23, 2025 at 11:11:53AM -0400, Andres Freund wrote:
> Unrelated to the above, another question about io_uring:
> 
> commit da722699 wrote:
> > +/*
> > + * Need to submit staged but not yet submitted IOs using the fd, otherwise
> > + * the IO would end up targeting something bogus.
> > + */
> > +void
> > +pgaio_closing_fd(int fd)
> 
> An IO in PGAIO_HS_STAGED clearly blocks closing the IO's FD, and an IO in
> PGAIO_HS_COMPLETED_IO clearly doesn't block that close.  For io_method=worker,
> closing in PGAIO_HS_SUBMITTED is okay.  For io_method=io_uring, is there a
> reference about it being okay to close during PGAIO_HS_SUBMITTED?  I looked
> awhile for an authoritative view on that, but I didn't find one.  If we can
> rely on io_uring_submit() returning only after the kernel has given the
> io_uring its own reference to all applicable file descriptors, I expect it's
> okay to close the process's FD.  If the io_uring acquires its reference later
> than that, I expect we shouldn't close before that later time.

I'm fairly sure io_uring has its own reference for the file descriptor by the
time io_uring_enter() returns [1].  What io_uring does *not* reliably tolerate
is the issuing process *exiting* before the IO completes, even if there are
other processes attached to the same io_uring instance.

AIO v1 had a posix_aio backend, which, on several platforms, did *not*
tolerate the FD being closed before the IO completes. Because of that
IoMethodOps had a closing_fd callback, which posix_aio used to wait for the
IO's completion [2].


I've added a test case exercising this path for all io methods. But I can't
think of a way that would catch io_uring not actually holding a reference to
the fd with a high likelihood - the IO will almost always complete quickly
enough to not be able to catch that. But it still seems better than not at all
testing the path - it does catch at least the problem of pgaio_closing_fd()
not doing anything.

Greetings,

Andres Freund

[1] See
  https://github.com/torvalds/linux/blob/586de92313fcab8ed84ac5f78f4d2aae2db92c59/io_uring/io_uring.c#L1728
  called from
  https://github.com/torvalds/linux/blob/586de92313fcab8ed84ac5f78f4d2aae2db92c59/io_uring/io_uring.c#L2204
  called from
  https://github.com/torvalds/linux/blob/586de92313fcab8ed84ac5f78f4d2aae2db92c59/io_uring/io_uring.c#L3372
  in the io_uring_enter() syscall

[2] https://github.com/anarazel/postgres/blob/a08cd717b5af4e51afb25ec86623973158a72ab9/src/backend/storage/aio/aio_posix.c#L738