Horton

Re: AIO v2.5

Thomas Munro <thomas.munro@gmail.com>

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From: Thomas Munro <thomas.munro@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Noah Misch <noah@leadboat.com>, Antonin Houska <ah@cybertec.at>, pgsql-hackers@postgresql.org, Heikki Linnakangas <hlinnaka@iki.fi>, Robert Haas <robertmhaas@gmail.com>, Jakub Wartak <jakub.wartak@enterprisedb.com>, Jelte Fennema-Nio <postgres@jeltef.nl>
Date: 2025-03-24T01:11:24Z
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 aio: Fix reference to outdated name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 Mon, Mar 24, 2025 at 5:59 AM Andres Freund <andres@anarazel.de> wrote:
> On 2025-03-23 08:55:29 -0700, Noah Misch wrote:
> > 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.

It is a bit strange that the documentation doesn't say that
explicitly.  You can sorta-maybe-kinda infer it from the fact that
io_uring didn't originally support cancelling requests at all, maybe a
small clue that it also didn't cancel them when you closed the fd :-)
The only sane alternative would seem to be that they keep running and
have their own reference to the *file* (not the fd), which is the
actual case, and might also be inferrable at a stretch from the
io_uring_register() documentation that says it reduces overheads with
a "long term reference" reducing "per-I/O overhead".  (The distant
third option/non-option is a sort of late/async binding fd as seen in
the Glibc user space POSIX AIO implementation, but that sort of
madness doesn't seem to be the sort of thing anyone working in the
kernel would entertain for a nanosecond...)  Anyway, there are also
public discussions involving Mr Axboe that discuss the fact that async
operations continue to run when the associated fd is closed, eg from
people who were surprised by that when porting stuff from other
systems, which might help fill in the documentation gap a teensy bit
if people want to see something outside the source code:

https://github.com/axboe/liburing/issues/568

> 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].

Just for the record while remembering this stuff: Windows is another
system that took the cancel-on-close approach, so the Windows IOCP
proof-of-concept patches also used that AIO v1 callback and we'll have
to think about that again if/when we want to get that stuff
going on AIO v2.  I recall also speculating that it might be better to
teach the vfd system to pick another victim to close instead if an fd
was currently tied up with an asynchronous I/O for the benefit of
those cancel-on-close systems, hopefully without any happy-path
book-keeping.  But just submitting staged I/O is a nice and cheap
solution for now, without them in the picture.