Horton

Re: AIO v2.2

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: pgsql-hackers@postgresql.org
Date: 2025-01-01T04:03:33Z
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 aio: Fix reference to outdated name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 a new version of the AIO patchset.


The biggest changes are:

- The README has been extended with an overview of the API. I think it gives a
  good overview of how the API fits together.  I'd be very good to get
  feedback from folks that aren't as familiar with AIO, I can't really see
  what's easy/hard anymore.


- The read/write patches and the bounce buffer patches are split out, so that
  there's no dependency between the first few AIO patches and the "don't dirty
  while IO is going on" patcheset [1].


- Retries for partial IOs (i.e. short reads) are now implemented. Turned out
  to take all of three lines and adding one missing variable initialization.


- I added quite a lot of function-header and file-header comments. There's
  more to be done here, but see also the TODO section below.


- IO stats are now tracked. Specifically, the "time" for an IO is now the time
  spent waiting for an IO, as discussed around [2].  I haven't updated the
  docs yet.


- There now is a fastpath for executing AIO "synchronously", i.e. preparing an
  IO and immediately submitting it.


- Previously one needed very large effective_io_concurrency values to get
  sufficient asynchronous IO for sequential scans, as read_stream.c limited
  max_pinned_buffers to effective_io_concurrency * 4. Unless
  effective_io_concurrency was very high, that'd only allow a single IO to be
  in-flight, due to io_combine_limit buffers getting merged into one IO.

  Instead the pin limit is now capped by effective_io_concurrency *
  io_combine_limit.

  Right now that's part of one larger "hack up read_stream.c" commit, Thomas
  said he'd take a look at how to do this properly.  This is probably
  something we could and should commit separately.


- io_method = sync has been made more similar to the way IO happens today. In
  particular, we now continue to issue prefetch requests and the actual IO is
  done only within WaitReadBuffers().


- When using buffered IO with io_uring, there previously was a small
  regression, due to more IO happening in the process context with io_uring
  (instead of in a kernel thread). While one could argue that it's better to
  not increase CPU usage beyond one process, I don't find that sufficiently
  convincing. To work around that I added a heuritic that tells IO uring to
  execute IOs using it's worker infrastructure. That seems to have fixed this
  problem entirely.


- IO worker infrastructure was cleaned up


- I pushed a few minor preliminary commits a while ago


- lots of other smaller stuff



The biggest TODOs are:

- Right now the API between bufmgr.c and read_stream.c kind of necessitates
  that one StartReadBuffers() call actually can trigger multiple IOs, if
  one of the buffers was read in by another backend, before "this" backend
  called StartBufferIO().

  I think Thomas and I figured out a way to evolve the interface so that this
  isn't necessary anymore:

  We allow StartReadBuffers() to memorize buffers it pinned but didn't
  initiate IO on in the buffers[] argument. The next call to StartReadBuffers
  then doesn't have to repin thse buffers. That doesn't just solve the
  multiple-IOs for one "read operation" issue, it also make the - very common
  - case of a bunch of "buffer misses" followed by a "buffer hit" cleaner, the
  hit wouldn't be tracked in the same ReadBuffersOperation anymore.


- Right now bufmgr.h includes aio.h, because it needs to include a reference
  to the AIO's result in ReadBuffersOperation. Requiring a dynamic allocation
  would be noticeable overhead, so that's not an option. I think the best
  option here would be to introduce something like aio_types.h, so fewer
  things are included.


- There's no obvious way to tell "internal" function operating on an IO handle
  apart from functions that are expected to be called by the issuer of an IO.

  One way to deal with this would be to introduce a distinct "issuer IO
  reference" type.  I think that might be a good idea, it would also make it
  clearer that a good number of the functions can only be called by the
  issuer, before the IO is submitted.

  This would also make it easier to order functions more sensibly in aio.c, as
  all the issuer functions would be together.

  The functions on AIO handles that everyone can call already have a distinct
  type (PgAioHandleRef vs PgAioHandle*).


- While I've added a lot of comments, I only got so far adding them. More are
  needed.


- The naming around PgAioReturn, PgAioResult, PgAioResultStatus needs to be
  improved


- The debug logging functions are a bit of a mess, lots of very similar code
  in lots of places. I think AIO needs a few ereport() wrappers to make this
  easier.


- More tests are needed. None of our current test frameworks really makes this
  easy :(.


- Several folks asked for pg_stat_aio to come back, in "v1" that showed the
  set of currently in-flight AIOs.  That's not particularly hard - except
  that it doesn't really fit in the pg_stat_* namespace.


- I'm not sure that effective_io_concurrency as we have it right now really
  makes sense, particularly not with the current default values. But that's a
  mostly independent change.


Greetings,

Andres Freund

[1] https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf%40gcnactj4z56m
[2] https://postgr.es/m/tp63m6tcbi7mmsjlqgxd55sghhwvjxp3mkgeljffkbaujezvdl%40fvmdr3c6uhat