Horton

Re: AIO v2.2

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Heikki Linnakangas <hlinnaka@iki.fi>
Cc: pgsql-hackers@postgresql.org
Date: 2025-01-08T22:56:04Z
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 aio: Fix reference to outdated name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 2025-01-07 22:09:56 +0200, Heikki Linnakangas wrote:
> On 07/01/2025 18:11, Andres Freund wrote:
> > > I didn't quite understand the point of the prepare callbacks. For example,
> > > when AsyncReadBuffers() calls smgrstartreadv(), the
> > > shared_buffer_readv_prepare() callback will be called. Why doesn't
> > > AsyncReadBuffers() do the "prepare" work itself directly; why does it need
> > > to be in a callback?
> >
> > One big part of it is "ownership" - while the IO isn't completely "assembled",
> > we can release all buffer pins etc in case of an error. But if the error
> > happens just after the IO was staged, we can't - the buffer is still
> > referenced by the IO. For that the AIO subystem needs to take its own pins
> > etc.  Initially the prepare callback didn't exist, the code in
> > AsyncReadBuffers() was a lot more complicated before it.
> >
> >
> > > I assume it's somehow related to error handling, but I didn't quite get
> > > it. Perhaps an "abort" callback that'd be called on error, instead of a
> > > "prepare" callback, would be better?
> >
> > I don't think an error callback would be helpful - the whole thing is that we
> > basically need claim ownership of all IO related resources IFF the IO is
> > staged. Not before (because then the IO not getting staged would mean we have
> > a resource leak), not after (because we might error out and thus not keep
> > e.g. buffers pinned).
>
> Hmm. The comments say that when you call smgrstartreadv(), the IO handle may
> no longer be modified, as the IO may be executed immediately. What if we
> changed that so that it never submits the IO, only adds the necessary
> callbacks to it?

> In that world, when smgrstartreadv() returns, the necessary details and
> completion callbacks have been set in the IO handle, but the caller can
> still do more preparation before the IO is submitted. The caller must ensure
> that it gets submitted, however, so no erroring out in that state.
>
> Currently the call stack looks like this:
>
> AsyncReadBuffers()
> -> smgrstartreadv()
>   -> mdstartreadv()
>     -> FileStartReadV()
>       -> pgaio_io_prep_readv()
>         -> shared_buffer_readv_prepare() (callback)
>         <- (return)
>       <- (return)
>     <- (return)
>   <- (return)
> <- (return)
>
> I'm thinking that the prepare work is done "on the way up" instead:
>
> AsyncReadBuffers()
> -> smgrstartreadv()
>   -> mdstartreadv()
>     -> FileStartReadV()
>       -> pgaio_io_prep_readv()
>       <- (return)
>     <- (return)
>   <- (return)
> -> shared_buffer_readv_prepare()
> <- (return)
>
> Attached is a patch to demonstrate concretely what I mean.

I think this would be somewhat limiting. Right now it's indeed just bufmgr.c
that needs to do a preparation (or "moving of ownership") step - but I don't
think it's necessarily going to stay that way.

Consider e.g. a hypothetical threaded future in which we have refcounted file
descriptors. While AIO is ongoing, the AIO subsystem would need to ensure that
the FD refcount is increased, otherwise you'd obviously run into trouble if
the issuing backend errored out and released its own reference as part of
resowner release.

I don't think the approach you suggest above would scale well for such a
situation - shared_buffer_readv_prepare() would again need to call to
smgr->md->fd.  Whereas with the current approach md.c (or fd.c?) could just
define its own prepare callback that increased the refcount at the right
moment.


There's a few other scenarios I can think of:

- If somebody were - no idea what made me think of that - to write an smgr
  implementation where storage is accessed over the network, one might need to
  keep network buffers and sockets alive for the duration of the IO.

- It'd be rather useful to have support for asynchronously extending a
  relation, that often requires filesystem journal IO and thus is slow. If
  you're bulk loading, or the extension lock is contented, it'd be great if we
  could start the next relation extension *before* it's needed and the
  extension has to happen synchronously.  To avoid deadlocks, such an
  asynchronous extension would need to be able to release the lock in any
  other backend, just like it's needed for the content locks when
  asynchronously writing.  Which in turn would require transferring ownership
  of the relevant buffers *and* the extension lock.  You could mash this
  together, but it seems like a separate callback woul make it more
  composable.


Does that make any sense to you?


> This adds a new pgaio_io_stage() step to the issuer, and the issuer needs to
> call the prepare functions explicitly, instead of having them as callbacks.
> Nominally that's more steps, but IMHO it's better to be explicit. The same
> actions were happening previously too, it was just hidden in the callback. I
> updated the README to show that too.
>
> I'm not wedded to this, but it feels a little better to me.

Right now the current approach seems to make more sense to me, but I'll think
about it more. I might also have missed something with my theorizing above.


Greetings,

Andres Freund