Horton

Re: AIO v2.5

Noah Misch <noah@leadboat.com>

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From: Noah Misch <noah@leadboat.com>
To: Andres Freund <andres@anarazel.de>
Cc: 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>, Antonin Houska <ah@cybertec.at>
Date: 2025-04-04T21:18:02Z
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 aio: Fix reference to outdated name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

On Fri, Apr 04, 2025 at 03:16:18PM -0400, Andres Freund wrote:
> On 2025-04-03 12:40:23 -0700, Noah Misch wrote:
> > On Thu, Apr 03, 2025 at 02:19:43PM -0400, Andres Freund wrote:

> > In the general case, we could want client requests as follows:
> > 
> > - If completor==definer and has not dropped pin:
> >   - Make defined before verifying page.  That's all.  It might be cleaner to
> >     do this when first retrieving a return value from io_uring, since this
> >     just makes up for what Valgrind already does for readv().
> 
> Yea, I think it's better to do that in io_uring. It's what I have done in the
> attached.
> 
> 
> > - If completor!=definer or has dropped pin:
> >   - Make NOACCESS in definer when definer cedes its own pin.
> 
> That's the current behaviour for shared buffers, right?

Yes.

> >   - For io_method=worker, make UNDEFINED before starting readv().  It might be
> >     cleanest to do this when the worker first acts as the owner of the AIO
> >     subsystem pin, if that's a clear moment earlier than readv().
> 
> Hm, what do we need this for?

At the time, we likely didn't need it:

- If the worker does its own PinBuffer*()+unpin, we don't need it.  Those
  functions do the Valgrind client requests.
- If the worker relies on the AIO-subsystem-owned pin and does neither regular
  pin nor regular unpin, we don't need it.  Buffers are always "defined".
- If the worker relies on the AIO-subsystem-owned pin to skip PinBuffer*() but
  uses regular unpin code, then the buffer may be NOACCESS.  Then one would
  need this.  But this would be questionable for other reasons.

Your proposed change to set NOACCESS in buffer_readv_complete_one() interacts
with things further, making the UNDEFINED necessary.

> >   - Make DEFINED in completor before verifying page.  It might be cleaner to
> >     do this when the completor first retrieves a return value from io_uring,
> >     since this just makes up for what Valgrind already does for readv().
> 
> I think we can't rely on the marking during retrieving it from io_uring, as
> that might have happened in a different backend for a temp buffer. That'd only
> happen if we got io_uring events for *another* IO that involved a shared rel,
> but it can happen.

Good point.  I think the VALGRIND_MAKE_MEM_DEFINED() in
pgaio_uring_drain_locked() isn't currently needed at all.  If
completor-subxact==definer-subxact, PinBuffer() already did what Valgrind
needs.  Otherwise, buffer_readv_complete_one() does what Valgrind needs.

If that's right, it would still be nice to reach the right
VALGRIND_MAKE_MEM_DEFINED() without involving bufmgr.  That helps future,
non-bufmgr AIO use cases.  It's tricky to pick the right place for that
VALGRIND_MAKE_MEM_DEFINED():

- pgaio_uring_drain_locked() is problematic, I think.  In the localbuf case,
  the iovec base address is relevant only in the ioh-defining process.  In the
  shmem completor!=definer case, this runs only in the completor.

- A complete_local callback solves those problems.  However, if the
  AIO-defining subxact aborted, then we shouldn't set DEFINED at all, since
  the buffer mapping may have changed by the time of complete_local.

- Putting it in the place that would call pgaio_result_report(ERROR) if
  needed, e.g. ProcessReadBuffersResult(), solves the problem of the buffer
  mapping having moved.  ProcessReadBuffersResult() doesn't even need this,
  since PinBuffer() already did it.  Each future AIO use case will have a
  counterpart of ProcessReadBuffersResult() that consumes the result and
  proceeds with tasks that depend on the AIO.  That's the place.

Is that right?  I got this wrong a few times while trying to think through it,
so I'm not too confident in the above.

> > > Not quite sure if we should mark
> > > the entire IOV is efined or just the portion that was actually read - the
> > > latter is additional fiddly code, and it's not clear it's likely to be helpful?
> > 
> > Seems fine to do the simpler way if that saves fiddly code.
> 
> Can't quite decide, it's just at the border of what I consider too
> fiddly... See the change to method_io_uring.c in the attached patch.

It is at the border, as you say, but I'd tend to keep it.


> Subject: [PATCH v1 1/3] localbuf: Add Valgrind buffer access instrumentation

Ready for commit


> Subject: [PATCH v1 2/3] aio: Make AIO compatible with valgrind

See above about pgaio_uring_drain_locked().

> related code until it is pinned bu "user" code again. But it requires some

s/bu/by/

> + * Return the iovecand its length. Currently only expected to be used by

s/iovecand/iovec and/

> @@ -361,13 +405,16 @@ pgaio_uring_drain_locked(PgAioUringContext *context)
>  		for (int i = 0; i < ncqes; i++)
>  		{
>  			struct io_uring_cqe *cqe = cqes[i];
> +			int32		res;
>  			PgAioHandle *ioh;
>  
>  			ioh = io_uring_cqe_get_data(cqe);
>  			errcallback.arg = ioh;
> +			res = cqe->res;
> +
>  			io_uring_cqe_seen(&context->io_uring_ring, cqe);
>  
> -			pgaio_io_process_completion(ioh, cqe->res);
> +			pgaio_uring_io_process_completion(ioh, res);

I guess this is a distinct cleanup, done to avoid any suspicion of cqe being
reused asynchronously after io_uring_cqe_seen().  Is that right?


> Subject: [PATCH v1 3/3] aio: Avoid spurious coverity warning

Ready for commit