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: 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: 2025-04-04T19:16:18Z
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 aio: Fix reference to outdated name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,

Sorry for the slow work on this. The cycle times are humonguous due to
valgrind being so slow...


On 2025-04-03 12:40:23 -0700, Noah Misch wrote:
> On Thu, Apr 03, 2025 at 02:19:43PM -0400, Andres Freund wrote:
> > The best fix for that one would, I think, be to have method_io_uring() iterate
> > over the IOV and mark the relevant regions as defined?  That does fix the
> > issue at least and does seem to make sense?
> 
> Makes sense.  Valgrind knows that read() makes its target bytes "defined".  It
> probably doesn't have an io_uring equivalent for that.

Correct - and I think it would be nontrivial to add, because there's not easy
syscall to intercept...


> I expect we only need this for local buffers, and it's unclear to me how the
> fix for (4a) didn't fix this.

At that time I didn't apply the fix in 4a) to local buffers, because local
buffers, in HEAD, don't have the valgrind integration. Without that marking
the buffer as NOACCESS would cause all sorts of issues, because it'd be
considered inaccessible even after pinning.  As you analyzed, that then ends
up considered undefined due to the MemoryContextAlloc().


> 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?


>   - 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?


>   - 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.



> > 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.


> > Questions:
> > 
> > 1) It'd be cleaner to implement valgrind support in localbuf.c, so we don't
> >    need to have special-case logic for that. But it also makes the change less
> >    localized and more "impactful", who knows what kind of skullduggery we have
> >    been getting away with unnoticed.
> > 
> >    I haven't written the code up yet, but I don't think it'd be all that much
> >    code to add valgrind support to localbuf.
> 
> It would be the right thing long-term, and it's not a big deal if it causes
> some false positives initially.  So if you're leaning that way, that's good.

It was easy enough.

I saw one related failure, FlushRelationBuffers() didn't pin temporary buffers
before flushing them. Pinning the buffers fixed that.

I don't think it's a real problem to not pin the local buffer during
FlushRelationBuffers(), at least not today. But it seems unnecessarily odd to
not pin it.


I wish valgrind had a way to mark the buffer as inaccessible and then
accessible again, without loosing the defined-ness information...


Greetings,

Andres Freund